The Motor Effect: How Electric Motors Work

A wire that pushes itself

Place a wire between the poles of a magnet and switch on a current. The wire suddenly jumps — it feels a force and moves. Nothing is touching it; the push comes purely from the invisible interaction between the current and the magnetic field. This is the motor effect, and it is the principle behind every electric motor on Earth, from a phone's vibrate buzzer to a high-speed train.

This lesson assumes you already know that a current creates a magnetic field — if not, start with electromagnets. Here we look at what happens when that magnetic field meets another magnetic field.

Why there is a force

A current-carrying wire is surrounded by its own circular magnetic field. When you put that wire inside the field of a permanent magnet, the two fields overlap and interact. On one side of the wire the two fields point the same way and reinforce, making the field strong; on the other side they oppose and partly cancel, making it weak.

Magnetic field lines behave a bit like stretched elastic bands that try to straighten out and that push apart when crowded together. The "crowded" strong side pushes the wire towards the weak side. The result is a force on the wire, at right angles to both the current and the field. For more on how magnets and field lines behave, see magnets and magnetism.

Fleming's left-hand rule

The directions of the field, the current and the resulting force are all mutually perpendicular (each at right angles to the other two). To find the direction of the force, use Fleming's left-hand rule:

Hold the thumb, first finger and second finger of your left hand so they are mutually at right angles, like the corner of a box. Then:

• First finger → Field (N to S),
• seCond finger → Current (conventional current, + to −),
• thuMb → Motion (the force / direction the wire moves).

Point your first finger along the field and your second finger along the current, and your thumb automatically points the way the wire will be pushed. A handy memory aid: Field, Current, Motion follow the order first–second–thumb.

A vital consequence: reverse the current, or reverse the magnets, and the force flips direction. Reverse both together and the force returns to its original direction.

How big is the force? F = BIL

The size of the force depends on three things — the strength of the field, the size of the current, and the length of wire in the field:

F = B × I × L

where:

• F is the force in newtons (N),
• B is the magnetic flux density (field strength) in teslas (T),
• I is the current in amperes (A),
• L is the length of wire inside the field in metres (m).

This equation only holds when the current is at right angles to the field. If the wire lies along the field, the force is zero; at angles in between, only the perpendicular part counts.

Worked examples

Example 1 — find the force. A 0.05 m length of wire carries a current of 4 A at right angles to a magnetic field of 0.3 T. F = B × I × L = 0.3 × 4 × 0.05 = 0.06 N.

Example 2 — find the field strength. A 0.10 m wire carrying 2.5 A experiences a force of 0.15 N when perpendicular to a field. Find B. Rearrange: B = F ÷ (I × L) = 0.15 ÷ (2.5 × 0.10) = 0.15 ÷ 0.25 = 0.6 T.

Example 3 — find the current. What current is needed to produce a 0.8 N force on a 0.2 m wire sitting in a 1.0 T field at right angles? I = F ÷ (B × L) = 0.8 ÷ (1.0 × 0.2) = 4 A.

From a single wire to a spinning motor

A single wire just twitches once. To make continuous rotation, real motors use a coil of wire on an axle between magnets:

1. Current flows through the coil. One side of the coil carries current one way, the other side the opposite way.
2. Because the two sides carry current in opposite directions through the same field, Fleming's rule gives forces in opposite directions on the two sides — one side is pushed up, the other down. This pair of forces makes the coil rotate.
3. After a half-turn the coil would be pushed back the other way and just oscillate. To prevent this, a split-ring commutator swaps the current direction in the coil every half-turn, so the force always drives the coil the same way round. The coil spins continuously.

To make the motor stronger you can: increase the current, use a stronger magnet, add more turns to the coil, or place a soft-iron core inside the coil to concentrate the field. These are exactly the ways engineers make motors more powerful.

Try it yourself! 🧪

Build a spinning "homopolar" motor — the simplest motor there is, powered by a single AA cell.

You need: one AA battery (1.5 V), a small but strong neodymium disc magnet, and about 20 cm of bare copper wire.

1. Stand the battery upright with its flat negative end on the magnet, so the magnet sticks to the bottom.
2. Bend the copper wire into a symmetrical shape (a heart or loop) that balances on the top (+) terminal and has its two ends just brushing the sides of the magnet.
3. Let go. Current flows from the top terminal, down the wire, through the magnet — and the motor effect spins the wire round and round.

You are watching a current in a magnetic field produce continuous motion, exactly as F = BIL predicts.

⚠️ Safety: Use only a single 1.5 V cell — never mains electricity. The wire and battery get hot quickly because this is almost a short circuit, so run it only for a few seconds at a time and disconnect when done. Neodymium magnets are strong and can pinch fingers or snap together — keep them away from phones, cards and small children who might swallow them.

What we learned

A current-carrying wire in a magnetic field feels a force — the motor effect. The force is at right angles to both the current and the field, its direction is given by Fleming's left-hand rule, and its size is F = B × I × L. Reversing the current or the field reverses the force. By using a coil and a commutator that flips the current each half-turn, this effect is turned into the continuous spin of an electric motor.