CURRAN.SCIENCE
KS3 Physics Resources
KS3 Physics · Year 8 · All Lessons
All 19 lessons — complete notes, questions and model answers
| All matter consists of atoms. |
| Atoms contain three types of smaller particles: protons, neutrons and electrons. |
| Protons are positively charged. Electrons are negatively charged. Neutrons have no charge. |
| Objects that are charged can affect other charged objects using the non-contact force of electrostatic charge. |
| When two objects have the same charge, they will repel one another. |
| If the objects are of opposite charge, they will attract each other. |
| Generally, the atom has a neutral charge as it has an equal number of protons and electrons. |
| If an atom loses an electron it becomes positively charged. If it gains an electron it becomes negatively charged. |
| Charged atoms are called ions. |
| Protons and electrons are usually found gathered together in atoms, which in turn make up objects and materials. |
| Protons and neutrons are held at the centre of the atom in the nucleus — it is not easy to change their number. |
| Electrons are on the outside of atoms in electron shells, making it possible for a small number to be removed or added. |
| When two materials are rubbed together, friction causes one material to lose electrons and the other to gain electrons. |
| If a material loses electrons, it becomes positively charged overall. |
| If a material gains electrons, it becomes negatively charged overall. |
| This process of gaining or losing electrons is called ionisation. |
| When we describe how positive or negative an object is overall, we say we want to know its charge. |
| We measure charge in Coulombs (C). One Coulomb equals approximately 6.2 billion billion electrons. |
| Typical charges: a charged rod ≈ 7 × 10⁻⁹ C; a simple circuit carries 1 C per second; a phone battery holds about 7,200 C. |
| Sometimes an object can appear charged even when no electrons have been added or removed — this is induced charge. |
| A positively charged object attracts electrons to the nearby surface of an uncharged object. |
| A negatively charged object repels electrons away from the nearby surface of an uncharged object. |
| When electrons have been attracted to or repelled from a surface, we say the uncharged object has an induced charge. |
| All charged objects have an electric field around them — a region where other charges experience a force. |
| Electric fields are shown as diagrams with arrows — the direction shows which way a positive charge would move. |
| The closer together the arrows, the stronger the field. |
| The field is strongest close to the charged object. |
| An electric field exists between an electron and a proton, with arrows pointing from positive to negative. |
| An electrical conductor is a material that allows the flow of charge (current) to move through it. |
| An electrical insulator is a material that does not allow the flow of charge to move through it. |
| A semiconductor is a material that conducts current only partly — between an insulator and a conductor. |
| Metals have delocalised electrons (free to move throughout the metal) — this is why metals conduct electricity. |
| The more insulating a material, the higher its resistance to the flow of electrical current. |
| Conductors: copper, iron, graphite, salt water. Insulators: rubber, plastic, wood, glass. |
| We can test whether a material is a conductor or an insulator by placing it in a simple circuit with a bulb. |
| If the bulb lights brightly, the material is a good conductor. If it does not light at all, the material is an insulator. |
| The resistance of a material describes how much it opposes the flow of electrical current. |
| High resistance = insulator; low resistance = conductor. |
| When two insulating materials are rubbed together, friction causes electrons to be transferred from one material to the other. |
| The material that loses electrons becomes positively charged overall. |
| The material that gains electrons becomes negatively charged overall. |
| This build-up of charge on an object that cannot easily conduct is called static charge. |
| Static charge cannot easily flow away because insulators do not allow electrons to move freely. |
| Examples: rubbing a plastic rod with a cloth, or shuffling across a carpet in socks. |
| If a charged insulating material is brought close to a conducting material, earthing (discharge) can take place. |
| For a positively charged material: electrons jump from the conductor to the positively charged insulator. |
