An electric current and a magnetic field are two sides of the same coin. In this mini-lesson you'll plot the field of a bar magnet and a coil, build an electromagnet, use Fleming's Left Hand Rule to find the force in a motor, induce a current to generate electricity, and step voltage up and down with a transformer — exactly the CCEA Unit 2.4 content, nothing more.
Outside a magnet, field lines always point from N to S. They are closest together (strongest) at the poles.
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2.4.1 · Magnetic field of a bar magnet
The field around a bar magnet
A magnetic field is the region around a magnet where it can exert a force. We draw it with field lines. Two rules you must recall:
Field lines always point out of the N pole and into the S pole — so N → S outside the magnet.
The lines are closest together at the poles, where the field is strongest, and spread apart (weaker) further away.
You can map the field with a plotting compass: place it near the magnet, mark where the needle points, move it on, and join the dots. The compass needle always lines up along the field, with its red (N-seeking) end pointing the way the field points.
A plotting compass sits along the field line; its red end shows the field direction.
Misconception buster. Field lines are not "drawn S → N", and they don't both leave from the middle. Outside the magnet they always go N to S. Inside the magnet they actually run S → N to make a closed loop, but at GCSE we draw and describe the outside direction.
Quick check
Which way do the lines point?
?A plotting compass is placed just outside a bar magnet, level with the gap between the poles. Its red (N-seeking) end points towards the magnet's S pole. What does this tell you about the field there?
2.4.2 · Magnetic field of a current-carrying coil
A current makes a magnetic field
Whenever a current flows, it produces a magnetic field around it. Around a straight wire the field is a set of concentric circles. We use the right-hand grip rule: point your right thumb along the conventional current, and your curled fingers show the way the circular field points.
The dot (⊙) means current coming out of the page; the cross (⊗) means current going in. Reverse the current and the field reverses.
Wind the wire into a coil (solenoid) and the circular fields add up to give a field just like a bar magnet — with a N pole at one end and a S pole at the other. Which end is N depends on the direction of the current: looking at an end, if the current flows anticlockwise it is a N pole; clockwise makes a S pole.
A solenoid's field is like a bar magnet's. Reverse the current and the N and S ends swap over.
Watch out. The field of a single straight wire is circles, not a line, and it has no poles. Only when you coil the wire do you get bar-magnet-style N and S ends.
Quick check
Find the pole
?You look straight at one end of a current-carrying coil and see the conventional current flowing clockwise. Which pole are you looking at, and how could you reverse it?
An electromagnet is a coil wound on a soft-iron core. Switch the current on and it is magnetic; switch it off and it loses its magnetism — that "on/off" control is what makes electromagnets so useful. CCEA's Prescribed Practical P9 investigates what makes the field stronger. The strength depends on:
The current in the coil — more current, stronger field.
The number of turns on the coil — more turns, stronger field.
The core material — a soft-iron core makes the field far stronger than air.
Count how many paperclips the electromagnet holds to measure its strength.
Why soft iron? Soft iron is easily magnetised and easily demagnetised, so the electromagnet switches cleanly on and off. A steel core would stay magnetised (a permanent magnet) — no good for switching.
Will it get stronger?
Stronger, weaker or no change?
For each change to an electromagnet, choose the effect on the strength of its magnetic field.
2.4.4 · The motor effect & Fleming's Left Hand Rule
The motor effect
Put a current-carrying wire in a magnetic field and the wire feels a force. This is the motor effect. The force is at right angles to both the current and the field, and its direction is given by Fleming's Left Hand Rule:
thuMb = Motion (force) · First finger = Field · seCond finger = Current
Hold your left hand with thumb, first and second fingers mutually at right angles. First finger points along the field (N → S); seCond finger points along the conventional current; your thuMb then points the way the wire is pushed.
Fleming's Left Hand Rule: Force ⟂ Field ⟂ Current.
