Electromagnetism Basics: Motor Effect and Induction Explained

Quick answer: Electromagnetism is one relationship seen from two sides. A moving charge always makes a magnetic field around it. Put a current-carrying wire inside another magnetic field and it feels a force, F = BIL, which is the motor effect. Change the field through a coil instead and you push a current back out of it, which is induction.

History is my favourite subject and English is my strongest, so physics is where I genuinely have to work for marks. Electromagnetism was the topic I kept "revising" by rereading the textbook and kept scoring mediocre marks on anyway. Two realisations fixed it: I was learning eight separate facts when there are really three, and nearly every mark I lost was a direction or unit mark, not a hard physics mark.

Everything comes from one idea: charge in motion

There is no separate magnetism topic. Every magnetic effect on your syllabus comes from moving charge, and the whole chapter is three consequences of that.

1. A current creates a magnetic field around itself.
2. A current sitting inside somebody else's magnetic field feels a force.
3. A magnetic field that changes through a coil creates a voltage in that coil.

Even a bar magnet fits: its field comes from electron motion inside the iron atoms, all lined up. So sort every question first. Given current, asked for force? Number 2. Given movement, asked for a meter reading? Number 3. That step took me from guessing to knowing which equation to reach for.

Field patterns you have to be able to draw

Two patterns come up over and over, and the drawing marks are free if you practise them once properly.

A straight current-carrying wire produces concentric circles around it. Use the right-hand grip rule: right thumb along the conventional current, fingers curl the way the field lines point. The circles get further apart as you move away, because the field weakens.

A solenoid produces a field that looks exactly like a bar magnet's: lines running out of one end, round the outside, back in the other, roughly uniform inside. Grip the coil with your right hand so your fingers follow the current in the turns, and your thumb points to the north end.

To make an electromagnet stronger, increase the current, add more turns per unit length, or add a soft iron core. A mark I lost twice: I wrote "increase the voltage". Examiners want current. Write current.

Drawing marks go for arrows on every field line, lines that never cross, correct spacing, and labelled N and S poles. If you cannot picture one, sketch it and check it in Explain before you memorise it backwards.

The motor effect: a full worked example

The force on a current-carrying conductor in a magnetic field is F = BIL, where B is flux density in tesla, I is current in amps and L is the wire length inside the field in metres.

Worked example. A wire carries 3.0 A through a field of flux density 0.15 T. The length of wire inside the field is 8.0 cm. Find the force.

1. Convert first: 8.0 cm = 0.080 m. This is where the easy mark goes.
2. Substitute: F = 0.15 x 3.0 x 0.080.
3. Calculate: F = 0.036 N.
4. State the direction using Fleming's left-hand rule.

Two things examiners love. The force is zero when the current runs parallel to the field. And at Cambridge International A-Level and AP the wire may sit at an angle, so you use F = BIL sin(theta), with theta between wire and field.

In a motor, the two sides of the coil carry current in opposite directions, so the forces oppose and the coil turns. The split-ring commutator reverses the current every half turn so rotation continues the same way. If a question asks "explain why the coil keeps rotating", the commutator is the answer they want.

Fleming's left hand and right hand, without mixing them up

Left hand for motors, right hand for generators. To stop confusing them, think cause and effect rather than memorising a phrase.

• Given current and field, asked which way something moves: motion is the effect, so it is a motor. Left hand.
• Given motion and field, asked which way current flows: current is the effect, so it is a generator. Right hand.

For both, the fingers mean the same thing: First finger is Field (north to south), seCond finger is Current (conventional, positive to negative), thuMb is Motion or force. All three at 90 degrees to each other.

An exam tip that feels silly and works: hold your hand up and rotate your wrist until the first finger points along the field on the diagram. I lost direction marks for a whole year trying to do it in my head.

Induction, Lenz's law and a worked emf calculation

Induced emf equals the number of turns multiplied by the rate of change of magnetic flux. Flux through a coil is B x A x cos(theta) in webers, where theta is the angle between the field and the normal to the coil.

Worked example. A coil of 200 turns has area 0.0040 m^2. The field through it falls from 0.50 T to 0.10 T in 0.20 s. Find the induced emf.

1. Flux per turn at the start: 0.50 x 0.0040 = 0.0020 Wb.
2. Flux per turn at the end: 0.10 x 0.0040 = 0.00040 Wb.
3. Change in flux per turn: 0.0016 Wb.
4. Rate of change: 0.0016 / 0.20 = 0.0080 Wb per second.
5. Induced emf = 200 x 0.0080 = 1.6 V.

Lenz's law is the minus sign: the induced current always opposes the change that produced it. Push a north pole into a coil and the coil's near face becomes a north pole to push back. That is conservation of energy. You do work against the repulsion, and that work becomes the electrical energy. If it helped you instead, you would get energy for free.

Three ways to increase induced emf: move faster, use a stronger magnet, use more turns. Held still, the flux is not changing, so the emf is zero. That last one is a classic two-mark question.

Transformers are the same physics repackaged: Vp / Vs = Np / Ns. With 1150 primary turns and 60 secondary turns on a 230 V supply, Vs = 230 x 60 / 1150 = 12 V. They only work on alternating current, because steady direct current gives no flux change and so no emf. Practise these on the physics hub or check your working in the math solver.

The marks I actually kept dropping

• Not converting cm to m before substituting into F = BIL.
• Writing "the magnet makes electricity". It induces a potential difference. Use the mark-scheme word.
• Naming Fleming's rule but never applying it, so no direction reaches the answer.
• Forgetting that a stationary magnet inside a coil induces nothing at all.

Test yourself

1. A 12 cm length of wire carrying 2.5 A sits perpendicular to a 0.20 T field. What is the force on it?
2. A magnet is pushed north-pole-first into a coil. Which pole appears on the near face of the coil, and why?
3. Which hand rule do you use to find the direction of the current induced in a wire that is being moved through a field?

FAQ

Why is it a left hand for motors but a right hand for generators?

Because the two situations have opposite cause and effect. In a motor, current and field are the inputs and motion is the output. In a generator, motion and field are the inputs and current is the output. Ask which quantity the question wants produced, then pick the hand.

Does the right-hand grip rule use conventional current or electron flow?

Conventional current, running from positive to negative. Every rule in this topic uses conventional current unless a question says otherwise, so never switch to electron flow halfway through a paper.

Why does a transformer not work with direct current?

A steady direct current produces a steady magnetic field in the core. Induction needs changing flux, and steady means no change, so no emf appears in the secondary coil. Alternating current constantly reverses, so the flux constantly changes.

How much of this do I need at GCSE compared with A-Level?

At GCSE and IGCSE you need the field patterns, F = BIL, both hand rules, the motor, the generator and the transformer equation. At Cambridge International A-Level and AP you add flux, flux linkage, quantitative Faraday's law and forces on individual moving charges.

In short: learn the three consequences of moving charge, practise the two field patterns until they are automatic, and rotate your actual hand for every direction question instead of guessing. Then test whether it stuck with a quick quiz rather than rereading the chapter, because rereading is exactly what fooled me into thinking I knew it.