DefinitionA standard drawing used to show a component in a circuit diagram. Circuit diagrams are drawn with a pencil and ruler, using straight lines for the wires.
02
Complete circuit
DefinitionA circuit with a full path and no gaps, so charge can flow all the way round. If there is a gap, the current is zero.
03
Series circuit
DefinitionA circuit with only one route around it, so the current must flow through every component. The current is the same everywhere and the supply p.d. is shared.
04
Parallel circuit
DefinitionA circuit with more than one route, so the current does not have to flow through every component. Each branch gets the full supply p.d. and the current is shared.
05
Charge
DefinitionA property carried by electrons as they are pushed around a circuit by a cell or power supply. Measured in coulombs (C).
06
Current
DefinitionThe rate of flow of charge — how much charge flows past a point each second. Measured in amperes (A) with an ammeter, connected in series.
07
Ammeter
DefinitionThe instrument used to measure current. Always connected in series with the component whose current is being measured.
08
Potential difference
DefinitionThe energy transferred per unit charge between two points. Measured in volts (V) with a voltmeter, connected in parallel.
09
Voltmeter
DefinitionThe instrument used to measure potential difference. Always connected in parallel across the component.
10
Resistance
DefinitionHow much a component opposes the flow of charge. Measured in ohms (Ω). Greater resistance means a smaller current for a given p.d.
11
Ohm’s law
DefinitionFor an ohmic conductor at constant temperature, current is directly proportional to potential difference. The I–V graph is a straight line through the origin.
12
Ohmic conductor
DefinitionA component whose resistance stays constant, as long as its temperature does not change.
13
I–V characteristic
DefinitionA graph of current against potential difference for a component. Its shape identifies the component.
14
Filament lamp
DefinitionA lamp whose resistance increases as it heats up, because the hotter ions vibrate more and electrons collide with them more often. Its I–V graph is an S-shape.
15
Diode
DefinitionA component that only lets current flow in one direction (forward). It has a very high resistance in the reverse direction.
16
LDR
DefinitionLight-dependent resistor. Its resistance decreases as light intensity increases. Used in street lights and security lights.
17
Thermistor
DefinitionA resistor whose resistance decreases as temperature increases. Used in thermostats, fire alarms and engine temperature sensors.
18
Sensing circuit
DefinitionA circuit that uses a sensor (LDR or thermistor) in series with a fixed resistor, so a change in the surroundings changes the p.d. and switches another device on or off.
19
Direct current (d.c.)
DefinitionCurrent that flows in one direction only. Supplied by cells and batteries.
20
Alternating current (a.c.)
DefinitionCurrent that repeatedly changes direction. Mains electricity is a.c.
21
UK mains supply
DefinitionAbout 230 V at a frequency of 50 Hz — the current changes direction 50 times every second.
22
Live wire
DefinitionThe brown wire, which carries the alternating p.d. from the supply (about 230 V). It is the dangerous one.
23
Neutral wire
DefinitionThe blue wire, which completes the circuit and stays near 0 V.
24
Earth wire
DefinitionThe green-and-yellow safety wire. If a fault makes the metal case live, it carries the current safely away to earth.
25
Fuse
DefinitionA thin wire inside a glass or ceramic case that melts if the current gets too large, breaking the circuit. Fit the smallest rating above the normal working current.
26
Double insulated
DefinitionAn appliance with a plastic case that cannot become live, so it needs only two wires and no earth wire.
27
Power
DefinitionThe energy transferred each second, measured in watts (W). 1 W = 1 J/s. Electrical power: P = V I or P = I2 R.
28
National Grid
DefinitionThe system of cables and transformers that carries electricity from power stations to consumers, at a high p.d. and a low current.
29
Transformer
DefinitionA device that changes the size of an alternating potential difference. Step-up increases it; step-down decreases it. For an ideal transformer, VpIp = VsIs.
30
Static electricity
DefinitionA build-up of charge that stays on the surface of an insulator after electrons have been transferred by friction. Only electrons move.
31
Electric field
DefinitionA region where a charge feels a force. Field lines go from positive to negative, never cross, and are closest where the field is strongest.
Lesson 1 · Circuit Symbols and Diagrams
Do Now
Q1
State the equation that links density, mass and volume.
Model Answerdensity = mass ÷ volume (ρ = m ÷ V). Density is in kg/m3 (or g/cm3), mass in kg, volume in m3.
Q2
Name the three states of matter.
Model Answersolid, liquid and gas.
Q3
State the principle of conservation of energy.
Model Answerenergy cannot be created or destroyed — it can only be transferred from one store to another, or dissipated to the surroundings.
Q4
Stretch: I want to make my torch brighter so I add lots of lamps in series. Does it work? Or should I try something else?
Model Answerno. Adding lamps in series adds resistance, so the current falls, and the supply p.d. is shared between more lamps — every lamp gets dimmer. Connect the lamps in parallel instead, so each one gets the full supply p.d.
Part 1 · Circuit Symbols
We use circuit symbols to show the different components (parts) of an electrical circuit.
Learn the standard symbols for each component shown below.
Fig 1.1 — Standard circuit symbols
Questions — Circuit Symbols
Q1
Draw the circuit symbol for each component: a) a cell b) a battery c) a lamp (bulb) d) an open switch e) an ammeterf) a voltmeter g) a resistor h) a variable resistor i) a fuse j) a diodek) an LED l) an LDR m) a thermistor(13 marks)
Model Answersymbols as shown in Fig 1.1 — one mark for each correctly drawn symbol.
Part 2 · Series and Parallel Circuits
Circuit diagrams are drawn with straight lines, using a pencil and ruler for the connecting wires.
For the circuit to work it must be complete — there must be no gaps for the current to cross.
Fig 1.2 — A complete circuit and an incomplete circuit
There are two types of circuit: series and parallel.
In a series circuit there is only one route around the circuit, so the current must flow through every component.
In a parallel circuit there is more than one route, so the current does not have to flow through every component.
Current always prefers to take the path of least resistance.
Fig 1.3 — A series circuit and a parallel circuit
Questions — Series and Parallel Circuits
Q2
What do we use to draw the connecting wires in a circuit diagram? (1 mark)
Model Answera pencil and ruler (straight lines).
Q3
What does it mean for a circuit to be complete? (1 mark)
Model Answerthere is a full path with no gaps, so current can flow all the way round.
Q4
How many routes are there around a series circuit? (1 mark)
Model Answerone.
Q5
In which type of circuit does the current have to flow through every component? (1 mark)
Model Answera series circuit.
Q6
Explain the difference between a series and a parallel circuit. (2 marks)
Model Answerin series there is one loop and the current flows through every component; in parallel there are separate branches and current does not have to flow through every component.
Q7
Look at Fig 1.3. State how many components are connected in the series circuit and how many separate branches the parallel circuit has. (2 marks)
Model Answerseries: two components (lamps A and B). Parallel: three branches.
Q8
A bulb in one branch of a parallel circuit is removed. Explain what happens to a bulb in another branch. (2 marks)
Model Answerit stays lit, because current can still flow round the other complete branch.
Q9
In a parallel circuit, explain why the branch with the lowest resistance carries the most current. (2 marks)
Model Answerthe potential difference is the same across each branch (they are in parallel); since I = V ÷ R, a lower resistance gives a larger current for the same p.d., so more current flows through the lowest-resistance branch.
Part 3 · Drawing and Measuring Circuits
When asked to draw a circuit, decide whether it is series or parallel, then place the components on a rectangle of wire.
Ammeters are always placed in series with the component whose current is being measured.
Voltmeters are always placed in parallel across the component whose potential difference is being measured.
Fig 1.4 — Ammeter in series, voltmeter in parallel
Drawing a circuit
Decide: series or parallel?
Draw the cell/battery first.
Add components: ammeter in series, voltmeter in parallel.
Questions — Drawing and Measuring Circuits
Q10
How is an ammeter connected in a circuit? (1 mark)
Model Answerin series.
Q11
How is a voltmeter connected in a circuit? (1 mark)
Model Answerin parallel.
Q12
Draw a series circuit containing two cells, a closed switch and a lamp. (3 marks)
Model Answertwo cells, a closed switch and a lamp all connected in one loop.
Q13
Draw a circuit with one cell and two lamps connected in parallel with each other. (3 marks)
Model Answerone cell connected to two lamps on separate parallel branches.
Q14
Draw a circuit to measure the current through, and the potential difference across, a single resistor. (3 marks)
Model Answercell, resistor and ammeter in series; voltmeter connected in parallel across the resistor (see Fig 1.4).
