How a motor turns

Definition

Electric motor

An electric motor is a device that uses the forces on current-carrying conductors in a magnetic field to make a coil rotate.

Definition

Turning effect

A turning effect is the rotation produced when forces act on an object in such a way that they twist it about an axis rather than push it in a straight line.

A simple motor is built from a rectangular coil of wire mounted on an axle so that it can spin, with the coil sitting in the field between the poles of a permanent magnet.
Current is fed in at one side of the coil and leaves at the other, so the current travels all the way round the loop.
Because the current goes round a loop, it runs along one long side of the coil in one direction and back along the other long side in the opposite direction. This single fact is what makes a motor work.
Both long sides lie across the same magnetic field, so both experience a force. The field is the same for both, but the current directions are opposite, so the two forces point in opposite directions.
One long side is pushed upwards while the other is pushed downwards, and each force acts at a distance from the axle.
A pair of equal forces acting in opposite directions on opposite sides of an axle does not push the coil anywhere. It twists the coil instead, producing a turning effect, and the coil rotates.
The short sides of the coil run along the field rather than across it, so they contribute no useful force and can be ignored in the explanation.
Reversing the current alone reverses both forces and so reverses the direction of rotation. Reversing the magnetic field alone does the same. Reversing both together leaves the motor spinning the way it did before.
Energy is transferred electrically from the supply to the kinetic energy store of the rotating coil, because the current-carrying sides are pushed by the magnetic field.

Key Idea

A motor rotates because the currents in the two sides of the coil are in opposite directions, so the forces on those sides are in opposite directions and produce a turning effect on the coil.

Keeping the coil turning

Definition

Split-ring commutator

A split-ring commutator is a rotating contact divided into two halves that reverses the current in the coil of a direct current motor every half turn.

A coil connected straight to a battery would not keep spinning. It would turn half a revolution and then stall.
The reason is that after half a turn the two sides have swapped places. The side that was on the left is now on the right, but the force on it still points the same way as before.
The turning effect would then act the wrong way round, pushing the coil back towards where it came from, so it would rock to and fro and settle.
A split-ring commutator solves this. It is a metal ring cut into two halves, mounted on the axle and turning with the coil, with each half connected to one end of the coil.
Two fixed carbon brushes press against the ring and carry the current in from the supply while the ring spins past them.
Every half turn, the gaps in the ring pass the brushes and each half of the ring meets the opposite brush, so the current through the coil is reversed.
Reversing the current reverses both forces at exactly the moment the sides have swapped places, so the turning effect keeps acting the same way round and the coil carries on rotating.
The switching is automatic and mechanical. Nothing in the circuit has to be operated by hand, because the commutator turns with the coil and reverses the current at the right instant on its own.
The point worth writing in an answer is not the construction of the ring. It is that the current is reversed every half turn so that the coil keeps turning in the same direction.

Making a motor turn faster

Increasing the current in the coil increases the force on each side, so the turning effect and the rotation rate both rise.
Using stronger magnets raises the magnetic flux density in the gap, which again increases the force on each side of the coil.
Adding more turns to the coil puts more current-carrying wire in the field, so the forces add and the turning effect grows.
Making the coil wider moves each force further from the axle, which increases the turning effect without changing the size of either force.
Reducing friction at the bearings and keeping the coil light both help, because less of the turning effect is wasted overcoming resistive forces.
These are the ideas behind real motors. A cordless drill draws a large current through many turns wound on an iron armature between strong magnets, while a model motor uses a small current and a light coil.

Rotation rates and unit conversions

The speed of a motor is quoted as a rotation rate, the number of complete turns made in a given time.
Manufacturers usually quote rotations per minute, while physics calculations need rotations per second, so a conversion is often the first step. $s1\ \text{min} = 60\ \text{s}$
To go from rotations per minute to rotations per second, divide by $60$ . To go the other way, multiply by $60$ .
The time for one rotation is the reciprocal of the rate in rotations per second: $\dfrac{1}{n}$
where $t$ is the time for one rotation in seconds and $n$ is the rotation rate in rotations per second.
Rotation rate and the time for one rotation are inverses of each other, so a faster motor has a shorter time per rotation.
Read the unit carefully. A rate of $rotations/s60\ \text{rotations/s}$ means sixty turns every second, not sixty seconds for one turn.