| For a negatively charged material: electrons jump from the insulator to the conductor. |
| As a result, the charged material becomes neutral — it has no overall charge. |
| This is called earthing or discharge. |
| A static electric shock occurs when charge suddenly moves from a charged object to a person through the air or by contact. |
| Charge is measured in Coulombs (C). |
| When a rod is rubbed with a cloth, approximately 7 × 10⁻⁹ C is transferred. |
| In a simple circuit with a lightbulb, about 1 C (6.2 billion billion electrons) moves around each second. |
| A charged rod can attract small uncharged pieces of paper because of induced charge. |
| A charged balloon can stick to a wall for the same reason. |
| Charged objects can deflect a thin stream of water — the water molecules are attracted towards the charged rod. |
| Electric current is the amount of charge flowing past a given point each second. |
| We can compare current in a wire to current in a river — in a river, current is the amount of water flowing past a point each second. |
| The size of the current depends on the amount of charge and the time it takes to flow. |
| Current is measured in Amperes (A). |
| We measure current using an ammeter, which must be placed in series in the circuit. |
| Current can be calculated using the equation: |
| I = Q / t where I = current (A), Q = charge (C), t = time (s) |
| Rearranging: Q = I × t and t = Q / I |
| Example: 100 C flows past a point in 50 s → I = 100/50 = 2 A. |
| Example: Lightning — 4,000 C in 0.2 s → I = 4000/0.2 = 20,000 A. |
| Adding more components to a series circuit decreases the current (more resistance). |
| Potential Difference (PD), also known as voltage, is the energy transferred per coulomb of charge between two points. |
| Potential difference is measured in Volts (V). |
| A cell increases the energy of charges flowing through it — it provides a positive PD. |
| A bulb or resistor decreases the energy of charges — the PD across it represents energy transferred out. |
| V = E / Q where V = PD (V), E = energy transferred (J), Q = charge (C). |
| We measure PD using a voltmeter connected in parallel — across the component. |
| Circuits are drawn using standard circuit symbols so they can be understood worldwide. |
| Key symbols: cell, battery, bulb, resistor, voltmeter (V), ammeter (A), switch. |
| A series circuit has all components connected in one single loop. |
| A parallel circuit has components connected in separate branches. |
| In a series circuit, if one component fails, the whole circuit breaks. |
| In a parallel circuit, if one branch fails, current can still flow through other branches. |
| Scientists use models to explain things that are difficult to observe directly. |
| In the rope model, a loop of rope represents an electrical circuit. |
| The rope represents the electrons flowing around the circuit. |
| The person pulling the rope represents the cell (energy source). |
| The person gripping the rope represents a component that transfers energy (e.g. a bulb). |
| Pulling harder (more force) represents a higher voltage — the rope moves faster. |
| Weakness: rope is continuous; electrons are discrete particles. Cannot show parallel circuits. |
| In the shopaholic model, shoppers walking around shops represent electrons moving around a circuit. |
| The bank represents the cell — it gives out money (energy) to the shoppers (electrons). |
| The shops represent components (bulbs/resistors) — they receive money (energy) from the shoppers. |
| In a series circuit: shoppers visit all shops in turn, spending a portion of their money at each. |
| In a parallel circuit: shoppers choose one of several shops — each still spends all their money in one shop. |
| Weakness: shoppers move at a fixed speed regardless of how much money they have — unlike real circuits. |
| In a series circuit, electrical components are connected one after another in a single loop. |
| An electron passes through every component on its way round the circuit. |
| If one component fails (e.g. a bulb goes out), the circuit is broken and current stops. |
| Rule 1 — Current in series: The current is the same everywhere in a series circuit. |
| Rule 2 — Voltage in series: The PDs across the components add up to the total PD supplied by the cell(s). |
| In a parallel circuit, electrical components are connected alongside one another in separate branches. |
| An electron passes through only one of the parallel components on its way round the circuit. |
| If one branch fails, current can still flow through the other branches. |