Exam-Style Question
The circuit diagram below contains a switch, a battery, a diode and a lamp, connected in one loop.
(a)
Use words from the box to label the components A, B and C in the circuit diagram. (3 marks)
cell diode lamp resistor switch
Model AnswerA = switch, B = battery, C = diode.
(b)
A student wants to investigate how the current through a 5 Ω resistor changes as the potential difference across it changes. Draw a circuit diagram she could use, using the correct symbols for each part of the circuit. (3 marks)
Model Answera cell (or battery), the 5 Ω resistor and an ammeter connected in series, with a voltmeter connected in parallel across the resistor (see Fig 1.4).
Total: 6 marks
Exit Ticket
1
Draw the circuit symbol for a lamp.
Model Answera circle with a cross through it.
2
How many routes are there around a parallel circuit?
Model Answermore than one — one for each branch.
3
Where must a voltmeter be connected to measure the p.d. across a resistor?
Model Answerin parallel, across the resistor.
Lesson 2 · Charge and Current
Do Now
Q1
What do we call a circuit that has no gaps for current to flow?
Model Answera complete circuit (a closed circuit).
Q2
What happens to the density of a substance when it melts from solid to liquid?
Model Answerfor most substances the density decreases slightly, because the mass stays the same but the particles spread out a little, increasing the volume. (Water is the exception — ice is less dense than liquid water.)
Q3
Name one renewable energy resource.
Model Answerany one of: wind, solar, hydroelectric, tidal, wave, geothermal, biofuel.
Q4
Stretch: If twice as much charge flows through a wire in the same amount of time, what do you think happens to the current? Explain your reasoning.
Model Answerthe current doubles. Current is the rate of flow of charge, so twice the charge in the same time means twice as much charge passes each second.
Part 1 · Charge and Current
Charge: negatively charged electrons are pushed around a circuit by a cell or power supply. Charge has units of coulombs (C).
Current is the rate of flow of charge — how much charge flows past a point each second.
Current has units of amperes (A) and is measured with an ammeter, which must be placed in series.
For charge to flow, the circuit must be complete and there must be a source of potential difference (e.g. a cell).
The same current flows everywhere in a single series loop, because charge is not used up — it is only transferred.
Current is conserved: the charge that flows into a component each second equals the charge that flows out.
Questions — Charge and Current
Q1
What is the name given to the rate of flow of charge? (1 mark)
Model Answercurrent.
Q2
State the unit of charge and the unit of current. (2 marks)
Model Answercharge: coulombs (C). Current: amperes (A).
Q3
What kind of particle flows around a metal circuit? (1 mark)
Model Answerelectrons.
Q4
Why must an ammeter be connected in series? (2 marks)
Model Answerso that all the charge flowing through the component also flows through the ammeter, allowing it to be measured.
Q5
State two things that are needed for charge to flow in a circuit. (2 marks)
Model Answera complete circuit and a source of potential difference (e.g. a cell).
Q6
Explain what happens to the current in a circuit if there is a break in it. (2 marks)
Model Answerthe circuit is no longer complete, so no charge can flow and the current is zero.
Q7
State two things that affect the size of the current in a circuit. (2 marks)
Model Answerthe potential difference of the supply and the total resistance of the circuit.
Q8
Explain why the current is the same at every point in a series circuit. (2 marks)
Model Answercharge is conserved (not used up); the charge entering a component each second equals the charge leaving it.
Part 2 · The Charge Equation
Charge, current and time are linked by the equation on the AQA equation sheet:
charge flow = current × time Q = I t
where Q is charge in coulombs (C), I is current in amperes (A) and t is time in seconds (s).
If a value is given in minutes or hours it must be converted into seconds before substituting.
Example (finding charge)
A current of 2 A flows for 30 s. Calculate the charge that flows.
V
I = 2 A t = 30 s Q = ?
E
Q = I t
S
Q = 2 × 30
S
Q = 60
U
C (coulombs)
Questions — The Charge Equation
Q9
Calculate the charge in each case (show your working with VESSU): a) I = 3 A, t = 10 sb) I = 0.5 A, t = 60 sc) I = 12 A, t = 5 sd) I = 0.2 A, t = 200 s(4 marks)
Model Answera) 30 C b) 30 C c) 60 C d) 40 C
Q10
A bed lamp is switched on for 10 minutes and works on a current of 0.5 A. How much charge flows? (3 marks)
Model Answer
Answer
t = 10 min = 600 s
V
I = 0.5 A t = 600 s Q = ?
E
Q = I t
S
Q = 0.5 × 600
S
Q = 300
U
C (coulombs)
Q11
A phone charger delivers a current of 1.2 A for 45 minutes. Calculate the charge delivered. (3 marks)
Model Answer
Answer
t = 45 min = 2700 s
V
I = 1.2 A t = 2700 s Q = ?
E
Q = I t
S
Q = 1.2 × 2700
S
Q = 3240
U
C (coulombs)
Part 3 · Finding Current and Time
The charge equation Q = I t can be rearranged to find current or time if the other two quantities are known.
Divide charge by time to find current; divide charge by current to find time.
Example (finding current)
4 C of charge passes a point in 8 s. Calculate the current.
V
Q = 4 C t = 8 s I = ?
E
Q = I t
S
4 = I × 8
S
I = 48 I = 0.5
U
A (amperes)
Example (finding time)
A kettle element passes a current of 5 A. How long does it take for 400 C of charge to flow?
V
Q = 400 C I = 5 A t = ?
E
Q = I t
S
400 = 5 × t
S
t = 4005 t = 80
U
s (seconds)
Questions — Finding Current and Time
Q12
Calculate the current in each case: a) Q = 240 C, t = 60 sb) Q = 90 C, t = 30 sc) Q = 50 C, t = 2 s(3 marks)
Model Answera) 4 A b) 3 A c) 25 A
Q13
Calculate the time in each case: a) Q = 20 C, I = 4 Ab) Q = 100 C, I = 2 Ac) Q = 6 C, I = 0.5 Ad) Q = 1000 C, I = 25 A(4 marks)
Model Answera) 5 s b) 50 s c) 12 s d) 40 s
Q14
During electrolysis a current of 6 A passes and 1.2 kC of charge flows. How long did the experiment last? (3 marks)
Model Answer
Answer
Q = 1.2 kC = 1200 C
V
Q = 1200 C I = 6 A t = ?
E
Q = I t
S
1200 = 6 × t
S
t = 12006 t = 200
U
s (seconds)
Exam-Style Question
The plug of an electrical appliance contains a fuse. The diagram below shows the structure of the fuse.
(a)
Which component in Fig 1.1 is the correct circuit symbol for a fuse? (1 mark)
Model Answerthe rectangle symbol (as shown in Fig 1.1).
(b)
Write down the equation that links charge flow, current and time. (1 mark)
Model AnswerQ = I t (charge flow = current × time).
(c)
The fuse wire melts when 1.52 coulombs of charge flows through the fuse in 0.40 seconds. Calculate the current at which the fuse wire melts. (3 marks)
Model AnswerI = Q ÷ t = 1.52 ÷ 0.40 = 3.8 A.
Total: 5 marks
Source: AQA GCSE Physics (8463) — June 2018, Paper 1 Foundation (8463/1F).
Exit Ticket
1
State the equation that links charge flow, current and time.
Model AnswerQ = I t.
2
What is the unit of charge?
Model Answerthe coulomb (C).
3
A current of 2 A flows for 1 minute. How much charge flows?
Model AnswerQ = I t = 2 × 60 = 120 C.
Lesson 3 · Resistance and Ohm’s Law
Do Now
Q1
What is the unit of electric charge?
Model Answerthe coulomb (C).
Q2
Define specific heat capacity.
Model Answerthe energy needed to raise the temperature of 1 kg of a substance by 1 °C.
Q3
State the equation that links power, energy and time.
Model Answerpower = energy transferred ÷ time (P = E ÷ t).
Q4
Stretch: A cell pushes charge around a circuit. If you added a second identical cell in series with it, what do you predict would happen to the current, and why?
Model Answerthe current would increase (roughly double). Two cells in series give twice the potential difference, and since I = V ÷ R, a larger p.d. across the same resistance drives a larger current.
Part 1 · Potential Difference and Resistance
Potential difference (p.d. or voltage) is the energy transferred per unit charge between two points. It is measured in volts (V) using a voltmeter in parallel.
Resistance opposes the flow of charge. It is measured in ohms (Ω).
The greater the resistance of a component, the smaller the current for a given potential difference.