This is exactly how a d.c. electric motor works: a coil carrying current sits between magnet poles. One side is pushed up and the other down, so the coil spins. (CCEA does not require the detail of the split-ring commutator.)
Misconception buster. The force is biggest when the wire is at 90° to the field, and zero when the wire is parallel to the field. CCEA treats this qualitatively — there is no F = BIL equation to learn at GCSE.
Quick check
Use Fleming's Left Hand Rule
?A wire carries current straight towards you (out of the page). The magnetic field points to the right. Using your left hand — First finger right (field), seCond finger out of the page (current) — which way is the wire pushed?
2.4.5 · a.c. and d.c.
a.c. and d.c.
There are two kinds of current:
Direct current (d.c.) flows in one direction only. A cell or battery gives d.c.
Alternating current (a.c.) keeps changing direction, back and forth. Mains electricity (and a generator) gives a.c.
On a cathode-ray oscilloscope (CRO) you can tell them apart from the trace:
d.c.: a flat line above the centre. a.c.: a repeating wave crossing the centre line.
Sources. d.c. → cells, batteries, solar cells. a.c. → the mains supply and a.c. generators. This matters next: induction naturally produces a.c., and transformers only work with a.c.
Quick check
Reading the trace
?An oscilloscope shows a trace that rises above and falls below the centre line, repeating over and over. What current is this, and name a source of it?
2.4.6 · Electromagnetic induction
Inducing a current
Move a magnet in and out of a coil connected to a centre-zero meter and a current is induced — the needle flicks. From this experiment CCEA expects you to recall:
Pushing the magnet in and pulling it out give currents in opposite directions (the needle flicks the other way).
Moving the magnet faster induces a bigger current.
When the magnet is stationary — even right inside the coil — there is no induced current.
Spinning the magnet near the coil induces an alternating current — this is the basis of generating electricity.
An induced current only flows while the magnetic field through the coil is changing.
Misconception buster. Induction needs a changing field. A magnet sitting still inside a coil — however strong — induces nothing. It's the movement (the change) that matters, and faster change means a bigger induced voltage.
Quick check
When is a current induced?
?A strong bar magnet is held perfectly still deep inside a coil joined to a centre-zero meter. The reading is zero. Why?
2.4.8 · The a.c. generator
Generating electricity
An a.c. generator uses induction the other way round to the motor. In its simplest form it is a coil of wire rotated between the poles of a magnet. As the coil spins, the field through it constantly changes, so an alternating voltage is induced.
Spin a coil between magnet poles and you induce an alternating voltage — that's how power stations make electricity.
In a power station, a turbine (driven by steam, falling water or wind) spins the generator. Spinning it faster or using a stronger field / more turns gives a bigger induced voltage.
Two coils, no movement. You can also induce a current by switching a current on/off in a neighbouring coil — the changing current changes the field. There's no moving magnet, but the field still changes. This is the basis of the transformer (next).
Quick check
Boosting the output
?An a.c. generator is a coil spinning between magnet poles. Which change would increase the size of the induced voltage?
2.4.9 · The transformer
Step-up and step-down transformers
A transformer changes the size of an alternating voltage. It is built from two coils wound on a shared soft-iron core:
The primary coil takes the input (alternating) voltage.
The secondary coil gives the output (alternating) voltage.
The iron core links the two coils, carrying the changing magnetic field from primary to secondary.
The alternating current in the primary makes a constantly changing field in the core. That changing field passes through the secondary and induces an alternating voltage in it. The turns decide the output:
Step-up:more turns on the secondary than the primary → output voltage is bigger.
Step-down:fewer turns on the secondary → output voltage is smaller.
More turns on the secondary than the primary = a step-up transformer (output voltage higher).
Misconception buster. A transformer only works with a.c. A steady d.c. gives an unchanging field in the core, so nothing is induced in the secondary. No changing field → no transformer action.
