Magnetism Exam Qs

a) generator effect

b) wire cuts through the magnetic field, a p.d. was induced across the wire, the circuit was complete so current was induced

the pressure variations in the sound waves cause the diaphragm to vibrate

the diaphragm causes the coil to vibrate

the coil repeatedly changes direction, inducing an alternating current

• the coil moves/cuts through the magnetic field

• a p.d is induced across the coil

• there is a complete circuit so a current is induced

• each half revolution, the split ring commutator switches brushes/contacts

• so the direction of the induced current does not reverse every half rotation

• there is no induced current

• so no magnetic field

• to oppose the movement of the coil

• the alternating current causes a changing magnetic field around the primary coil

• the magnetic field changes direction in the core

• this induces an alternating potential difference across the secondary coil, causing an alternating current

What assumption is made when using Fleming’s Left Hand Rule?

the wire/force is at right angles to the magnetic field

Difference between generator and motor effect

Motor effect: uses electricity to cause motion

Generator effect: uses motion to cause electricity

In a step down transformer, is the number of turns on the secondary coil greater, the same as or smaller than the number of turns on the primary coil?

less than

• there is a magnetic field around the magnet

• this magnetic field moves

• and cuts through the coil

• so a p.d is induced across the coil

• the coil forms a complete circuit

• so a current is induced

A student measured the strength of an electromagnet by counting the number of paper clips the electromagnet could hold.

Explain why it was important that all the clips were the same size (2)

• so the paper clips have the same weight/mass

• which allows the results for different numbers of turns to be compared fairly/ allows a fair test

Explain what is meant by the following statement: ‘A non-contact force acts on each magnet’ (when two magnets are brought close together) (2)

the magnets are not touching

but each experiences a force

Describe how to plot the magnetic field pattern of a bar magnet (3)

• place a plotting compass near the pole of the magnet and mark the direction that the compass points

• move the compass around the bar magnet marking at intervals the direction the compass points

• join the points up and add an arrow pointing from the north pole to the south pole

• closing switch S causes a current in the coil

• a magnetic field is created

• a force of attraction acts on the ball-bearing

• so the ball bearing accelerates towards the iron rod

• a varying/alternating current is passed through the coil of the loudspeaker

  • This creates a changing magnetic field around the coil

• As the current is constantly changing direction, the direction of the magnetic field will be constantly changing

• The magnetic field around the coil interacts with the field from the permanent magnet

• The interacting magnetic fields will exert a force on the coil

• As the magnetic field is constantly changing direction, the force exerted on the coil will constantly change direction

  • this makes the coil vibrate

• the vibrating coil causes the speaker cone to vibrate

  • this makes the air vibrate, creating sound waves

1. relay/solenoid

2. a current flows through the coil, the arm is attracted to the electromagnet, the contacts are pushed together