Questions — Potential Difference and Resistance
Q1
Define potential difference. (1 mark)
Model Answerthe energy transferred per unit charge between two points in a circuit.
Q2
State the unit of potential difference and the unit of resistance. (2 marks)
Model Answerpotential difference: volts (V). Resistance: ohms (Ω).
Q3
How should a voltmeter be connected to measure the p.d. across a lamp? (1 mark)
Model Answerin parallel, across the lamp.
Q4
State what is meant by the resistance of a component. (1 mark)
Model Answerhow much the component opposes the flow of charge (current).
Q5
Two resistors are connected to the same supply. Explain which carries the larger current: the 5 Ω resistor or the 20 Ω resistor. (2 marks)
Model Answerthe 5 Ω resistor, because lower resistance allows a larger current for the same p.d.
Q6
Describe how potential difference and current can both be measured for a single resistor. (3 marks)
Model Answerconnect an ammeter in series with the resistor to measure current, and a voltmeter in parallel across the resistor to measure p.d.
Part 2 · The Resistance Equation
Potential difference, current and resistance are linked by the AQA equation:
potential difference = current × resistance V = I R
where V is in volts (V), I is in amperes (A) and R is in ohms (Ω).
Example (finding p.d.)
A current of 3 A flows through a 4 Ω resistor. Calculate the p.d.
V
I = 3 A R = 4 Ω V = ?
E
V = I R
S
V = 3 × 4
S
V = 12
U
V (volts)
Example (finding resistance)
A 12 V supply drives a current of 4 A. Calculate the resistance.
V
V = 12 V I = 4 A R = ?
E
V = I R
S
12 = 4 × R
S
R = 124 R = 3
U
Ω (ohms)
Questions — The Resistance Equation
Q7
Calculate the potential difference in each case: a) I = 2 A, R = 5 Ωb) I = 0.5 A, R = 20 Ωc) I = 3 A, R = 8 Ωd) I = 0.2 A, R = 50 Ω(4 marks)
Model Answera) 10 V b) 10 V c) 24 V d) 10 V
Q8
Calculate the current in each case: a) V = 12 V, R = 6 Ωb) V = 230 V, R = 46 Ω(2 marks)
Model Answera) 2 A b) 5 A
Q9
A 9 V battery is connected to a resistor and a current of 0.3 A flows. Calculate the resistance. (3 marks)
Model Answer
Answer
V
V = 9 V I = 0.3 A R = ?
E
V = I R
S
9 = 0.3 × R
S
R = 90.3 R = 30
U
Ω (ohms)
Q10
A resistor of 8 Ω carries a current of 1.5 A. Calculate the potential difference across it. (2 marks)
Model AnswerV = I R = 8 × 1.5 = 12 V.
Part 3 · Ohm’s Law
Ohm’s law: for an ohmic conductor at constant temperature, the current is directly proportional to the potential difference.
This means the resistance stays constant, so a graph of current against potential difference is a straight line through the origin.
An ohmic conductor only obeys Ohm’s law while its temperature does not change.
Questions — Ohm’s Law
Q11
State Ohm’s law. (2 marks)
Model Answerfor an ohmic conductor at constant temperature, current is directly proportional to potential difference.
Q12
Sketch the shape of the current–potential difference graph for an ohmic conductor. (1 mark)
Model Answera straight line through the origin.
Q13
What does ‘directly proportional’ mean for current and potential difference? (2 marks)
Model Answerif the p.d. doubles, the current doubles; the ratio V/I stays constant.
Q14
Why must the temperature be kept constant for a conductor to be ohmic? (2 marks)
Model Answerbecause resistance changes with temperature; Ohm’s law only holds while resistance (and so temperature) is constant.
Q15
An ohmic resistor has 6 V across it and a current of 0.5 A. The p.d. is increased to 12 V. State the new current and explain your answer. (3 marks)
Model Answer1.0 A. The current is proportional to the p.d., and the p.d. has doubled, so the current doubles.
Exam-Style Question
A 12 V filament bulb is connected to a 12 V power supply. The graph shows how the current changes after the bulb is switched on.
(a)
After 0.10 seconds, the bulb works at its normal brightness. What is the current through the bulb when it is working at normal brightness? (1 mark)
Model Answer1.7 A (reading the plateau of the graph).
(b)
The bulb works at normal brightness for 30 seconds before it is switched off. Calculate the charge that flows through the bulb in this time. Give the unit. (3 marks)
Model AnswerQ = I t = 1.7 × 30 = 51 C.
(c)
Calculate the energy transferred by the bulb when it is working at normal brightness for 30 seconds. (2 marks)
Model AnswerE = Q V = 51 × 12 = 612 J.
(d)
Between 0.02 s and 0.08 s, both the resistance and the temperature of the filament increase. Explain, in terms of the electrons and ions in the filament, why this happens. (2 marks)
Model Answeras current flows, the filament heats up; the hotter ions vibrate more, so electrons collide with them more often, which increases the resistance.
Total: 8 marks
Exit Ticket
1
State the equation that links potential difference, current and resistance.
Model AnswerV = I R.
2
What is the unit of resistance?
Model Answerthe ohm (Ω).
3
What shape is the I–V graph for an ohmic conductor?
Model Answera straight line through the origin.
Lesson 4 · Resistance of Components
Do Now
Q1
What is the unit of resistance?
Model Answerthe ohm (Ω).
Q2
What is meant by the internal energy of a substance?
Model Answerthe total kinetic energy and potential energy of all the particles in the substance.
Q3
Name one non-renewable energy resource.
Model Answerany one of: coal, oil, natural gas, nuclear fuel (uranium).
Q4
Stretch: A filament lamp gets hot when current flows through it. Predict what happens to its resistance as it heats up, and explain your thinking.
Model Answerthe resistance increases. The hotter metal ions vibrate more, so the electrons collide with them more often and find it harder to move through the filament.
Part 1 · I–V Characteristics
An I–V characteristic is a graph of current against potential difference for a component.
Ohmic resistor: straight line through the origin — resistance is constant.
Filament lamp: as current increases the filament heats up, so its resistance increases — the line curves (an S-shape).
Diode: current only flows in one direction (forward). It has a very high resistance in the reverse direction.
Fig 4.1 — I–V characteristics for a resistor, a filament lamp and a diode
Questions — I–V Characteristics
Q1
What is plotted on each axis of an I–V characteristic? (2 marks)
Model Answerx-axis: potential difference (V). y-axis: current (I).
Q2
Describe the shape of the I–V graph for an ohmic resistor. (1 mark)
Model Answera straight line through the origin.
Q3
Explain why the resistance of a filament lamp increases as the current increases. (3 marks)
Model Answera larger current heats the filament more; the hotter metal ions vibrate more, so electrons collide with them more often, increasing resistance.
Q4
Describe how the current through a diode depends on the direction of the potential difference. (2 marks)
Model Answerin the forward direction, current flows easily; in the reverse direction, almost no current flows (very high resistance).
Q5
Compare the resistance of a filament lamp at low and high potential difference. (2 marks)
Model Answerresistance is lower at low p.d. (lamp cooler) and higher at high p.d. (lamp hotter).
Part 2 · Required Practical: Investigating Resistance
Aim: investigate how the resistance of a wire depends on its length.
Set up a cell, ammeter and the test wire in series, with a voltmeter in parallel across the wire.
Measure the p.d. and current for several lengths of wire and calculate resistance using R = V ÷ I.
Result: resistance is directly proportional to the length of the wire.
Method
Connect ammeter in series, voltmeter in parallel.
Record V and I for each length.
Calculate R = V ÷ I and plot R against length.
Questions — Required Practical
Q6
Where should the ammeter and voltmeter be placed in this circuit? (2 marks)
Model Answerammeter in series with the wire; voltmeter in parallel across the wire.
Q7
State the equation used to calculate resistance from p.d. and current. (1 mark)
Model AnswerR = V ÷ I.
Q8
A length of wire has 6 V across it and a current of 0.5 A through it. Calculate its resistance. (3 marks)
Model AnswerR = V ÷ I = 6 ÷ 0.5 = 12 Ω.
Q9
State the relationship between the resistance of a wire and its length. (1 mark)
Model Answerresistance is directly proportional to length.
Q10
A 0.4 m wire has a resistance of 8 Ω. Predict the resistance of a 1.2 m length of the same wire and explain your reasoning. (3 marks)
Model Answer24 Ω. Length has tripled, and resistance is directly proportional to length, so resistance triples too.
Part 3 · Control Variables
To make the investigation a fair test, keep the material and cross-sectional area (thickness) of the wire the same.
