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GCSE Standard Questions: The Motor Effect

Q4. The circle (with the cross in it) below represents a straight wire carrying a current. The cross shows that the current is into the plane of the paper.

(a) Complete the diagram to show the magnetic field pattern around the wire.

Marks were awarded for:

drawing at least three circles
clockwise arrows on the circles

See the right hand grip rule

[2 marks]

(b) The magnetic flux density 10 cm from the wire is 4 microtesla.

Which of the following is the same as 4 microtesla?

Tick one selection.

Selection
 Tick here
4 x 10-2 T
  
4 x 10-3 T
  
4 x 10-6 T
4 x 10-9 T
  

 

[1 mark]

(c) Here is a diagram of a simple electric motor.

When there is a current in the coil, the coil rotates continuously. Explain why.

The current creates a magnetic field around the wires of the coil. The field from the coil interacts with the field of the permanent magnet. That makes each of the sides of the coil parallel to the magnet experience a force. This force pair acts in opposite directions making the coil rotate.

Each half-revolution of the coil the two halves of the rotating commutator swap from one carbon brush to the other, making the direction of current flow through the coil reverse in direction.This then causes the opposite side of the coil experience the turning force its counterpart had, and the coil continues to rotate.

[4 marks]

(Total 7 marks)