This mini-lesson covers OCR Gateway Topic P3 — Electricity and Topic P4 — Magnetism & magnetic fields together: from static charge and circuits through to magnets, the motor effect and transformers.
Make a guess: where did the rod's charge come from?
Mission 1Where did the charge come from?
Mission 2What does a bulb take from the charge?
Mission 3What notices that it is getting dark?
Mission 4Hairdryer or bulb: which transfers more energy?
Mission 5Why does a paperclip never get pushed away?
Mission 6Which way does the wire jump?
Work through each screen, answer the questions as you go (some wordy, some calculations) and collect ⭐ stars. Higher-tier-only ideas are clearly flagged. Press Start when you're ready.
P3.1 · Static and charge
Static electricity
Rub two insulators together and electrons (negative) are transferred from one to the other. The one that gains electrons becomes negative; the one that loses them is left positive.
Only electrons move. Like charges repel; unlike charges attract — a non-contact force.
Watch out: a positive object is not made by moving positive charge in — it is made by electrons leaving. Static builds up on insulators, because the charge cannot flow away through them; charge on an earthed conductor simply flows away.
Mission 1 check
Where did the charge come from?
?A balloon is rubbed on a woollen jumper. Afterwards the balloon is negatively charged. Where did its charge come from?
P3.1 · Electric fields & sparks
Electric fields & field lines
A charged object is surrounded by an electric field — the region where it exerts a force on another charge. We draw field lines that point in the direction a positive test charge would be pushed: away from + and towards –.
The field is stronger where the lines are closer together (near the charge).
Sparking: if charge builds up enough, the field can become strong enough to push electrons through the air — you see a spark as the charge suddenly flows.
Quick check
What makes it positive?
?A perspex rod is rubbed with a cloth and becomes positively charged. What has happened?
Mission 2 of 6
What does a bulb take from the charge?
Make a guess: what does the bulb take from the charge?
P3.1 · Charge & current
Current is a flow of charge
An electric current is the rate of flow of charge. Charge needs a source of potential difference and a complete (closed) circuit to flow.
Q = I tcharge (C, coulombs) = current (A, amps) × time (s, seconds)
In a single closed loop the current is the same value at every point — charge is not used up as it goes round.
Worked example
A current of 3 A flows for 2 minutes (120 s).
Q = I t = 3 × 120 = 360 C
Calculate
Your turn — charge flow
1A current of 0.5 A flows through a torch bulb for 40 s. Calculate the charge that passes through it.
C
Hint: Q = I t = 0.5 × 40.
P3.2 · p.d. & resistance · practical
Potential difference & resistance
Potential difference (p.d., or voltage, in volts) is the energy transferred per unit charge — it pushes the current round. Resistance opposes the current:
V = I Rp.d. (V, volts) = current (A) × resistance (Ω, ohms)
Practical (PAG): connect a component in series with an ammeter and a variable resistor, with a voltmeter in parallel across it. Vary the p.d., record V and I, and find R = V ÷ I. Investigate how the length of a wire changes its resistance.
Worked example
A resistor carries 0.25 A when the p.d. across it is 6 V.
R = V ÷ I = 6 ÷ 0.25 = 24 Ω
P3.2 · Measuring in circuits
Where the meters go
Two rules you must never mix up:
An ammeter measures current, so it goes in series (in the loop).
A voltmeter measures p.d., so it goes in parallel (across the component).
Conventional current flows from + to – outside the cell. The voltmeter loops across R; the ammeter sits in the loop.Calculate
Your turn — resistance
2A voltmeter reads 12 V across a heater and the ammeter reads 4 A. Calculate the resistance of the heater.
Ω
Hint: R = V ÷ I = 12 ÷ 4.
Mission 2 check
What does a bulb take from the charge?
?A car headlamp is lit. An ammeter in the wire going into the lamp reads 4 A. What does an ammeter in the wire coming out read, and what does the lamp take?