Take readings quickly and switch off between readings so the wire does not heat up and change resistance.
Questions — Control Variables
Q11
State two variables that must be controlled to make this a fair test. (2 marks)
Model Answerthe material of the wire and its cross-sectional area (thickness).
Q12
Name the independent variable and the dependent variable in this investigation. (2 marks)
Model Answerindependent: length of wire. Dependent: resistance.
Q13
Explain why the wire should not be allowed to get hot during the experiment. (2 marks)
Model Answerheating changes the resistance of the wire, which would make the results inaccurate.
Q14
Suggest why the experiment is repeated and a mean resistance calculated for each length. (2 marks)
Model Answerto reduce the effect of random error and get a more accurate/reliable result.
Q15
A student’s results do not give a straight line through the origin. Suggest one reason why. (2 marks)
Model Answerthe wire heated up during readings (changing its resistance), or there was a measurement error.
Exam-Style Question 1
A student investigated how the resistance of a piece of nichrome wire varies with length. The diagram below shows part of the circuit the student used.
(a)
Complete the diagram by adding an ammeter and a voltmeter. Use the correct circuit symbols. (3 marks)
Model Answerammeter added in series in the main loop; voltmeter added in parallel across the test wire:
(b)
Describe how the student would obtain the data needed for the investigation. Your answer should include a risk assessment for one hazard in the investigation. (6 marks)
Model Answerset up the circuit, measure the p.d. and current for several different lengths of wire, and calculate R = V ÷ I for each. A hazard is the wire becoming hot; the risk is reduced by switching off between readings and not touching the wire.
(c)
Why would switching off the circuit between readings have improved the accuracy of the investigation? (1 mark)
Model Answerthe temperature of the wire would not change, so its resistance would not change, giving more accurate readings.
(d)
The student used crocodile clips to make connections to the wire. They could have used a piece of equipment called a ‘jockey’ instead. The photos below show a crocodile clip and a jockey in contact with a wire.Suggest how using a jockey would have affected the accuracy and the resolution of the length measurement, compared with using a crocodile clip. (2 marks)
Model Answerthe jockey allows a continuously variable length to be selected, giving a higher resolution for the length measurement; contact quality is also more consistent, which can improve accuracy.
Total: 12 marks
Exam-Style Question 2
A student wants to investigate how the current through a filament lamp affects its resistance.
(a)
Use the components below to draw a circuit diagram she could use: a 12 V battery, a variable resistor, a filament lamp, a voltmeter and an ammeter. (2 marks)
Model Answerbattery, variable resistor, filament lamp and ammeter in series; voltmeter in parallel across the filament lamp:
(b)
Describe how the student could use her circuit to investigate how the current through the filament lamp affects its resistance. (4 marks)
Model Answeruse the variable resistor to change the current; record the ammeter and voltmeter readings for several settings; calculate R = V ÷ I for each and see how R changes with I.
(c)
The student’s results are shown below.Describe how the resistance of the filament lamp changes as the current through it increases. (1 mark)
Model Answerthe resistance increases as the current increases.
(d)
Use the graph to estimate the resistance of the filament lamp when a current of 0.10 A passes through it. (1 mark)
Model Answerabout 5–6 Ω (accept values consistent with extrapolating the curve back towards the y-axis).
(e)
The current–potential difference graphs of three components are shown below.Use answers from the box to identify each component. (3 marks)
diode filament lamp light-dependent resistor resistor at constant temperature thermistor
Model AnswerGraph A = thermistor. Graph B = resistor at constant temperature. Graph C = diode.
Total: 11 marks
Exit Ticket
1
Describe the shape of the I–V graph for a filament lamp.
Model Answeran S-shaped curve — the line gets shallower as the p.d. increases, because resistance rises.
2
Name the independent variable in the resistance-of-a-wire practical.
Model Answerthe length of the wire.
3
State one control variable in the resistance-of-a-wire practical.
Model Answerthe material of the wire, or its cross-sectional area (thickness).
Lesson 5 · LDRs, Thermistors and Sensing Circuits
Do Now
Q1
Describe the shape of the I–V graph for an ohmic resistor.
Model Answera straight line through the origin.
Q2
What is meant by the specific latent heat of a substance?
Model Answerthe energy needed to change the state of 1 kg of a substance without changing its temperature.
Q3
What is meant by the efficiency of an energy transfer?
Model Answerthe fraction (or percentage) of the input energy that is transferred usefully — useful output ÷ total input.
Q4
Stretch: A street light needs to switch on by itself when it gets dark. What property of a component would need to change as light levels fall for this to work?
Model Answerits resistance. A component whose resistance rises as the light falls (an LDR) can change the p.d. in the circuit and trigger a switch.
Part 1 · LDRs and Thermistors
An LDR (light-dependent resistor) has a resistance that decreases as light intensity increases.
A thermistor has a resistance that decreases as temperature increases.
Both are examples of components whose resistance is not constant.
Fig 5.1 — How resistance varies for an LDR (left) and a thermistor (right)
Questions — LDRs and Thermistors
Q1
What happens to the resistance of an LDR as the light gets brighter? (1 mark)
Model Answerit decreases.
Q2
What happens to the resistance of a thermistor as it gets hotter? (1 mark)
Model Answerit decreases.
Q3
Name the variable that controls the resistance of (a) an LDR and (b) a thermistor. (2 marks)
Model Answer(a) light intensity. (b) temperature.
Q4
An LDR is in bright sunlight and then moved into shade. Describe and explain the change in its resistance. (2 marks)
Model Answerresistance increases, because light intensity has decreased and LDR resistance increases as light intensity falls.
Q5
Using Fig 5.1, describe the shape of the resistance–temperature graph for a thermistor. (2 marks)
Model Answera curve that falls steeply at first then levels off as temperature increases.
Q6
State one way an LDR and a thermistor are similar and one way they are different. (2 marks)
Model Answersimilar: both have resistance that is not constant / both decrease in resistance with increasing stimulus. Different: LDR responds to light, thermistor responds to temperature.
Part 2 · Sensing Circuits
LDRs and thermistors are used in sensing circuits that respond automatically to their surroundings.
The sensor is connected in series with a fixed resistor; as the sensor’s resistance changes, the share of the potential difference across each component changes.
This changing p.d. is used to switch another device (a lamp, heater or alarm) on or off.
Fig 5.2 — An LDR (X) in a light-sensing circuit
Questions — Sensing Circuits
Q7
What is a sensing circuit? (1 mark)
Model Answera circuit that uses a sensor (e.g. LDR or thermistor) to respond automatically to a change in its surroundings.
Q8
In Fig 5.2, the LDR is in series with a fixed resistor. State what happens to the LDR’s resistance in the dark. (1 mark)
Model Answerit increases.
Q9
Name one component used as the sensor in (a) a light sensor and (b) a temperature sensor. (2 marks)
Model Answer(a) LDR. (b) thermistor.
Q10
Explain how the potential difference across the LDR changes as it gets darker. (3 marks)
Model Answeras it gets darker, the LDR’s resistance increases; since it is in series with a fixed resistor, it takes a larger share of the total p.d., so the p.d. across the LDR increases.
Q11
Suggest one use for a thermistor sensing circuit in the home, and explain how it works. (3 marks)
Model Answera thermostat: as temperature falls, thermistor resistance increases, changing the p.d. in the circuit, which switches the heating on.
Part 3 · Using LDRs and Thermistors
LDRs are used in circuits that switch lights on automatically when it gets dark (e.g. street lights, security lights).
Thermistors are used in temperature monitors such as thermostats, fire alarms and engine temperature sensors.
A small change in resistance is turned into a change in potential difference that controls an output device.
Questions — Using LDRs and Thermistors
Q12
Name one practical use of an LDR. (1 mark)
Model Answera street light (or security light).
Q13
Name one practical use of a thermistor. (1 mark)
Model Answera thermostat (or fire alarm, or engine temperature sensor).
Q14
Explain how an LDR can be used to switch a street light on when it gets dark. (3 marks)
Model Answeras it gets dark, the LDR’s resistance increases; this changes the p.d. across it, which is used to trigger a switch that turns the light on.
Q15
A fire alarm uses a thermistor. Explain how a rise in temperature could trigger the alarm. (3 marks)
Model Answeras temperature rises, the thermistor’s resistance falls, changing the p.d. in the circuit; at a set point this triggers the alarm circuit.
Q16
Suggest why an ordinary fixed resistor could not be used as the sensor in these circuits. (2 marks)
Model Answerits resistance does not change with light or temperature, so it could not detect a change in surroundings.