Example

Converting a rotation rate

A small electric motor is quoted as running at $rotations/min3600\ \text{rotations/min}$ . Its rate in rotations per second and the time for one rotation are both required.
There are $s60\ \text{s}$ in a minute, so divide the quoted rate by $60$ .
$\dfrac{3600}{60} = 60\ \text{rotations/s}$
The motor therefore makes $60$ complete turns in every second.
$\dfrac{1}{n} = \dfrac{1}{60} = 0.0167\ \text{s}$
To two significant figures the time for one rotation is $s0.017\ \text{s}$ .

Example

Using proportional reasoning on a motor

A motor turns at $rotations/min1200\ \text{rotations/min}$ when the current is $A0.50\ \text{A}$ . The current is raised to $A1.5\ \text{A}$ and the rotation rate is found to rise in the same ratio.
Find the ratio of the two currents first.
$1.50.50=3.0\dfrac{1.5}{0.50} = 3.0$
The current is three times larger, so the rotation rate is also three times larger.
$1200 \times 3.0 = 3600\ \text{rotations/min}$
Converting gives $rotations/s\dfrac{3600}{60} = 60\ \text{rotations/s}$ , so the time for one rotation falls from $s0.050\ \text{s}$ to $s0.017\ \text{s}$ .

Practical

Constructing a simple electric motor

Aim: to build a working coil motor, to show that the forces on the two sides of the coil produce rotation, and to test how the current, the magnets and the number of turns affect how fast it spins.
Apparatus: about $m1\ \text{m}$ of enamelled copper wire, a cylindrical former such as a battery to wind the coil on, two paper clips bent into cradles, a wooden block or cork base, two strong slab or neodymium magnets, a $V1.5\ \text{V}$ cell in a holder or a low-voltage supply, a rheostat, an ammeter, a switch, fine sandpaper, sticky tape and a stopwatch.
Variables: the current, the strength of the magnets or the number of turns is the independent variable, the rotation rate is the dependent variable, and the coil, the base, the magnet positions and everything not being changed are kept the same.
Method, building the coil:
- Wind about ten neat turns of enamelled wire round the former, then slide the coil off and leave a straight tail of a few centimetres projecting from each side along the same line.
- Loop each tail once through the coil to bind the turns together, so the coil holds its shape and stays balanced when it spins.
- Check that the two tails lie exactly opposite each other on a straight line through the middle of the coil, because a lopsided axle makes the motor wobble and stall.
Method, making the contacts:
- Sand the enamel completely off one tail all the way round, so that this end always makes contact.
- Sand the enamel off only the upper half of the other tail, laying the wire flat and rubbing one side only.
- That half-stripped tail is a simple commutator. It carries current for half of every rotation and breaks the circuit for the other half, so the coil is only pushed while the push is in the useful direction.
- Tape the two paper-clip cradles to the base, rest the tails in them, and check that the coil spins freely by flicking it with a finger.
Method, running and testing:
- Place the magnets directly below the coil, or one on each side of it, with unlike poles facing so that the field runs straight across the coil.
- Connect the paper clips in series with the switch, ammeter, rheostat and cell, close the switch and give the coil a gentle starting flick.
- Once it is spinning steadily, time $20$ rotations with the stopwatch and divide to find the time for one rotation, repeating three times for a mean.
- Change one variable at a time and repeat the timing: raise the current with the rheostat, then swap in stronger magnets, then rebuild the coil with twenty turns instead of ten.
- Finally turn the magnets over so the field is reversed, and note that the coil now spins the other way.
Expected pattern: the coil spins steadily once started, it turns faster with a larger current, with stronger magnets and with more turns, and reversing either the current or the magnets reverses the direction of rotation.
Processing: convert each mean time for one rotation into a rate in rotations per second using $\dfrac{1}{t}$ , and then into rotations per minute by multiplying by $60$ so the results can be compared with a manufacturer's figure.
Watch out: enamel left on the fully stripped tail so no current flows, both tails stripped all the way round so the coil rocks instead of turning, tails that are not in line, magnets too far away, and timing a single rotation instead of twenty, which makes reaction time a large share of the reading.
Safety: the coil and the contacts get hot, so switch off between runs and never leave the circuit closed with the coil stationary. Keep fingers and hair clear of the spinning coil, take care with the sanded wire ends, and handle neodymium magnets carefully because they can pinch skin.

