Topic 7: Electromagnetism

Question 1

Describe the direction of magnetic fields in a magnet

Answer 1

Out from the north, in through the south

Question 2

Where is the magnetic field in a magnet most strongest?

Answer 2

The poles

Question 3

What do magnets do?

Answer 3

Attraction and repulsion

Question 4

How do you change the size of the induced potential difference?

Answer 4

• change the strength of the magnetic field - larger potential difference
• move the wire or magnets more quickly - bigger potential difference
• shape the wire into a coil - bigger potential difference

Question 5

Describe the generator effect (electromagnetic induction)

Answer 5

• magnetic field around a permanent magnet
• wire moves within the magnetic field
• induces p.d in the wire, which results in an alternating current
• this wire has to be part of a complete circuit

Question 6

What is an alternator?

Answer 6

A coil of wire rotating in a magnetic field

Question 7

Describe how alternators generate an alternating current

Answer 7

• the coil moves through the magnetic field
• a potential difference is induced (across the coil)
• there is a complete circuit, so a current is induced (in the coil)
• every half turn, the potential difference reverses direction
• so every half turn the current changes direction

Question 8

Describe how a sound wave is produced in loud speakers and microphones

Answer 8

• the current creates a magnetic field (around the coil)
• (which) interacts with the permanent magnetic field
• producing a (resultant) force causing the coil/ cone to move
• (when the) direction of the current (AC) reverses, the direction of the (resultant) force reverses, producing a sound wave

Question 9

Describe how in both transformers of the National Grid, how an alternating current is caused

Answer 9

• an alternating current in primary coil
• induces an alternating magnetic field in the core
• induces an alternating p.d. in secondary coil
• causing an alternating current, if it is a complete circuit

Question 10

Describe how sound waves are produced in a loud speaker

Answer 10

• the alternating current creates a magnetic field around the cone
• which interacts with the magnetic field in permanent magnet
• this generates force on the cone - causing it to vibrate
• when the direction of the alternating current reverses, the direction of the resultant force reverses producing a sound wave

Question 11

Describe how dynamos generate direct current

Answer 11

Dynamos work in the same way as alternators, apart from one important difference
They have a split-ring commutator instead of slip rings
This swaps the connection every half turn to keep the current flowing in the same direction

Question 12

Describe how a solenoid can increase the strength of the magnetic field that the wire produces

Answer 12

• you can increase the strength of the magnetic field that the wire produces by wrapping the wire into a coil called a solenoid
• this happens because the field lines around each loop of wire line up with each other
• this results in lots of field lines pointing in the same direction that are very close to each other
• the magnetic field inside a solenoid is strong and uniform

Question 13

Explain why when there’s a current in the coil, the coil rotates continuously

Answer 13

• the sides of the coil experience a force in opposite directions
• the forces cause moments that act in the same direction
• the carbon brush rotates as the force acts on it
• the split-ring commutator swaps the contacts every half turn to keep the motor rotating in the same direction
• the commutator reverses the current