Exam-Style Question 1
The diagram shows a simple light-sensing circuit. The graph, supplied by the manufacturer, shows how the resistance of the component labelled X varies with light intensity.
(i)
What is component X? (1 mark)
Model Answera light-dependent resistor (LDR).
(ii)
Use the graph to find the resistance of component X when the light intensity is 20 lux. (1 mark)
Model Answerabout 22 kΩ (accept 20–24 kΩ).
(iii)
The light intensity increases. State and explain what happens to the resistance of component X. (2 marks)
Model Answerthe resistance decreases, because the resistance of an LDR falls as the light intensity increases.
Total: 4 marks
Exam-Style Question 2
The diagram shows a temperature-sensing circuit used to control a heating system in a house.
(a)
What quantity does the ammeter measure? (1 mark)
Model Answerthe current in the circuit.
(b)
Explain how the readings on both meters change when the environmental conditions change. (6 marks)
Model Answeras the temperature falls, the thermistor’s resistance increases; this increases the p.d. across the thermistor (shown on the voltmeter) and decreases the current in the circuit (shown on the ammeter). As the temperature rises, the opposite happens: resistance falls, the voltmeter reading falls and the ammeter reading rises.
(c)
The current in the circuit is 3.5 mA when the potential difference across the thermistor is 4.2 V. Calculate the resistance of the thermistor. (3 marks)
Model AnswerR = V ÷ I = 4.2 ÷ 0.0035 = 1200 Ω.
(d)
Calculate the charge that flows through the thermistor in 5 minutes when the current is 3.5 mA. (3 marks)
Model AnswerQ = I t = 0.0035 × 300 = 1.05 C.
(e)
The circuit can be modified to turn lights on and off by replacing the thermistor with a light-dependent resistor (LDR). Draw the circuit symbol for an LDR in the space below. (1 mark)
Model Answera rectangle with an arrow pointing into it (as shown in Fig 1.1).
Total: 14 marks
Exit Ticket
1
What happens to the resistance of an LDR as light intensity increases?
Model Answerit decreases.
2
What happens to the resistance of a thermistor as temperature increases?
Model Answerit decreases.
3
Name one device that uses a thermistor.
Model Answera thermostat, a fire alarm, or an engine temperature sensor.
Lesson 6 · Series and Parallel Circuits
Do Now
Q1
What happens to the resistance of an LDR as light intensity increases?
Model Answerit decreases.
Q2
Explain, in terms of particles, why gas pressure increases when a gas is heated at constant volume.
Model Answerthe particles gain kinetic energy and move faster, so they collide with the walls more often and with greater force, increasing the pressure.
Q3
State the equation for kinetic energy.
Model Answerkinetic energy = ½ × mass × (speed)2 (Ek = ½ m v2).
Q4
Stretch: Two identical lamps are connected in series to a battery. Predict how the brightness of each lamp compares to a single lamp connected alone to the same battery, and explain your reasoning.
Model Answereach lamp is dimmer. The two lamps share the supply p.d. (half each) and the total resistance is doubled, so the current is smaller and each lamp transfers less energy per second.
Part 1 · Series Circuits
In a series circuit the components are connected in one loop, one after another.
The current is the same at every point.
The total potential difference of the supply is shared between the components.
The total resistance is the sum of the individual resistances: Rtotal = R1 + R2 + …
Fig 6.1 — A series circuit
Questions — Series Circuits
Q1
In a series circuit, what is true about the current at every point? (1 mark)
Model Answerit is the same everywhere.
Q2
How is the supply potential difference shared in a series circuit? (1 mark)
Model Answerit is shared between the components.
Q3
Two resistors of 3 Ω and 5 Ω are connected in series. Calculate the total resistance. (1 mark)
Model Answer8 Ω.
Q4
Calculate the total resistance of these series combinations: a) 4 Ω and 6 Ωb) 10 Ω, 20 Ω and 30 Ωc) 2.5 Ω and 7.5 Ω(3 marks)
Model Answera) 10 Ω b) 60 Ω c) 10 Ω
Q5
A 12 V supply is connected to two identical lamps in series. State the p.d. across each lamp and explain your answer. (2 marks)
Model Answer6 V each, because the supply p.d. is shared equally between two identical components.
Q6
Two resistors, 4 Ω and 8 Ω, are connected in series to a 6 V supply. Calculate the current in the circuit. (3 marks)
Model AnswerR = 12 Ω; I = V ÷ R = 6 ÷ 12 = 0.5 A.
Part 2 · Parallel Circuits
In a parallel circuit the components are connected on separate branches.
The potential difference across each branch is the same and equal to the supply p.d.
The current is shared between the branches; the branch currents add up to the total current from the supply.
Adding more parallel branches decreases the total resistance — it becomes less than the smallest single resistance.
Fig 6.2 — A parallel circuit
Questions — Parallel Circuits
Q7
What is true about the potential difference across each branch of a parallel circuit? (1 mark)
Model Answerit is the same across each branch, and equal to the supply p.d.
Q8
How does the current from the supply relate to the branch currents? (1 mark)
Model Answerthe supply current equals the sum of the branch currents.
Q9
Explain what happens to the total resistance when an extra branch is added in parallel. (2 marks)
Model Answerthe total resistance decreases, because adding a branch gives current more paths to flow through.
Q10
A 6 V supply is connected to two lamps in parallel. State the p.d. across each lamp. (1 mark)
Model Answer6 V (the same as the supply).
Q11
In a parallel circuit the supply current is 0.9 A. One branch carries 0.4 A. Calculate the current in the other branch. (2 marks)
Model Answer0.5 A (since 0.4 + 0.5 = 0.9 A).
Q12
Explain why the lamps in a house are wired in parallel rather than in series. (3 marks)
Model Answereach lamp gets the full supply p.d. so works at normal brightness, and each can be switched on/off independently; if one breaks, the others stay on.
Part 3 · Comparing the Two
Series: same current; shared p.d.; if one component breaks, the whole circuit stops.
Parallel: same p.d.; shared current; if one branch breaks, the others keep working.
Series adds resistance; parallel reduces it.
Questions — Comparing the Two
Q13
State one advantage of a parallel circuit over a series circuit. (1 mark)
Model Answerif one branch breaks, the others keep working (accept: each component gets full supply p.d.).
Q14
In a string of old fairy lights, one bulb breaks and they all go out. What type of circuit is this? Explain. (2 marks)
Model Answerseries — there is only one loop, so a break anywhere stops current flowing through all the bulbs.
Q15
Compare what happens to the current and the p.d. for components in series and in parallel. (4 marks)
Model Answerseries: current is the same, p.d. is shared. Parallel: p.d. is the same, current is shared.
Q16
Two 6 Ω resistors are connected (i) in series and (ii) in parallel. State which arrangement has the larger total resistance and give the series value. (3 marks)
Model Answerseries has the larger total resistance; series total = 12 Ω (parallel would be 3 Ω).
Exam-Style Question
The diagram shows a simple type of car rear window heater. The six heating elements are exactly the same and are connected in series. Each heating element has a resistance of 5 Ω. The current passing through each element is 0.4 A.
(a)
Calculate the total resistance of the six heating elements. Show clearly how you work out your answer. (2 marks)
Model AnswerRtotal = 6 × 5 = 30 Ω (resistances in series add up).
(b)
Why is the current passing through each element the same? (1 mark)
Model Answerthey are all in series, and current is the same everywhere in a series circuit.
(c)
What is the total current passing through the whole circuit? (1 mark)
Model Answer0.4 A (same as through each element).
(d)
How is the 12-volt potential difference of the car battery shared between the six heating elements? (1 mark)
Model Answerit is shared equally between the six identical elements (2 V across each).
Total: 5 marks
Exit Ticket
1
In which type of circuit is the current the same everywhere?
Model Answera series circuit.
2
Two 10 Ω resistors are connected in series. What is the total resistance?
Model Answer20 Ω.
3
Why are house lights wired in parallel?
Model Answerso each lamp gets the full supply p.d. and can be switched independently; if one breaks the others stay on.
Lesson 7 · Mains Electricity and Wiring
Do Now
Q1
In a series circuit, what is true about the current at every point?
Model Answerit is the same everywhere.
Q2
What is the difference between evaporation and boiling?
Model Answerevaporation happens only at the surface of a liquid and at any temperature; boiling happens throughout the liquid at a fixed boiling point.
Q3
State the equation for gravitational potential energy.
Model AnswerGPE = mass × gravitational field strength × height (Ep = m g h).