Exam technique

Explaining how a motor works

This is usually an extended answer, so write it as a chain of short linked sentences: current flows in the coil, the coil is in a magnetic field, the two sides carry current in opposite directions, the forces on them act in opposite directions, and those forces produce a turning effect.
The step most often missed is the one about opposite current directions. Without it the opposite forces have no cause and the explanation loses marks.
Finish with the commutator sentence: it reverses the current every half turn so the coil keeps rotating in the same direction.
Use the words current, magnetic field, force, opposite directions, turning effect and rotation, since these are the terms the mark scheme is written around.
For a rate conversion, show the division by $60$ as a separate line of working so that the method mark stands even if the final figure is wrong.

Common Mistake

Do not confuse a motor with a generator. A motor takes a current and produces movement, while a generator takes movement and produces a potential difference.
Do not write that the forces on the two sides cancel out. They act on opposite sides of the axle, so they twist the coil instead of cancelling.
Do not say that electricity turns into force. Energy is transferred electrically to the kinetic energy store of the coil.
Do not say the commutator reverses the magnetic field. It reverses the current in the coil, while the magnets stay exactly as they are.
Do not read $rotations/s60\ \text{rotations/s}$ as sixty seconds per rotation. It is sixty rotations in one second.

Self review

Explain why the forces on the two long sides of a motor coil act in opposite directions.
Explain how those two forces make the coil rotate rather than move in a straight line.
Describe what a split-ring commutator does and explain why a motor needs one.
Give three changes that would make a motor turn faster, with a reason for each.
Convert a rotation rate of $rotations/min2400\ \text{rotations/min}$ into rotations per second and find the time for one rotation.
State what happens to the direction of rotation if both the current and the magnetic field are reversed.

How a motor turns

Definition

Electric motor

An electric motor is a device that uses the forces on current-carrying conductors in a magnetic field to make a coil rotate.

Definition

Turning effect

A turning effect is the rotation produced when forces act on an object in such a way that they twist it about an axis rather than push it in a straight line.

A simple motor is built from a rectangular coil of wire mounted on an axle so that it can spin, with the coil sitting in the field between the poles of a permanent magnet.
Current is fed in at one side of the coil and leaves at the other, so the current travels all the way round the loop.
Because the current goes round a loop, it runs along one long side of the coil in one direction and back along the other long side in the opposite direction. This single fact is what makes a motor work.
Both long sides lie across the same magnetic field, so both experience a force. The field is the same for both, but the current directions are opposite, so the two forces point in opposite directions.
One long side is pushed upwards while the other is pushed downwards, and each force acts at a distance from the axle.
A pair of equal forces acting in opposite directions on opposite sides of an axle does not push the coil anywhere. It twists the coil instead, producing a turning effect, and the coil rotates.
The short sides of the coil run along the field rather than across it, so they contribute no useful force and can be ignored in the explanation.
Reversing the current alone reverses both forces and so reverses the direction of rotation. Reversing the magnetic field alone does the same. Reversing both together leaves the motor spinning the way it did before.
Energy is transferred electrically from the supply to the kinetic energy store of the rotating coil, because the current-carrying sides are pushed by the magnetic field.

Key Idea

A motor rotates because the currents in the two sides of the coil are in opposite directions, so the forces on those sides are in opposite directions and produce a turning effect on the coil.

Keeping the coil turning

Definition

Split-ring commutator

A split-ring commutator is a rotating contact divided into two halves that reverses the current in the coil of a direct current motor every half turn.

A coil connected straight to a battery would not keep spinning. It would turn half a revolution and then stall.
The reason is that after half a turn the two sides have swapped places. The side that was on the left is now on the right, but the force on it still points the same way as before.
The turning effect would then act the wrong way round, pushing the coil back towards where it came from, so it would rock to and fro and settle.
A split-ring commutator solves this. It is a metal ring cut into two halves, mounted on the axle and turning with the coil, with each half connected to one end of the coil.
Two fixed carbon brushes press against the ring and carry the current in from the supply while the ring spins past them.
Every half turn, the gaps in the ring pass the brushes and each half of the ring meets the opposite brush, so the current through the coil is reversed.
Reversing the current reverses both forces at exactly the moment the sides have swapped places, so the turning effect keeps acting the same way round and the coil carries on rotating.
The switching is automatic and mechanical. Nothing in the circuit has to be operated by hand, because the commutator turns with the coil and reverses the current at the right instant on its own.
The point worth writing in an answer is not the construction of the ring. It is that the current is reversed every half turn so that the coil keeps turning in the same direction.

