The motor effect and electric motorsIB MYP Physics: Revision notes
Section 1
The motor effect
When a wire carrying a current is placed in a magnetic field, it experiences a force. This is called the motor effect. It happens because the magnetic field of the current interacts with the field of the magnet.
The force is at right angles to both the field and the current. It is greatest when the wire is at right angles to the field, and zero if the wire is parallel to the field lines.
If the current or the field is reversed, the force reverses. If both are reversed, the force stays the same.
Section 2
Fleming's left-hand rule
Fleming's left-hand rule gives the direction of the force. Hold the thumb and first two fingers of your left hand at right angles to each other:
- First finger: the direction of the magnetic Field (north to south)
- seCond finger: the direction of the Current (conventional current, positive to negative)
- Thumb: the direction of the Thrust (force) and motion
Example: field pointing right, current pointing away from you. The thumb points down, so the force on the wire is downwards.
Use the left hand for the motor effect, and remember that the current direction is the conventional current from positive to negative.
Section 3
Size of the force
The force on a conductor increases if you:
- increase the current
- use a stronger magnetic field
- use a longer length of wire in the field
The force is zero if the wire is parallel to the field.
Section 4
The simple d.c. motor
A d.c. motor turns electrical energy into kinetic energy. A rectangular coil sits between the poles of a magnet. The current flows in opposite directions along the two sides of the coil, so the forces on them are opposite (one up, one down) and the coil turns.
A split-ring commutator and carbon brushes connect the coil to the supply. Every half turn the commutator reverses the current in the coil, so the forces keep turning the coil in the same direction. Without it the coil would stop after half a turn.
Section 5
Making a motor turn faster
A motor turns faster, or with more force, if you:
- increase the current (a larger voltage)
- use stronger magnets
- add more turns to the coil
Changing the polarity of the supply, or swapping the magnet poles, makes the motor turn the other way.
Say 'increase the current', not just 'more power'. Examiners look for the factor that changes the force.
Section 6
Loudspeakers
A loudspeaker uses the motor effect. A coil of wire is attached to a paper cone and placed in the field of a permanent magnet. A varying alternating current from an amplifier flows in the coil, so the force on the coil keeps changing direction. The coil and the cone vibrate in and out, pushing on the air to make sound waves.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The motor effect and electric motors
- In a demonstration in Istanbul, a copper rod rests on two horizontal metal rails between the poles of a strong horseshoe magnet. When a current from a d.c. supply is passed through the rod, the rod rolls along the rails.State two changes that would increase the force on the rod.2 marks
- A straight horizontal wire passes at right angles between the poles of a large magnet. The north pole is on the left and the south pole is on the right, so the magnetic field points to the right. The conventional current in the wire flows away from the observer.The poles of the magnet are swapped and the current is also reversed. State the direction of the force on the wire and explain your answer.2 marks
- Students in Nairobi build a model d.c. motor. A rectangular coil of wire is placed between two magnets and connected to a battery through a split-ring commutator and carbon brushes.Explain why the coil starts to turn when the battery is connected.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).