Q4
Stretch: Why do you think a metal-cased appliance needs a safety wire connected to its case, but a plastic-cased appliance does not?
Model Answermetal conducts, so a fault could make the case live and give a shock; the earth wire carries that current safely away. Plastic is an insulator, so a plastic case cannot become live.
Part 1 · a.c. and d.c.
Direct current (d.c.) flows in one direction only. Cells and batteries provide d.c.
Alternating current (a.c.) repeatedly changes direction. Mains electricity is a.c.
UK mains supply is about 230 V with a frequency of 50 Hz (it changes direction 50 times each second).
Fig 7.1 — Alternating current (a.c.) and direct current (d.c.)
Questions — a.c. and d.c.
Q1
What does d.c. stand for, and in how many directions does it flow? (2 marks)
Model Answerdirect current; it flows in one direction only.
Q2
Give one source of direct current. (1 mark)
Model Answera cell or battery.
Q3
State the potential difference and frequency of the UK mains supply. (2 marks)
Model Answerabout 230 V, 50 Hz.
Q4
Explain the difference between a.c. and d.c. (2 marks)
Model Answerd.c. flows in one direction only; a.c. repeatedly reverses direction.
Q5
The mains frequency is 50 Hz. State what this means. (1 mark)
Model Answerthe current changes direction 50 times every second.
Q6
Using Fig 7.1, describe how you can tell from a graph whether a supply is a.c. or d.c. (2 marks)
Model Answera.c. is a wave that goes above and below zero; d.c. is a flat line that stays on one side of zero.
Part 2 · The Three-Pin Plug
A mains cable contains three wires: live (brown), neutral (blue) and earth (green and yellow).
The live wire carries the alternating p.d. from the supply (about 230 V) — it is the dangerous one.
The neutral wire completes the circuit and is near 0 V.
The earth wire is a safety wire that carries current away if a fault makes the case live.
Fig 7.2 — Inside a UK three-pin plug
Questions — The Three-Pin Plug
Q7
State the colour of each wire: (a) live (b) neutral (c) earth. (3 marks)
Model Answer(a) brown. (b) blue. (c) green and yellow.
Q8
Which wire carries the dangerous alternating potential difference? (1 mark)
Model Answerthe live wire.
Q9
State the approximate potential difference of (a) the live wire and (b) the neutral wire. (2 marks)
Model Answer(a) about 230 V. (b) about 0 V.
Q10
Explain the job of the earth wire. (2 marks)
Model Answerit is a safety wire that carries current away to earth if a fault makes the case live, preventing electric shock.
Q11
Explain why touching the live wire is dangerous even when an appliance is switched off at the socket’s neutral side. (2 marks)
Model Answerthe live wire is still connected to the alternating supply p.d., so it can still give a shock even if the neutral side is switched off.
Part 3 · Fuses and Earthing
A fuse contains a thin wire that melts if the current gets too large, breaking the circuit.
Together, the earth wire and fuse protect the user: a fault sends a large current through the earth wire, which melts the fuse and disconnects the live supply.
Double-insulated appliances have a plastic case and do not need an earth wire.
Questions — Fuses and Earthing
Q12
What is inside a fuse? (1 mark)
Model Answera thin wire that melts if the current is too large.
Q13
What happens to a fuse if too much current flows? (1 mark)
Model Answerthe fuse wire melts and breaks the circuit.
Q14
Explain how the earth wire and fuse work together to keep the user safe if the metal case becomes live. (3 marks)
Model Answera fault sends a large current through the earth wire (least resistance path); this large current melts the fuse, disconnecting the live supply and making the appliance safe.
Q15
An appliance has a plastic case and only two wires. State why it does not need an earth wire. (2 marks)
Model Answerit is double insulated; the plastic case cannot become live, so no earth wire is needed.
Q16
A 3 A and a 13 A fuse are available for a device that normally draws 2 A. State which fuse should be fitted and explain why. (3 marks)
Model Answerthe 3 A fuse, because it is the smallest rating above the normal working current, so it will melt quickly if a fault increases the current.
Exam-Style Question 1
Use numbers from the box to complete the following sentences.
12 50 110 230
(a)
In the UK, the mains electricity supply is ________ volts. The frequency of the UK mains electricity supply is ________ hertz. (2 marks)
Model Answer230 volts; 50 hertz.
(b)(i)
A hairdryer designed to be used with the UK mains supply has a plastic case. The cable connecting the hairdryer to the plug does not have an earth wire.Why does the hairdryer not need a cable with an earth wire? (1 mark)
Model Answerit is double insulated — the plastic case cannot become live, so no earth wire is needed.
(b)(ii)
Which one of the following materials are the two wires inside the cable made from? Draw a ring around your answer. (1 mark)
aluminium copper steel
Model Answercopper.
Total: 4 marks
Exam-Style Question 2
The diagram shows the three pins in a mains plug. The pins connect with the live, neutral and earth terminals in a socket.
(a)
State which pin is: the live pin, the neutral pin, the earth pin. (3 marks)
Inside the plug, the fuse is held between two metal caps, and the cable is held in place by a cable grip where it enters the plug. Name one material that could be used for the fuse’s outer body, and explain why it must not conduct electricity. (2 marks)
Model Answerceramic (or glass/plastic); it must not conduct electricity so that current only flows through the fuse wire, which melts to break the circuit when the current is too large.
(b)(ii)
Complete the sentence: the cable grip is used to hold the ____________ firmly in place. (2 marks)
Model Answerthe cable (the outer insulation/sheath of the cable), stopping it being pulled out of the plug.
(b)(iii)
The plug is used with an electric fire. Which part of the electric fire is connected to the earth pin? (1 mark)
Model Answerthe metal case (body) of the fire.
Total: 8 marks
Exit Ticket
1
State the colour of the live wire.
Model Answerbrown.
2
What is the frequency of the UK mains supply?
Model Answer50 Hz.
3
What happens inside a fuse when the current gets too large?
Model Answerthe thin fuse wire melts and breaks the circuit.
Lesson 8 · Electrical Power and Energy
Do Now
Q1
State the potential difference and frequency of the UK mains supply.
Model Answerabout 230 V and 50 Hz.
Q2
State the equation that links density, mass and volume.
Model Answerdensity = mass ÷ volume (ρ = m ÷ V).
Q3
Name three of the energy stores.
Model Answerany three of: kinetic, gravitational potential, elastic potential, chemical, thermal (internal), magnetic, electrostatic, nuclear.
Q4
Stretch: Two identical bulbs are connected to the same battery: one directly, one through a length of resistance wire. Predict which bulb will glow brighter, and explain your thinking.
Model Answerthe bulb connected directly. The resistance wire adds resistance in series, so the current is smaller and some of the supply p.d. is dropped across the wire, leaving less for the bulb — so it transfers less energy per second.
Part 1 · Electrical Power
Power is the energy transferred each second, measured in watts (W). 1 W = 1 J/s.
Electrical power can be calculated with two AQA equations:
power = potential difference × current P = V I
power = (current)2 × resistance P = I2 R
Example (P = V I)
A lamp works at 230 V and draws 0.20 A. Calculate its power.
V
V = 230 V I = 0.20 A P = ?
E
P = V I
S
P = 230 × 0.20
S
P = 46
U
W (watts)
Example (P = I²R)
A 4 A current flows through a 10 Ω heater. Calculate the power.
V
I = 4 A R = 10 Ω P = ?
E
P = I2 R
S
P = 42 × 10
S
P = 16 × 10 = 160
U
W (watts)
Questions — Electrical Power
Q1
What is electrical power, and what is its unit? (2 marks)
Model Answerthe energy transferred each second; measured in watts (W).
Q2
Calculate the power using P = V I: a) V = 12 V, I = 2 Ab) V = 230 V, I = 5 Ac) V = 6 V, I = 0.5 A(3 marks)
Model Answera) 24 W b) 1150 W c) 3 W
Q3
Calculate the power using P = I²R: a) I = 2 A, R = 5 Ωb) I = 3 A, R = 10 Ω(2 marks)
Model Answera) 20 W b) 90 W
Q4
A kettle is rated at 2300 W on a 230 V supply. Calculate the current it draws. (3 marks)
Model AnswerI = P ÷ V = 2300 ÷ 230 = 10 A.
Part 2 · Energy Transferred
The energy transferred by an appliance depends on its power and how long it is used:
energy = power × time E = P t
Energy can also be found from the charge that flows and the potential difference:
energy = charge flow × potential difference E = Q V
Energy is in joules (J), power in watts (W), time in seconds (s), charge in coulombs (C).