Mission 3 of 6
What notices that it is getting dark?
Make a guess: which part of the circuit notices the dark?
P3.2 · I–V characteristics
I–V graphs of components
Plotting current against p.d. reveals whether a component's resistance is constant (ohmic) or changes:
Ohmic resistor: straight line through the origin (R constant). Filament lamp: S-shaped — it heats up, so resistance rises and the line bends over. Diode: only conducts one way.
Why the lamp bends: as current rises the filament gets hotter, so its resistance increases — the curve flattens. Both the resistor and lamp lines pass through the origin.
Sort it
Read the graph
Tap the component that matches each I–V description.
P3.2 · LDR & thermistor
Sensing components
Two components change resistance in response to their surroundings:
An LDR (light-dependent resistor): resistance falls as light gets brighter.
A thermistor: resistance falls as temperature rises (it's hotter → lower R).
Both curves fall: more light → less LDR resistance; more heat → less thermistor resistance.Quick check
Sensing the surroundings
?A thermistor is placed in a beaker of water that is then heated up. What happens to its resistance?
Mission 3 check
What notices that it is getting dark?
?A child's night-light switches itself on when the bedroom goes dark. Which component senses the dark, and how does it change?
Mission 4 of 6
Hairdryer or bulb: which transfers more energy?
Make a guess: which one transfers more energy each day?
P3.2 · Series & parallel
Series vs parallel circuits
Two lamps drawn with the correct ⊗ lamp symbol and a cell (long line +, short line –).
Series: current same everywhere · p.d. shares out between components · total resistance is the sum (adding resistors increases R).
Parallel: p.d. is the same across each branch · current splits between branches · total resistance is less than the smallest resistor.
Quick check
Series rules
?Three identical lamps are connected in series with one cell. Which statement is correct?
P3.2 · Power & energy
Electrical power & energy
Power is the rate at which a device transfers energy. In a circuit it depends on p.d. and current:
P = V I = I² Rpower (W) = p.d. (V) × current (A) = current² (A²) × resistance (Ω)
The energy transferred over a time is then:
E = V I t = P tenergy (J) = p.d. (V) × current (A) × time (s) = power (W) × time (s)
Worked example
A 230 V appliance draws a current of 2 A.
P = V I = 230 × 2 = 460 W. In 60 s it transfers E = P t = 460 × 60 = 27 600 J.
Calculate
Your turn — electrical power
3A lamp has a p.d. of 12 V across it and a current of 3 A through it. Calculate its power.
W
Hint: P = V I = 12 × 3.
Calculate
Your turn — energy transferred
4A 2000 W kettle is switched on for 90 s. Calculate the energy it transfers.
J
Hint: E = P t = 2000 × 90.
Mission 4 check
Hairdryer or bulb: which transfers more energy?
?A 2400 W kettle is used for 150 s each day. A 6 W Wi-Fi router is on all day, which is 86 400 s. Which transfers more energy each day?
Mission 5 of 6
Why does a paperclip never get pushed away?
Make a guess: why does a magnet never push a paperclip away?
P4.1 · Permanent & induced magnets
Magnets & poles
Every magnet has a north and a south pole. Like poles repel; unlike poles attract — a non-contact force.
A permanent magnet produces its own magnetic field all the time.
An induced magnet only becomes magnetic when placed in a field (e.g. a steel paperclip near a magnet) — and the force between them is always attractive. It loses most of its magnetism when removed.
Earth's field: a freely-suspended dipping compass lines up with Earth's magnetic field, which is evidence that Earth's core is magnetic. The geographic and magnetic poles are not in the same place.
P4.1 · Magnetic field lines
Field around a bar magnet
Magnetic field lines always run from North to Southoutside the magnet. They are closest together at the poles — that is where the field is strongest. Arrows point the way a compass north would point.
