Magnetism and electromagnetism · System
How a motor works
A wire carrying a current inside a magnetic field is pushed. Arrange for the push to keep going the same way round, and you have a motor.
Start here
Swap the two wires and the motor runs backwards.
A small electric motor is connected to a battery and its shaft spins clockwise. Take both wires off, put them back the other way round, and the same motor spins anticlockwise at the same speed. Nothing inside it has been touched.
Why does swapping the wires reverse it?
The direction of the push on a current-carrying wire in a field depends on which way the current is going. Swapping the leads sends the current round the coil the other way, so both sides are pushed the opposite way and the coil turns the opposite way. The magnets inside have not moved, and they could not be flipped by a current — they are permanent magnets bonded to the case.
Put a wire that is carrying a current into a magnetic field, and the wire is pushed sideways — at right angles both to the field and to the current. This is the one new fact the whole lesson rests on, and it is easy to show: a loose wire between the poles of a horseshoe magnet jumps the moment the current is switched on.
Which way it is pushed depends on two things: the direction of the current, and the direction of the field. Reverse the current and the push reverses. Reverse the magnets and the push reverses. Reverse both and it goes back to where it started.
Now bend the wire into a rectangular loop and hang it on an axle between the poles. The current runs one way along the left-hand side of the loop and the other way along the right-hand side, so the two sides are pushed in opposite directions — one up, one down. A pair of opposite pushes on either side of an axle is a turning effect, and the loop turns.
There is a catch. After half a turn the two sides have swapped places, and the pushes now fight the rotation instead of driving it. The fix is the split-ring commutator: the loop's two ends are joined to two half-rings that press against fixed contacts, so every half turn the connection swaps and the current through the loop reverses. The push therefore keeps driving the same way round, and the motor keeps going.
At the bench · a coil on an axle between two magnets
Reverse one thing at a time.
Change a control to begin
The coil is drawn face on, with the field running across the page between the two magnets.
Commit first. A working motor has its battery leads swapped and its two magnets turned round, both at the same time. Which way does it now spin?
Current round the coil
The magnets
At the axle
2.0 A
The left-hand side is
—
—
Turning effect
—
where 100 is the strongest setting here
It spins
—
Does it keep going
—
The figure
Four parts, and what each one is for
The magnets
Provide a field across the gap. Turn them round and everything reverses.
The coil
Carries current up one side and down the other, so the two sides are pushed opposite ways and the coil turns.
The split ring
Swaps the connections every half turn, so the current through the coil reverses just as the coil passes upright. Without it the motor stops after half a turn.
The brushes
Fixed contacts that press on the split ring, so current can reach a part that is spinning.
Reverse the current: it runs the other way. Reverse the magnets: it runs the other way. Reverse both: it runs exactly as it did. That last one is the test of whether you have understood the rule rather than memorised a picture.
Key fact
A wire carrying a current in a magnetic field is pushed sideways, and the direction of the push depends on both the current direction and the field direction. In a motor a coil on an axle has its two sides pushed opposite ways, which turns it, and a split-ring commutator reverses the current every half turn so the turning effect keeps driving the same way round.
Think again
“The coil is pulled round because the magnets attract it.”
Switch the current off and the coil hangs there — the magnets do nothing to a coil of copper wire, because copper is not a magnetic material and there is nothing to attract. What acts is a force on the moving charge in the wire, and it appears only while a current flows. It is also at right angles to both the field and the current, which is not what attraction looks like: the coil is not pulled towards either magnet, it is pushed up on one side and down on the other.
“The split ring is what makes it turn.”
It is what makes it keep turning. Remove the split ring and the coil still starts — you can see it kick on the bench — but only for half a turn, because after that the sides have changed places and the same pushes are now fighting the rotation. The split ring does not create the turning effect; it reverses the current at the moment the turning effect would otherwise start working against you.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Predict the reversal
A motor is running clockwise. Its two magnets are turned round so the poles swap, and nothing else is changed. What happens?
Rung 2 · Diagnose the fault
A student builds a motor but fits two plain rings instead of a split ring. What do they see when the current is switched on?
Rung 3 · Explain
Explain why a coil carrying a current between two magnets turns, rather than simply being pushed to one side.
Rung 4 · Take it somewhere new
A cordless drill has a switch marked forward and reverse, and the motor inside has permanent magnets that cannot be moved. Explain what that switch must be doing, and explain why the drill still needs a split ring in both settings.
Key note
A wire carrying a current in a magnetic field is pushed sideways, at right angles to both. Which way it is pushed depends on the current direction and on the field direction, so reversing either one reverses the push and reversing both changes nothing. A motor is a coil on an axle in a field: the current runs up one side and down the other, the two sides are pushed opposite ways, and the coil turns. After half a turn the sides have swapped and the pushes would fight the rotation, so a split-ring commutator reverses the current every half turn and the motor keeps going.
Going further
Real motors have more than one coil, set at angles to each other, and the commutator has a segment for each. That fixes two problems at once: the turning effect of a single coil drops to nothing twice per turn, when the coil is upright and the pushes are pulling it apart rather than round, and a single-coil motor therefore runs unevenly and will not start at all from that position. With several coils, one of them is always well placed, and the machine starts from wherever it happens to be sitting.
Run the same machine backwards and it becomes a generator. Turn the coil by hand instead of feeding it current and a voltage appears across its ends, because moving a wire through a field pushes the charge in it along. Almost every power station on Earth is doing that, on an enormous scale, with steam or water or wind turning the coil.
Before this lesson
Connects to
At GCSE this becomes
- Fleming's left-hand rule, the force on a conductor worked out from flux density, current and length, and the generator effect.
Where to next
- Next: Density
Matter and the particle model
- Previous: Electromagnets
Ask Mr Badmus AI
Want to check which way a coil will turn?
Eye protection on. The coil gets hot within a minute, so switch off between tries and let it cool. Keep fingers, hair and loose sleeves clear of the spinning part. Use only the low-voltage supply your teacher gives you.
The bench is a teaching model. The coil is drawn face on and frozen in the horizontal position, where the turning effect is at its largest; a real coil's turning effect falls to nothing twice per turn as it passes upright, which is why real motors use several coils rather than one. The turning effect shown is a relative figure with the largest setting here set to 100, and it is taken as rising in proportion to the current; no value in newtons or newton metres is given, because the equation for the force on a conductor is beyond this stage. The magnets are treated as producing the same field whichever way round they are, the friction at the axle is treated as one fixed amount, and the coil is treated as one turn. A real motor without a split ring does not stop dead at the half turn but rocks to a halt over several swings.
Lesson content © MrBadmusAI.