Example (E = P t)
A 2000 W heater is on for 30 s. Calculate the energy transferred.
V
P = 2000 W t = 30 s E = ?
E
E = P t
S
E = 2000 × 30
S
E = 60000
U
J (joules)
Example (E = Q V)
12 C of charge moves through a p.d. of 5 V. Calculate the energy.
V
Q = 12 C V = 5 V E = ?
E
E = Q V
S
E = 12 × 5
S
E = 60
U
J (joules)
Questions — Energy Transferred
Q5
State the equation linking energy, power and time. (1 mark)
Model AnswerE = P t.
Q6
Calculate the energy transferred (E = P t): a) P = 100 W, t = 60 sb) P = 2000 W, t = 5 sc) P = 60 W, t = 120 s(3 marks)
Model Answera) 6000 J b) 10000 J c) 7200 J
Q7
Calculate the energy transferred (E = Q V): a) Q = 10 C, V = 6 Vb) Q = 50 C, V = 12 V(2 marks)
Model Answera) 60 J b) 600 J
Q8
A 1500 W microwave runs for 2 minutes. Calculate the energy it transfers. (3 marks)
Model AnswerE = P t = 1500 × 120 = 180000 J.
Q9
A charge of 8 C transfers 96 J of energy through a component. Calculate the potential difference across it. (2 marks)
Model AnswerV = E ÷ Q = 96 ÷ 8 = 12 V.
Part 3 · Power, Appliances and the Kilowatt-Hour
An appliance with a higher power transfers energy faster, so it costs more to run for the same time.
The current an appliance draws can be found from P = V I, which is used to choose the right fuse and cable.
A higher-power appliance draws a larger current from the same supply (I = P ÷ V).
Questions — Power and Appliances
Q10
Explain why a higher-power appliance transfers more energy in the same time. (2 marks)
Model Answerpower is energy transferred per second, so a higher power means more energy is transferred each second.
Q11
A 920 W toaster runs on 230 V. Calculate the current it draws. (3 marks)
Model AnswerI = P ÷ V = 920 ÷ 230 = 4 A.
Q12
Two heaters are rated 1000 W and 2000 W. State which heats a room faster and which costs more per hour, and explain. (3 marks)
Model Answerthe 2000 W heater heats faster and costs more per hour, because it transfers twice as much energy each second.
Q13
A 1380 W hairdryer is used for 5 minutes. Calculate the energy transferred in joules. (2 marks)
Model AnswerE = 1380 × 300 = 414000 J.
Exam-Style Question 1
An experimental solar-powered bike has solar cells connected to a battery. The solar cells charge up the battery. There is a switch on the handlebars; when the switch is closed, the battery drives a motor attached to the front wheel.
(a)
Use words from the list to complete the sentences below. Words may be used once, more than once, or not at all. (5 marks)
chemical electrical heat (thermal) kinetic light potential sound
(i) The solar cells transfer ________ energy to ________ energy.(ii) When the battery is being charged up, ________ energy is transferred to ________ energy.(iii) The motor is designed to transfer ________ energy to ________ energy.
Model Answer(i) light energy to electrical energy. (ii) electrical energy to chemical energy. (iii) electrical energy to kinetic energy.
(b)(i)
The cyclist stops pedalling for 10 seconds. During this time the motor transfers 1.5 kJ of energy. Calculate the power of the motor. (3 marks)
Model AnswerP = E ÷ t = 1500 ÷ 10 = 150 W.
(b)(ii)
Name one form of wasted energy which is produced when the motor is running. (1 mark)
Model Answerheat (thermal energy) — or sound.
Total: 9 marks
Exam-Style Question 2
A battery-powered drone has a battery that can store 97.5 kJ of energy. When the drone is hovering, the power output of the battery is 65.0 W.
(a)
Calculate the time for which the drone can hover. (3 marks)
Model Answert = E ÷ P = 97500 ÷ 65.0 = 1500 s.
(b)
The battery powers 4 motors in the drone. Each motor has a resistance of 1.60 Ω when the power input to each motor is 19.6 W. The 4 motors are connected in parallel with the battery. Calculate the current through the battery. (4 marks)
Model Answercurrent in one motor: P = I2R, so I = √(19.6 ÷ 1.60) = 3.5 A. Four identical motors in parallel: total current = 4 × 3.5 = 14 A.
Total: 7 marks
Exit Ticket
1
State the two equations for electrical power.
Model AnswerP = V I and P = I2 R.
2
What is 1 watt equal to in joules per second?
Model Answer1 W = 1 J/s.
3
A 100 W lamp is on for 60 s. How much energy does it transfer?
Model AnswerE = P t = 100 × 60 = 6000 J.
Lesson 9 · The National Grid and Transformers
Do Now
Q1
State the equation linking power, potential difference and current.
Model AnswerP = V I.
Q2
Describe the arrangement and movement of particles in a solid.
Model Answerthe particles are packed closely together in a regular arrangement and vibrate about fixed positions.
Q3
Explain why non-renewable energy resources are a problem for the future.
Model Answerthey will eventually run out because they are used faster than they are replaced, and burning them releases carbon dioxide and other pollutants.
Q4
Stretch: Electricity is transmitted through the National Grid at a very high voltage, even though high voltages are more dangerous. Why might this still be a sensible choice?
Model Answera higher p.d. means the same power can be delivered with a much smaller current. Power wasted as heat in the cables is I2R, so a smaller current wastes far less energy. The cables are kept high up and well insulated to manage the danger.
Part 1 · The National Grid
The National Grid is the system of cables and transformers that carries electricity from power stations to consumers.
Electricity is transmitted at a high potential difference and a low current.
A low current means less energy is wasted as heat in the cables (because power loss = I2R), making transmission efficient.
Fig 9.1 — The National Grid: from power station to consumer
Questions — The National Grid
Q1
What is the National Grid? (1 mark)
Model Answerthe system of cables and transformers that carries electricity from power stations to consumers.
Q2
State whether the grid transmits power at high or low potential difference, and at high or low current. (2 marks)
Model Answerhigh potential difference and low current.
Q3
Explain why transmitting at a low current reduces energy losses. (3 marks)
Model Answerpower loss in the cables = I2R; since loss depends on current squared, a smaller current greatly reduces the heat wasted in the cables.
Q4
State the energy transfer that causes losses in transmission cables. (1 mark)
Model Answerelectrical energy transferred to heat (thermal) energy.
Q5
Explain why a high transmission p.d. allows a low current to deliver the same power. (3 marks)
Model Answerpower = p.d. × current (P = V I); for the same power, increasing p.d. means current can be smaller.
Part 2 · Transformers
Transformers change the size of an alternating potential difference.
A step-up transformer increases the p.d. (used between the power station and the grid).
A step-down transformer decreases the p.d. (used before electricity reaches homes).
Fig 9.2 — A transformer with primary and secondary coils on an iron core
Questions — Transformers
Q6
What does a transformer do? (1 mark)
Model Answerchanges (steps up or down) the size of an alternating potential difference.
Q7
State where a step-up and a step-down transformer are used on the grid. (2 marks)
Model Answerstep-up: at the power station, before transmission. Step-down: before electricity reaches homes.
Q8
Why must the National Grid use alternating current with transformers? (2 marks)
Model Answertransformers only work with a changing (alternating) potential difference; they cannot step up or down direct current.
Q9
Name the parts of a transformer labelled in Fig 9.2. (3 marks)
Model Answerprimary coil, secondary coil, iron core.
Q10
Explain why the p.d. is stepped up before transmission and stepped down before reaching homes. (2 marks)
Model Answerstepping up reduces the current needed for transmission (less energy wasted); stepping down brings the p.d. to a safe, usable level for homes.
Part 3 · Transformer Calculations (Higher)
For an ideal transformer, the power in the primary coil equals the power in the secondary coil. The AQA equation is:
Vp Ip = Vs Is
where p = primary and s = secondary; V in volts, I in amperes.
Example (Higher)
A transformer has Vp = 230 V, Ip = 4 A and Vs = 920 V. Find Is.
V
Vp = 230 V Ip = 4 A Vs = 920 V Is = ?
E
Vp Ip = Vs Is
S
230 × 4 = 920 × Is
S
Is = 920920 Is = 1.0
U
A (amperes)
Questions — Transformer Calculations
Q11
State the transformer equation that links the primary and secondary coils. (1 mark)
Model AnswerVp Ip = Vs Is.
Q12
What is assumed about the power in an ideal transformer? (1 mark)
Model Answerthat the power in the primary coil equals the power in the secondary coil (no energy losses).