| Rule 3 — Current in parallel: The current from the cell splits between branches. Total current = sum of branch currents. |
| Rule 4 — Voltage in parallel: The PD across each parallel branch is the same as the cell PD. |
| This is why parallel wiring is used in homes — each appliance receives the full mains voltage. |
| In a series circuit, the ammeter reads the same wherever it is placed — confirming current is the same throughout. |
| The voltmeter reads the same across the cell and across a single component (if only one) — all the PD is used. |
| The voltmeter reads zero across a plain wire — no energy is transferred there. |
| Adding more cells in series increases the total PD and increases the current. |
| Adding more bulbs in series decreases the current and each bulb gets a smaller share of the total voltage. |
| As more bulbs are added in parallel, the ammeter in the main circuit reads higher — each new branch draws extra current. |
| The voltmeter across a single bulb in parallel stays the same — each branch receives the full cell PD. |
| This confirms Rules 3 and 4. |
| Adding bulbs in parallel does not share the voltage, but it does increase the total current. |
| Resistance is the opposition to the flow of electric current in a circuit. |
| Resistance is measured in Ohms (Ω). |
| Resistance is caused by electrons colliding with positive ions in the material as they flow. |
| A longer wire has more resistance — electrons must travel further and collide more often. |
| A thicker wire has less resistance — more paths available for electrons. |
| Resistance increases with temperature in most conductors — more vibrating ions cause more collisions. |
| R = V / I where R = resistance (Ω), V = potential difference (V), I = current (A) |
| Rearranging: V = I × R and I = V / R |
| Example: Lightbulb with 2 V and 0.2 A → R = 2 / 0.2 = 10 Ω. |
| Example: Metal ruler with 0.012 V and 1000 A → R = 0.000012 Ω (very low — conductor). |
| Example: Wet pencil wood with 10 V and 0.002 A → R = 5000 Ω (high — insulator). |
| Components with high resistance (bulbs, resistors) reduce the current in a circuit. |
| Components with very low resistance (wires) allow current to flow freely. |
| Adding a resistor in series reduces the current for a given PD. |
| A variable resistor allows the resistance to be adjusted, controlling the current. |
| Higher resistance = less current = dimmer bulb. |
| Power is the amount of energy transferred per second. |
| Power is measured in Watts (W). 1 W = 1 J of energy transferred per second. |
| We can find how quickly a device uses energy from its power rating (shown on the label). |
| P = E / t where P = power (W), E = energy (J), t = time (s). |
| Examples: Fridge (200 W), Large Bulb (36 W), Microwave (800 W), Tumble-dryer (3000 W). |
| P = I × V where P = power (W), I = current (A), V = potential difference (V). |
| Example: Kettle with 10 A at 230 V → P = 10 × 230 = 2300 W. |
| Example: Phone charger with 0.5 A at 5 V → P = 0.5 × 5 = 2.5 W. |
| Higher power rating → more energy used per second → more expensive to run. |
| UK mains electricity is at 230 V and alternates with a frequency of 50 Hz. |
| Electricity is dangerous because current passing through the body can cause burns and stop the heart. |
| A person is electrocuted when they form a link between the high-voltage supply and the ground (earth). |
| Birds on power lines are safe — they do not connect the high voltage to the ground. |
| Water greatly increases the risk — it lowers the resistance of the body, allowing more current to flow. |
| Common household hazards: damaged cables, overloaded sockets, electrical appliances near water, bare wires. |
| A fuse contains a thin wire that melts if too much current flows, breaking the circuit. |
| Fuses are rated in Amperes — the fuse melts if the current exceeds its rating. |
| A circuit breaker is an automatic switch that trips if the current is too high. It can be reset. |
| The earth wire (green/yellow) is connected to the casing of an appliance. |
| If a fault makes the casing live, the earth wire provides a low-resistance path to earth, causing a large current and blowing the fuse. |
| The neutral wire (blue) completes the circuit at 0 V. The live wire (brown) carries 230 V AC. |
| A magnet is an object that attracts or is attracted by other magnetic materials. |
| The magnetic elements are iron, cobalt and nickel. Their alloys are also magnetic. |
| Every magnet has a north pole and a south pole. |
| Like poles repel: north–north or south–south. |
| Unlike poles attract: north–south. |
| The north pole of a free magnet points towards geographic north. |
