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In an electric motor, current flows through a coil in a magnetic field, and the motor effect pushes the two sides in opposite directions. Free notes.
Understanding Electric Motors is part of The motor effect in GCSE AQA Higher Physics.
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In an electric motor, current flows through a coil in a magnetic field, and the motor effect pushes the two sides in opposite directions. Free notes.
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Use this lesson to stay inside The motor effect while connecting the detail back to the wider Magnetism and electromagnetism topic.
Read the core explanations from Understanding Electric Motors before testing yourself from memory.
Picture a wire carrying electric current sitting between the poles of a magnet. Something remarkable happens - the wire jumps sideways. This is a fascinating physical phenomenon driven by the motor effect, and it powers everything from your phone's vibration to electric car engines.
The motor effect occurs because moving electric charges (current) create their own magnetic field. When this field interacts with an external magnetic field, forces develop that push the conductor in a specific direction. The stronger the current or magnetic field, the greater the force.
This force is always perpendicular to both the current direction and the magnetic field direction. That's why the wire moves sideways rather than along the field lines or along the current path.
Many students think the force pushes the conductor along the magnetic field lines - it doesn't. The force acts at right angles to both the field and current, which is why Fleming's left-hand rule is so useful for working out directions.
Predicting which way the force acts seems impossible until you learn Fleming's left-hand rule. This simple hand gesture tells you the direction of force every time, without fail.
Hold your left hand with thumb, first finger, and middle finger all at right angles to each other. Point your first finger in the direction of the magnetic field (north to south). Point your middle finger in the direction of conventional current (positive to negative). Your thumb now points in the direction of motion - that's where the force pushes the conductor.
The memory trick is: First finger = Field, seCond finger = Current, thuMb = Motion. Notice how the letters F, C, and M appear in the right positions when you say 'seCond' instead of 'middle'.
Students often confuse this with Fleming's right-hand rule (used for generators) or use the wrong hand entirely. Always use your LEFT hand for motors - the motor effect pushes things to the LEFT of where you'd expect.
A single wire experiencing the motor effect moves once and stops. To create continuous rotation, we need something more clever - a rectangular coil of wire that can spin around an axis.
When current flows through a rectangular coil in a magnetic field, the two vertical sides experience forces in opposite directions. One side gets pushed up while the other gets pushed down, creating a turning effect or torque that makes the coil rotate.
The horizontal sides of the coil don't contribute to rotation because their forces act along the axis of rotation. Only the vertical sides create the turning moment that spins the coil around.
As the coil rotates, it reaches a position where it's perpendicular to the magnetic field. At this point, the forces on both sides act directly towards the centre, creating no turning effect. The coil would stop here without some clever engineering.
Here's the problem: after half a turn, the coil sides have swapped positions. The side that was being pushed up is now being pushed down, and vice versa. Without intervention, the coil would just rock back and forth rather than spinning continuously.
The solution is a commutator - a split ring that reverses the current direction in the coil every half turn. Just as the coil reaches the position where it would start turning backwards, the commutator flips the current direction, ensuring the forces continue pushing the coil in the same rotational direction.
The commutator consists of two metal semicircles separated by insulating gaps. Carbon brushes maintain electrical contact as the commutator rotates, but the split design means each brush connects to a different semicircle every half turn, effectively reversing the current.
Without a commutator, the motor would be useless - it would just vibrate back and forth. The commutator is what transforms oscillation into rotation, making practical electric motors possible.
A complete electric motor combines all these elements: a coil that can rotate in a magnetic field, a commutator to reverse current direction, and brushes to maintain electrical contact. When you connect this assembly to a battery, current flows through the coil, creating forces that spin it continuously.
The speed of rotation depends on several factors. Increasing the current increases the force on each side of the coil, making it spin faster. Strengthening the magnetic field has the same effect. Adding more turns to the coil increases the total force without changing the current.
Real motors often use electromagnets instead of permanent magnets to create stronger, controllable magnetic fields. They also use multiple coils arranged around the rotor to provide smoother rotation and higher torque.
Worked example: Motor Operation Sequence
Describe what happens when a simple motor is connected to a 6V battery.
Answer: The motor spins continuously, converting the 6V electrical supply into rotational kinetic energy.
Motor questions often test your understanding of the relationship between current, magnetic field, and force direction. Examiners love to give you a diagram and ask you to predict which way the coil will rotate.
The key is to apply Fleming's left-hand rule systematically to each side of the coil separately. Don't try to guess the overall rotation - work out the force on each vertical side, then determine which way these forces will turn the coil.
When explaining motor operation, examiners want to see specific physics terms. Use 'motor effect', 'Fleming's left-hand rule', 'commutator', and 'turning moment' in your answers. Avoid vague phrases like 'the coil spins because of magnetism'.
For calculations involving motor speed or efficiency, remember that electrical power input equals current times voltage (P = IV), while mechanical power output depends on torque and rotational speed. The difference represents energy lost as heat in the motor's resistance.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
In an electric motor, current flows through a coil in a magnetic field, and the motor effect pushes the two sides of the coil in opposite directions, creating a turning effect. A commutator reverses the current every half turn so the coil keeps rotating in the same direction.
An electric motor spins faster with a bigger current, a stronger magnetic field, or more turns on the coil. Each of these increases the force on the sides of the coil.
A commutator is a split ring that reverses the current direction in the coil every half turn. This keeps the forces pushing the coil in the same rotational direction; without it, the coil would just rock back and forth instead of spinning.
When current flows through a rectangular coil in a magnetic field, the two sides carry current in opposite directions, so the motor effect pushes one side up and the other down. These opposite forces create a turning effect that makes the coil rotate.
Carbon brushes keep electrical contact with the commutator as it rotates. Because the commutator is split into two metal semicircles, each brush connects to a different semicircle every half turn, which reverses the current in the coil.
The horizontal sides of a motor coil do not contribute to rotation because their forces act along the axis of rotation. Only the vertical sides create the turning moment that spins the coil.
When the coil is perpendicular to the magnetic field, the forces on both sides act directly towards the centre and create no turning effect. After half a turn the forces would reverse, so without a commutator the coil would rock back and forth.
A single current-carrying wire in a magnetic field jumps sideways, then stops; it moves once rather than spinning. A rectangular coil that can rotate around an axis is needed for continuous rotation.
Real motors often use electromagnets instead of permanent magnets to create stronger, controllable magnetic fields. They also use multiple coils around the rotor for smoother rotation and higher torque.
The motor effect force is always perpendicular to both the current direction and the magnetic field direction, which is why the wire moves sideways rather than along the field lines or along the current.
The rotation of a motor coil is found by applying Fleming's left-hand rule to each vertical side of the coil separately, then using the two forces to see which way they turn the coil.
When a simple motor is connected to a 6 V battery, current flows through the coil via the brushes and commutator, the motor effect creates opposite forces on the coil sides, and the coil starts rotating. The commutator reverses the current every half turn, so it spins continuously.
An electric motor's electrical power input equals current × voltage (P = IV), and the difference between this and the mechanical power output is energy lost as heat in the motor's resistance.