Making a motor turn faster

Increasing the current in the coil increases the force on each side, so the turning effect and the rotation rate both rise.
Using stronger magnets raises the magnetic flux density in the gap, which again increases the force on each side of the coil.
Adding more turns to the coil puts more current-carrying wire in the field, so the forces add and the turning effect grows.
Making the coil wider moves each force further from the axle, which increases the turning effect without changing the size of either force.
Reducing friction at the bearings and keeping the coil light both help, because less of the turning effect is wasted overcoming resistive forces.
These are the ideas behind real motors. A cordless drill draws a large current through many turns wound on an iron armature between strong magnets, while a model motor uses a small current and a light coil.

Rotation rates and unit conversions

The speed of a motor is quoted as a rotation rate, the number of complete turns made in a given time.
Manufacturers usually quote rotations per minute, while physics calculations need rotations per second, so a conversion is often the first step. $s1\ \text{min} = 60\ \text{s}$
To go from rotations per minute to rotations per second, divide by $60$ . To go the other way, multiply by $60$ .
The time for one rotation is the reciprocal of the rate in rotations per second: $\dfrac{1}{n}$
where $t$ is the time for one rotation in seconds and $n$ is the rotation rate in rotations per second.
Rotation rate and the time for one rotation are inverses of each other, so a faster motor has a shorter time per rotation.
Read the unit carefully. A rate of $rotations/s60\ \text{rotations/s}$ means sixty turns every second, not sixty seconds for one turn.

Example

Converting a rotation rate

A small electric motor is quoted as running at $rotations/min3600\ \text{rotations/min}$ . Its rate in rotations per second and the time for one rotation are both required.
There are $s60\ \text{s}$ in a minute, so divide the quoted rate by $60$ .
$\dfrac{3600}{60} = 60\ \text{rotations/s}$
The motor therefore makes $60$ complete turns in every second.
$\dfrac{1}{n} = \dfrac{1}{60} = 0.0167\ \text{s}$
To two significant figures the time for one rotation is $s0.017\ \text{s}$ .

Example

Using proportional reasoning on a motor

A motor turns at $rotations/min1200\ \text{rotations/min}$ when the current is $A0.50\ \text{A}$ . The current is raised to $A1.5\ \text{A}$ and the rotation rate is found to rise in the same ratio.
Find the ratio of the two currents first.
$1.50.50=3.0\dfrac{1.5}{0.50} = 3.0$
The current is three times larger, so the rotation rate is also three times larger.
$1200 \times 3.0 = 3600\ \text{rotations/min}$
Converting gives $rotations/s\dfrac{3600}{60} = 60\ \text{rotations/s}$ , so the time for one rotation falls from $s0.050\ \text{s}$ to $s0.017\ \text{s}$ .

Practical

Constructing a simple electric motor

Aim: to build a working coil motor, to show that the forces on the two sides of the coil produce rotation, and to test how the current, the magnets and the number of turns affect how fast it spins.
Apparatus: about $m1\ \text{m}$ of enamelled copper wire, a cylindrical former such as a battery to wind the coil on, two paper clips bent into cradles, a wooden block or cork base, two strong slab or neodymium magnets, a $V1.5\ \text{V}$ cell in a holder or a low-voltage supply, a rheostat, an ammeter, a switch, fine sandpaper, sticky tape and a stopwatch.
Variables: the current, the strength of the magnets or the number of turns is the independent variable, the rotation rate is the dependent variable, and the coil, the base, the magnet positions and everything not being changed are kept the same.
Method, building the coil:
- Wind about ten neat turns of enamelled wire round the former, then slide the coil off and leave a straight tail of a few centimetres projecting from each side along the same line.
- Loop each tail once through the coil to bind the turns together, so the coil holds its shape and stays balanced when it spins.
- Check that the two tails lie exactly opposite each other on a straight line through the middle of the coil, because a lopsided axle makes the motor wobble and stall.
Method, making the contacts:
- Sand the enamel completely off one tail all the way round, so that this end always makes contact.
- Sand the enamel off only the upper half of the other tail, laying the wire flat and rubbing one side only.
- That half-stripped tail is a simple commutator. It carries current for half of every rotation and breaks the circuit for the other half, so the coil is only pushed while the push is in the useful direction.
- Tape the two paper-clip cradles to the base, rest the tails in them, and check that the coil spins freely by flicking it with a finger.
Method, running and testing:
- Place the magnets directly below the coil, or one on each side of it, with unlike poles facing so that the field runs straight across the coil.
- Connect the paper clips in series with the switch, ammeter, rheostat and cell, close the switch and give the coil a gentle starting flick.
- Once it is spinning steadily, time $20$ rotations with the stopwatch and divide to find the time for one rotation, repeating three times for a mean.
- Change one variable at a time and repeat the timing: raise the current with the rheostat, then swap in stronger magnets, then rebuild the coil with twenty turns instead of ten.
- Finally turn the magnets over so the field is reversed, and note that the coil now spins the other way.
Expected pattern: the coil spins steadily once started, it turns faster with a larger current, with stronger magnets and with more turns, and reversing either the current or the magnets reverses the direction of rotation.
Processing: convert each mean time for one rotation into a rate in rotations per second using $\dfrac{1}{t}$ , and then into rotations per minute by multiplying by $60$ so the results can be compared with a manufacturer's figure.
Watch out: enamel left on the fully stripped tail so no current flows, both tails stripped all the way round so the coil rocks instead of turning, tails that are not in line, magnets too far away, and timing a single rotation instead of twenty, which makes reaction time a large share of the reading.
Safety: the coil and the contacts get hot, so switch off between runs and never leave the circuit closed with the coil stationary. Keep fingers and hair clear of the spinning coil, take care with the sanded wire ends, and handle neodymium magnets carefully because they can pinch skin.