| Inside magnetic materials there are tiny regions called magnetic domains — each domain acts like a tiny magnet. |
| In an unmagnetised material, domains point in random directions and cancel out. |
| When placed in a strong external magnetic field, the domains line up — the material becomes magnetised. |
| Iron is a temporary magnet — its domains randomise again when the external field is removed. |
| Steel is a permanent magnet — its domains remain aligned after the external field is removed. |
| Iron is used for temporary magnets in electromagnets; steel for permanent magnets in compasses. |
| A compass is a magnetised piece of steel on a pivot — it spins freely to align with a magnetic field. |
| The Earth has a weak magnetic field — a freely spinning compass needle aligns with it, pointing north. |
| Compasses were first used in ancient China, originally as lodestones on a string. |
| A compass can be used to navigate because it always shows which direction is north. |
| If a stronger magnet is placed near a compass, the needle aligns with the stronger field instead of Earth's. |
| This is why compasses should be kept away from magnets and electronic devices. |
| A magnetic field is a region where a magnetic material experiences a force. |
| Magnetic field lines go from north to south outside a magnet. |
| The closer together the field lines, the stronger the magnetic field. |
| The field is strongest at the poles of a magnet. |
| We can plot field lines using a compass — the needle points along the field line at each position. |
| Two unlike poles produce field lines that connect them (attraction). Like poles produce a neutral point between them. |
| Any moving charged particle generates a magnetic field — not only permanent magnets. |
| Any conductor carrying a current also has a magnetic field around it. |
| The magnetic field around a current-carrying wire is circular — concentric circles around the wire. |
| Unlike a bar magnet, the field around a straight wire has no distinct north or south pole. |
| The strength of the magnetic field increases as the current through the wire increases. |
| This discovery links electricity and magnetism — they are related phenomena. |
| A dot (·) in the wire represents current coming out of the page towards you. |
| A cross (×) in the wire represents current going into the page away from you. |
| This is based on imagining an arrow: dot = arrowhead coming towards you; cross = feathers going away. |
| Right-hand rule: point your right thumb in the direction of current; your fingers curl in the direction of the field circles. |
| An electromagnet is a magnet made from a coil of wire carrying a current — it can be switched on and off. |
| When current flows, a magnetic field is produced. When current is switched off, the field disappears. |
| Winding the wire into a loop concentrates the field inside the loop, making it stronger. |
| Adding more loops (a solenoid) significantly increases the field strength. |
| A solenoid produces a magnetic field similar to a bar magnet, with north and south poles. |
| Advantages over a permanent magnet: magnetism can be switched on/off and its strength can be varied. |
| Three ways to make an electromagnet stronger: (1) increase the current; (2) add more coils of wire; (3) add an iron core. |
| An iron core inside the solenoid significantly increases the field strength because iron "channels" the magnetic field. |
| Iron is used (not steel) because iron is a temporary magnet — it loses its magnetism when the current is switched off. |
| Steel would remain magnetised after switching off, meaning the electromagnet could not be fully turned off. |
| Scrapyard cranes: lift heavy metal objects; switching off drops the load. |
| Electric bells: electromagnet repeatedly attracts and releases a striker. |
| Door entry systems: electromagnet holds a door shut; switching off opens the door. |
| Loudspeakers and headphones: varying current produces varying field that moves a cone to produce sound. |
| Electric motors: electromagnets create forces on current-carrying coils to produce rotation. |
| When a current-carrying conductor is placed in an external magnetic field, it experiences a force. |
| This is called the motor effect. |
| The force is perpendicular to both the current direction and the magnetic field direction. |
| If the current is reversed, the force direction reverses. |
| If the magnetic field is reversed, the force direction reverses. |
| There is no force if the wire is parallel to the magnetic field. |