Q13
A transformer has Vp = 100 V, Ip = 6 A and Vs = 300 V. Calculate Is. (3 marks)
Model Answer
Answer
V
Vp = 100 V Ip = 6 A Vs = 300 V Is = ?
E
Vp Ip = Vs Is
S
100 × 6 = 300 × Is
S
Is = 600300 Is = 2.0
U
A (amperes)
Q14
The output power of a transformer is 1200 W. Assuming it is ideal, state the input power. (1 mark)
Model Answer1200 W (input power = output power for an ideal transformer).
Q15
A step-down transformer reduces 920 V to 230 V. The current in the primary coil is 1 A. Calculate the current in the secondary coil. (3 marks)
Model Answer
Answer
V
Vp = 920 V Ip = 1 A Vs = 230 V Is = ?
E
Vp Ip = Vs Is
S
920 × 1 = 230 × Is
S
Is = 920230 Is = 4.0
U
A (amperes)
Q16
Explain why the current is larger in the low-voltage coil of a step-down transformer. (2 marks)
Model Answerpower is conserved (VpIp = VsIs); a lower p.d. must be paired with a larger current to keep the power the same.
Exam-Style Question
The diagram shows how electricity is distributed from power stations to consumers.
(a)(i)
What name is given to the network of cables and transformers that links power stations to consumers? (1 mark)
Model Answerthe National Grid.
(a)(ii)
What does a step-up transformer do? (1 mark)
Model Answerit increases (steps up) the potential difference.
(a)(iii)
Explain why step-up transformers are used in the electricity distribution system. (2 marks)
Model Answera higher p.d. allows the same power to be transmitted with a smaller current; a smaller current reduces energy losses (heating) in the transmission cables, since power loss = I2R.
(b)
Most of the world’s electricity is generated in power stations that burn fossil fuels. State one environmental problem that burning fossil fuels produces. (1 mark)
Model Answerreleases carbon dioxide, contributing to global warming/climate change (or releases sulfur dioxide, causing acid rain).
(c)(i)
Electricity can also be generated using energy from the wind. A company wants to build a new wind farm, but not everyone thinks this is a good idea. What arguments could the company give to persuade people that a wind farm is a good idea? (2 marks)
Model Answerwind power is renewable and produces no carbon dioxide while generating electricity; it creates local jobs and income.
(c)(ii)
What reasons may be given by people who think that wind farms are not a good idea? (2 marks)
Model Answerturbines can be noisy, are considered unsightly, and only generate electricity when it is windy (unreliable supply).
Total: 9 marks
Exit Ticket
1
Why is electricity transmitted at a high p.d. and a low current?
Model Answerbecause power lost as heat in the cables is I2R, so a smaller current wastes far less energy.
2
What does a step-down transformer do?
Model Answerit decreases the potential difference.
3
State the transformer equation.
Model AnswerVp Ip = Vs Is.
Lesson 10 · Static Electricity and Electric Fields
Separate Physics only
Do Now
Q1
What is the National Grid?
Model Answerthe system of cables and transformers that carries electricity from power stations to consumers.
Q2
What happens to the internal energy of a substance during a change of state, even though its temperature does not change?
Model Answerthe internal energy increases (or decreases): the potential energy of the particles changes as bonds are broken or formed, while the kinetic energy — and so the temperature — stays the same.
Q3
Name one way energy is wasted during an energy transfer.
Model Answerany one of: heating the surroundings through friction or resistance, sound, or light that is not useful.
Q4
Stretch: If you rub a balloon on your hair, both the balloon and your hair become charged. Predict what happens when you then hold the balloon near your hair, and explain your thinking.
Model Answerthe hair is attracted to the balloon and stands up. Electrons move from the hair to the balloon, so the balloon becomes negative and the hair positive — opposite charges attract.
Part 1 · Static Charge
When two insulators are rubbed together, electrons are transferred from one to the other.
The material that gains electrons becomes negatively charged; the one that loses electrons becomes positively charged.
Only electrons move — never the positive charges.
This build-up of charge that stays on a surface is called static electricity.
Questions — Static Charge
Q1
What particles are transferred when two insulators are rubbed together? (1 mark)
Model Answerelectrons.
Q2
A rod loses electrons when rubbed. State the charge it gains. (1 mark)
Model Answerpositive.
Q3
Explain how a cloth becomes negatively charged when rubbed against a rod. (2 marks)
Model Answerelectrons are transferred from the rod to the cloth by friction, giving the cloth an overall negative charge.
Q4
Explain why the rod in the example above becomes positively charged. (2 marks)
Model Answerthe rod loses electrons to the cloth, leaving it with more protons than electrons, so it is positively charged.
Q5
State which particles can and cannot move during charging by friction. (2 marks)
Model Answerelectrons can move; positive charges (protons) cannot move.
Q6
Explain why a metal rod held in the hand cannot easily be charged by rubbing. (2 marks)
Model Answermetals are conductors, and the body is also a conductor connected to earth, so charge flows away through the hand instead of building up.
Part 2 · Forces Between Charges
Two charges exert a non-contact force on each other.
Like charges repel (two positives, or two negatives).
Opposite charges attract (one positive and one negative).
A charged object can also attract small uncharged objects by induction.
Questions — Forces Between Charges
Q7
State what happens between two negatively charged objects. (1 mark)
Model Answerthey repel.
Q8
State what happens between a positive and a negative charge. (1 mark)
Model Answerthey attract.
Q9
Why is the force between two charges described as a non-contact force? (2 marks)
Model Answerthe charges exert a force on each other without touching, through their electric fields.
Q10
Two balloons are rubbed with the same cloth and hung side by side. Predict and explain what happens. (3 marks)
Model Answerthey move apart (repel), because both balloons gain the same type of charge from the cloth, and like charges repel.
Q11
Explain how a charged comb can pick up small uncharged pieces of paper. (3 marks)
Model Answerthe charged comb induces an opposite charge on the near side of the paper; the attraction between the comb and the induced opposite charge is stronger than the repulsion from the far side, so the paper is attracted (induction).
Part 3 · Electric Fields
An electric field is a region where a charge feels a force.
Rules for electric fields
Field lines always go from positive to negative.
Field lines never cross.
The field is strongest where the lines are closest.
Fig 10.1 — Electric field lines around a positive and a negative charge
Questions — Electric Fields
Q12
What is an electric field? (1 mark)
Model Answera region where a charge feels a force.
Q13
State the direction of the field lines around a positive charge. (1 mark)
Model Answerpointing away from the charge.
Q14
How can you tell from a field diagram where the field is strongest? (2 marks)
Model Answerthe field is strongest where the field lines are closest together.
Q15
Describe the electric field around an isolated negative charge. (2 marks)
Model Answerradial field lines pointing inwards (towards the charge), getting closer together near the charge.
Q16
State whether the field around a positive charge points towards or away from it. (1 mark)
Model Answeraway from it.
Q17
Explain, in terms of the electric field, why the force on a small charge is larger when it is closer to a charged sphere. (3 marks)
Model Answerthe field is stronger nearer the sphere (field lines closer together); a stronger field exerts a larger force on the charge.
Exam-Style Question (Separate Only)
PET and PVC are two common types of plastic that can be recycled from household waste. They need to be separated in a recycling plant. The waste plastics are crushed into small chips and tumbled together. The PET chips become positively charged. The PVC chips become negatively charged.
(a)
Explain, in terms of electron transfer, how the PET chips become positively charged and the PVC chips become negatively charged. (2 marks)
Model Answerduring tumbling, friction between the chips transfers electrons from the PET chips to the PVC chips; PET loses electrons and becomes positive, PVC gains electrons and becomes negative.
(b)
The mixture is dropped onto a rotating drum and sticks to the outside of it. The mixture passes a metal rod that has a positive charge. The PVC chips leave the drum and fall into a collecting bin. Explain why the PVC chips leave the drum. (2 marks)
Model Answerthe PVC chips are negatively charged and the rod is positively charged, so the rod attracts them (opposite charges attract), pulling them away from the drum.
(c)
The PET chips stay on the drum until they reach a scraper and fall into a separate collecting bin. Both the bins and the scraper are connected to earth. Suggest why. (1 mark)
Model Answerto allow charge to flow away to earth, so the bins and scraper do not build up a charge that could interfere with the separation process.
Total: 5 marks
Exit Ticket
1
Which particles move when two insulators are rubbed together?
Model Answerelectrons.
2
What happens between two objects that carry the same type of charge?
Model Answerthey repel.
3
In which direction do electric field lines point around a positive charge?