Plot the pattern with a small plotting compass or iron filings. Lines never cross.Quick check
Which way do the lines point?
?On a field-line diagram of a bar magnet, in which direction do the arrows point outside the magnet?
Mission 5 check
Why does a paperclip never get pushed away?
?Bars A and B push each other apart. Bar C is pulled towards both ends of bar A, whichever end of C is used. What is bar C?
P4.1 · Electromagnetism
A current makes a magnetic field
When a current flows through a wire it creates a magnetic field of concentric circles around it. The field is stronger with a bigger current and closer to the wire.
Wind the wire into a coil — a solenoid — and the fields add up to give a strong, uniform field inside, and outside a field shaped like a bar magnet's. Add an iron core and you have an electromagnet (used in scrapyard cranes, relays and bells), whose strength you can switch on and off.
More turns, more current or an iron core all make the electromagnet stronger.Mission 6 of 6
Which way does the wire jump?
Make a guess: what makes the wire jump when the current flows?
P4.2 · Higher Tier only
The motor effect & F = BIL
Put a current-carrying wire in a magnetic field and the two fields interact, exerting a force on the wire (the motor effect). The force is greatest when the wire is at right angles to the field:
F = B I Lforce (N) = magnetic flux density (T, tesla) × current (A) × length (m)
Fleming's left-hand rule gives the direction. Hold your left hand so:
First finger = Field, seCond finger = Current, thuMb = Motion (force).
This force spins a coil in a d.c. motor. A split-ring commutator swaps the current direction every half-turn so the coil keeps rotating the same way.
Calculate
Your turn — force on a wire (HT)
5A wire of length 0.2 m carries a current of 5 A at right angles to a magnetic field of flux density 0.4 T. Calculate the force on it.
N
Hint: F = B I L = 0.4 × 5 × 0.2.
Mission 6 check
Which way does the wire jump?
?A wire sits between two magnet poles. The field points from left to right. The current flows into the page, away from you. Which way is the wire pushed?
P4.2 · Higher Tier only
Induction & transformers
The generator effect is the reverse of the motor effect: move a magnet near a coil (or a wire through a field) and a potential difference is induced across it, which can drive a current. This is used in an alternator to generate a.c. and a dynamo to generate d.c.
A microphone works the same way: sound waves vibrate a diaphragm attached to a coil in a magnet's field, inducing a p.d. that varies just like the sound. A loudspeaker or headphone does the reverse: a varying current in its coil feels a varying force (the motor effect), so the cone vibrates and makes sound (P4.2j).
A transformer uses this on an iron core: an alternating current in the primary coil makes a changing field that induces a p.d. in the secondary coil. The turns ratio sets the voltages:
Vp Ip = Vs Is(for a 100% efficient transformer — power in = power out)
More turns on the secondary → step-up (higher voltage); fewer → step-down.
Crucial misconception: a transformer only works with a.c. A steady d.c. current makes a steady field, which induces nothing — you need a changing field.
Calculate
Your turn — transformer (HT)
6A transformer has 200 turns on the primary and 50 turns on the secondary. The primary p.d. is 240 V. Calculate the secondary p.d.
V
Hint: V_s = V_p × (N_s ÷ N_p) = 240 × (50 ÷ 200).
Match-up
Match the rule to the idea
Tap a term on the left, then its match on the right.
Term
Means…
Recap
The equations to know
Charge: Q = I t
Potential difference: V = I R
Power: P = V I = I² R
Energy: E = V I t = P t
Motor effect (HT): F = B I L
Transformer (HT): Vp/Vs = Np/Ns and VpIp = VsIs
You've covered P3 Electricity (static, current, p.d., resistance, I–V graphs, circuits, power) and P4 Magnetism (magnets, fields, electromagnets, the motor effect and transformers). Press Finish to see your score.
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Mini-lesson complete!
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You've worked through Electricity & Magnetism for OCR Gateway GCSE Physics A. 🎉