| Fleming's left-hand rule: First finger = Field direction; seCond finger = Current direction; thuMb = force direction (Motion). |
| The size of the force depends on: (1) the current; (2) the magnetic field strength; (3) the length of wire in the field. |
| A top-pan balance measures the force — the change in mass reading is converted to force (F = m × g, g ≈ 10 N/kg). |
| Increasing the current increases the force proportionally. |
| An anomaly is a data point that does not fit the general pattern — identified on a graph as a point far from the line of best fit. |
| F = B × I × L where F = force (N), B = magnetic field strength (T), I = current (A), L = length of wire in field (m). |
| Example: 0.1 m wire in Earth's field (0.00005 T) with 2 A: F = 0.00005 × 2 × 0.1 = 0.00001 N. |
| Example: 0.1 m wire in 1 T field with 2 A: F = 1 × 2 × 0.1 = 0.2 N. |
| Remember: length must be in metres (divide cm by 100). |
| An electric motor uses the motor effect to convert electrical energy into kinetic (rotational) energy. |
| A coil of wire is placed inside a magnetic field. When current flows, each side of the coil experiences a force. |
| The force on the left side is in the opposite direction to the force on the right side. |
| This is because the current in the two sides travels in opposite directions. |
| The two opposite forces create a turning effect (torque) — the coil rotates. |
| All electric motors need: (1) a coil of wire; (2) a magnetic field; (3) a current flowing through the coil. |
| Electric motors are used wherever circular or rotational motion is needed. |
| Common applications: electric cars, electric fans, drills, washing machines. |
| Less obvious: hairdryers (spin the fan inside), lifts (wind cable up/down), roller shutters. |
| Increasing the current increases the speed/force of the motor. |
| Adding a resistor decreases the current and reduces the motor's speed. |
| Changing the direction of the current reverses the direction of rotation. |
| For continuous spinning, the current direction must be reversed every half turn. |
| This is achieved using a split-ring commutator — swaps the connections each half rotation. |
| Without the commutator, the coil would oscillate back and forth rather than rotating continuously. |
| Brushes maintain electrical contact between the stationary power supply and the rotating commutator. |
| More coils, higher current, or stronger magnets all increase the motor's power. |
| When a magnet is moved into or out of a coil of wire, a potential difference (voltage) is induced — this is electromagnetic induction. |
| A current is induced whenever the magnetic field through the coil changes. |
| Moving the magnet faster induces a larger potential difference. |
| Moving the south pole in produces a PD in the opposite direction to the north pole entering. |
| Moving the magnet from the opposite end also reverses the PD. |
| This principle is used in generators to produce electrical energy from kinetic energy. |
| A generator uses coils of wire and magnets to produce electricity. |
| In a generator, a coil of wire rotates inside a magnetic field — the changing field induces a current. |
| Direct current (DC) — current always flows in the same direction at a constant rate (e.g. from a cell). |
| Alternating current (AC) — the current repeatedly changes direction as the coil rotates. |
| Ways to increase the output voltage: increase rotation speed, increase number of coils, use stronger magnets. |
| UK mains electricity is AC at 230 V and 50 Hz — changes direction 50 times per second. |
| The national grid is the network of power lines and transformers that carries electricity from power stations to homes. |
| Electricity is generated in power stations at around 25,000 V using turbines and generators. |
| It is transmitted through high-voltage cables at up to 400,000 V. |
| Before entering homes, the voltage is reduced to 230 V for safe use. |
| High-voltage transmission is used because: higher voltage → lower current → less energy wasted as heat in cables. |
| Power = Current × Voltage. For the same power, higher voltage means lower current and less heating. |
| A transformer is a device that changes the voltage of an alternating current. |
| It consists of two coils of wire (the primary coil and the secondary coil) wound around an iron core. |
| A step-up transformer increases the voltage (more turns on secondary than primary). |
| A step-down transformer decreases the voltage (fewer turns on secondary than primary). |
| Transformers only work with alternating current (AC) — not with DC. |
| Vp / Vs = Np / Ns (ratio of voltages = ratio of turns) |