Exam technique

Explaining how a motor works

This is usually an extended answer, so write it as a chain of short linked sentences: current flows in the coil, the coil is in a magnetic field, the two sides carry current in opposite directions, the forces on them act in opposite directions, and those forces produce a turning effect.
The step most often missed is the one about opposite current directions. Without it the opposite forces have no cause and the explanation loses marks.
Finish with the commutator sentence: it reverses the current every half turn so the coil keeps rotating in the same direction.
Use the words current, magnetic field, force, opposite directions, turning effect and rotation, since these are the terms the mark scheme is written around.
For a rate conversion, show the division by $60$ as a separate line of working so that the method mark stands even if the final figure is wrong.

Common Mistake

Do not confuse a motor with a generator. A motor takes a current and produces movement, while a generator takes movement and produces a potential difference.
Do not write that the forces on the two sides cancel out. They act on opposite sides of the axle, so they twist the coil instead of cancelling.
Do not say that electricity turns into force. Energy is transferred electrically to the kinetic energy store of the coil.
Do not say the commutator reverses the magnetic field. It reverses the current in the coil, while the magnets stay exactly as they are.
Do not read $rotations/s60\ \text{rotations/s}$ as sixty seconds per rotation. It is sixty rotations in one second.

Self review

Explain why the forces on the two long sides of a motor coil act in opposite directions.
Explain how those two forces make the coil rotate rather than move in a straight line.
Describe what a split-ring commutator does and explain why a motor needs one.
Give three changes that would make a motor turn faster, with a reason for each.
Convert a rotation rate of $rotations/min2400\ \text{rotations/min}$ into rotations per second and find the time for one rotation.
State what happens to the direction of rotation if both the current and the magnetic field are reversed.

12.3.4 Electric motors

How a motor turns

Keeping the coil turning

Making a motor turn faster

Rotation rates and unit conversions

Converting a rotation rate

Using proportional reasoning on a motor

Constructing a simple electric motor

Explaining how a motor works

Review 12.3.4 Electric motors by teaching Genie

12.3.4 Electric motors

How a motor turns

Keeping the coil turning

Making a motor turn faster

Rotation rates and unit conversions

Converting a rotation rate

Using proportional reasoning on a motor

Constructing a simple electric motor

Explaining how a motor works

Review 12.3.4 Electric motors by teaching Genie

12.3.4 Electric motors

How a motor turns

Keeping the coil turning

Making a motor turn faster

Rotation rates and unit conversions

Converting a rotation rate

Using proportional reasoning on a motor

Constructing a simple electric motor

Explaining how a motor works

Review 12.3.4 Electric motors by teaching Genie

12.3.4 Electric motors Revision Guide

12.3.4 Electric motors

How a motor turns

Keeping the coil turning

Making a motor turn faster

Rotation rates and unit conversions

Converting a rotation rate

Using proportional reasoning on a motor

Constructing a simple electric motor

Explaining how a motor works

Review 12.3.4 Electric motors by teaching Genie

12.3.4 Electric motors Revision Guide