OCR GCSE Physics: Electricity and Magnetism (P3–P4)
OCR GCSE Physics: Electricity and Magnetism (P3–P4)
Topics P3 Electricity and P4 Magnetism of OCR Gateway Science A GCSE Physics (J249) complete Paper 1. P3 explains how charge flows and how circuits behave; P4 explains how electricity and magnetism are two sides of the same phenomenon — how currents make magnetic fields, how fields exert forces on currents, and how moving magnets generate electricity. Between them they power everything from a torch to the national grid, and they are among the most heavily tested — and most misconception-prone — parts of the course. This guide works through both topics with the definitions, equations and worked examples that earn marks, and it forms part of our complete OCR GCSE Physics revision guide.
P3 — Electricity
Charge and Current
An electric current is a flow of electric charge. In a metal, the charge carriers are free (delocalised) electrons; in an electrolyte, they are ions. Current, measured in amperes (A), is the rate of flow of charge:
Q=It
where Q is charge in coulombs (C), I is current and t is time in seconds. A key idea that trips students up: in a series circuit the current is the same everywhere — charge is not "used up" as it goes round. What the components do is transfer energy from the charge, not consume the charge itself.
Worked example. A current of 0.5 A flows for 2 minutes. The charge that passes is
Q=It=0.5×120=60 C
(note the conversion of 2 minutes to 120 seconds — a classic unit trap).
Potential Difference and Resistance
Potential difference (voltage), measured in volts (V), is the energy transferred per unit charge between two points. Resistance, measured in ohms (Ω), opposes the flow of charge. The three are linked by the most-used equation in the topic:
V=IR
For a fixed resistor at constant temperature, current is directly proportional to potential difference — this is Ohm's law. Not every component obeys it, though, which brings us to I–V characteristics.
Worked example. A resistor carries a current of 0.30 A when the potential difference across it is 6.0 V. Its resistance is
R=IV=0.306.0=20 Ω
I–V Characteristics
Plotting current against potential difference reveals how a component behaves — one of the required practicals:
- A fixed resistor (at constant temperature) gives a straight line through the origin: current is proportional to potential difference, so resistance is constant.
- A filament lamp gives a curve that flattens as the voltage rises: as the filament heats up, its resistance increases, so the current rises less and less steeply.
- A diode allows current in one direction only: the graph shows almost no current in the reverse direction and a rising current once the forward voltage is high enough.
Being able to sketch and interpret these three graphs, and explain the filament lamp's curve in terms of temperature and resistance, is reliably worth marks.
Series and Parallel Circuits
The behaviour of current, potential difference and resistance differs between the two circuit types:
| Quantity | Series circuit | Parallel circuit |
|---|---|---|
| Current | The same at every point | Splits between the branches; the branch currents add up to the total |
| Potential difference | Shared between components; the p.d.s add up to the supply | The same across each branch (equal to the supply) |
| Total resistance | Adding resistors increases the total resistance | Adding resistors in parallel decreases the total resistance |
The parallel result surprises students: adding another resistor in parallel gives the current more paths to flow through, so the overall resistance falls. This is why household circuits are wired in parallel — each appliance gets the full mains voltage and can be switched independently.
Power and Energy
Electrical power is the rate at which energy is transferred, measured in watts (W). Two equations give it:
P=VIP=I2R
The energy transferred by a component over a time t is then E=Pt. These relationships explain why a kettle (high power) transfers energy fast and why thick cables are used for high-current appliances to limit heating losses.
Worked example. A lamp operates at 230 V and draws a current of 0.26 A. Its power is
P=VI=230×0.26=59.8 W≈60 W
Mains Electricity and Safety
UK mains electricity is alternating current (a.c.) at about 230 V and 50 Hz — the current repeatedly reverses direction. A battery, by contrast, supplies direct current (d.c.), which flows one way. A three-pin plug has three wires, and knowing their colours and roles is essential:
- The live wire (brown) carries the alternating potential difference from the supply — it is the dangerous one.
- The neutral wire (blue) completes the circuit and stays near earth potential.
- The earth wire (green and yellow) is a safety wire that carries current to earth if a fault makes the metal case live, and works with the fuse to cut the supply.
A fuse contains a thin wire that melts and breaks the circuit if the current exceeds a safe value, protecting the appliance and the user. Explaining how the earth wire and fuse work together to make an appliance safe is a favourite exam question.
P4 — Magnetism
Magnets and Magnetic Fields
A magnetic field is the region around a magnet or a current-carrying wire where a magnetic force acts. Field lines run from north to south outside a magnet, and the field is strongest where the lines are closest together — at the poles. Permanent magnets produce their own field; induced magnets become magnetic only when placed in a field and lose most of their magnetism when removed. Like poles repel and unlike poles attract.
Electromagnetism
An electric current creates a magnetic field around it. Around a straight wire the field forms concentric circles; coiling the wire into a solenoid concentrates the field, producing a field like that of a bar magnet, strong and uniform inside the coil. Adding an iron core makes an electromagnet, whose great advantage is that it can be switched on and off and its strength varied by changing the current or the number of turns. Electromagnets are used in scrapyard cranes, relays, circuit breakers and loudspeakers.
The Motor Effect
When a current-carrying wire is placed in a magnetic field, the field of the wire interacts with the external field and the wire experiences a force — the motor effect. The size of this force is given by
F=BIL
where B is the magnetic flux density in tesla (T), I is the current and L is the length of wire in the field. The direction of the force is given by Fleming's left-hand rule (thumb = force/motion, first finger = field, second finger = current). This is the principle of the electric motor: a current-carrying coil in a magnetic field experiences forces that make it rotate.
Worked example. A wire of length 0.05 m carries a current of 3.0 A at right angles to a magnetic field of flux density 0.20 T. The force on it is
F=BIL=0.20×3.0×0.05=0.03 N
The Generator Effect (Higher Tier)
The motor effect run in reverse gives the generator effect: when a conductor moves through a magnetic field (or the field through the conductor), a potential difference is induced across it, and if the circuit is complete, a current flows. Moving the conductor faster, using a stronger field or adding more turns of wire all increase the induced potential difference. This is how generators and alternators produce electricity, and it underlies the microphone and the dynamo.
Transformers
A transformer changes the size of an alternating potential difference. It has two coils — a primary and a secondary — wound on an iron core. An alternating current in the primary produces a continually changing magnetic field in the core, which induces an alternating potential difference in the secondary. The voltages are related to the numbers of turns by
VsVp=NsNp
A step-up transformer (more turns on the secondary) increases the voltage; a step-down transformer decreases it. Transformers are central to the national grid: power is stepped up to very high voltage for transmission, which reduces the current and therefore the energy wasted as heat in the cables, then stepped down again for safe use in homes.
Worked example. A transformer has 100 turns on the primary and 500 turns on the secondary. If the primary voltage is 12 V, the secondary voltage is
Vs=Vp×NpNs=12×100500=60 V
so this is a step-up transformer.
How These Topics Connect
P3 and P4 are deeply linked: the current of P3 is what creates the magnetic fields of P4, and the generator effect and transformers of P4 are what deliver that electricity across the country. The power equations of P3 return in P8's treatment of the national grid, where the reason for high-voltage transmission — minimising I2R heating losses — ties the two topics together. Circuits reward genuine understanding over memorised rules, so practise reasoning about what happens to current, voltage and resistance when a circuit changes.
To drill these topics interactively, work through the Electricity course and the Magnetism course, each taking you from the foundations to exam-level questions with an AI tutor on hand. For calculation and six-mark technique, see the exam technique guide.