2.4.10 · The turns-ratio equation
The turns-ratio equation
The output voltage depends on the ratio of turns. CCEA gives the equation as:
Ns / Np = Vs / Vp
Np, Ns = turns on the primary and secondary · Vp, Vs = primary (input) and secondary (output) voltages. The same as Vp/Vs = Np/Ns.
Worked example — step-up
A transformer has 200 turns on the primary and 1000 turns on the secondary. The input voltage is 12 V. Find the output voltage.
Vs = Vp × (Ns / Np) = 12 × (1000 / 200) = 12 × 5 = 60 V
CCEA also tells you to treat a transformer as 100% efficient: the electrical power input equals the power output. So if the voltage is stepped up, the current must step down by the same factor (and vice-versa) — the power stays the same.
Higher Tier. The transformer equation and these calculations sit in CCEA's Higher Tier material (the induction, generation and transformer cluster, 2.4.6–2.4.11). Note CCEA does not print the power formula VpIp = VsIs — it states the idea in words: power in = power out.
Calculate · transformer
Find the output voltage
=A transformer has 500 turns on the primary and 1500 turns on the secondary. The input (primary) voltage is 20 V. Calculate the output (secondary) voltage.
V
Hint: Vs = Vp × (Ns / Np) = 20 × (1500 / 500).
Calculate · transformer
How many turns?
=A step-down transformer must change 230 V mains to 23 V. The primary has 2000 turns. How many turns are needed on the secondary?
turns
Hint: Ns / Np = Vs / Vp, so Ns = 2000 × (23 / 230).
Calculate · power in = power out
The current on the other side
=A 100% efficient transformer steps voltage up so the power output equals the power input. The primary takes 12 V at 5 A. The secondary output is 60 V. What is the secondary current?
A
Hint: power in = power out. Power in = 12 × 5 = 60 W. So 60 = 60 × Is.
2.4.11 · Transmission of electricity
Transformers and the transmission of electricity
Electricity is carried from power stations to homes through long cables. Transformers make this efficient:
A step-up transformer raises the voltage to a very high value for transmission across the country.
A step-down transformer lowers it again — to a safe voltage — before it reaches homes.
Why bother stepping the voltage up? Because power is delivered at high voltage and low current. A smaller current means less energy wasted as heat in the cables, so far more of the electricity reaches the consumer.
Step up for transmission (high V, low I, low loss); step down for safe use in homes.
Misconception buster. High voltage isn't dangerous "for the sake of it" — it's deliberate. High V → low I → low heat loss in the cables. Transformers need a.c., which is exactly why our grid runs on alternating current.
Quick check
Why high voltage?
?Electricity is sent across the country at very high voltage. What is the main reason transformers step the voltage up before transmission?
Sort it · needs a.c. or works on d.c.?
a.c. or d.c.?
Tap an item, then tap the box it belongs in.
🔁 Alternating (a.c.)
➡️ Direct (d.c.)
Quick check
When is the force biggest?
?A current-carrying wire lies parallel to the magnetic field between two magnet poles. What is the force on the wire?
Recap
The facts to know
Bar magnet field: lines run N → S outside, densest (strongest) at the poles.
Wire field: concentric circles (right-hand grip rule); a coil/solenoid acts like a bar magnet.
Electromagnet (P9): stronger with more current, more turns, an iron core.
Motor effect: force on a wire ⟂ to both field and current — Fleming's Left Hand Rule (thuMb = motion, First = field, seCond = current). Max at 90°, zero when parallel. Basis of the d.c. motor.
a.c. / d.c.: a.c. reverses (mains, generator); d.c. is one-way (cell). CRO: wave vs flat line.
Induction: a changing field induces a voltage; faster change → bigger; no change → nothing.
a.c. generator: a coil spun between magnet poles induces alternating voltage.
Transformer (a.c. only): Ns/Np = Vs/Vp; treat as 100% efficient (power in = power out).
Transmission: step up → high V, low I, low heat loss; step down for safe use in homes.
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