Topic 7: Electromagnetism Flashcards
Describe the direction of magnetic fields in a magnet
Out from the north, in through the south
Where is the magnetic field in a magnet most strongest?
The poles
What do magnets do?
Attraction and repulsion
How do you change the size of the induced potential difference?
- 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
Describe the generator effect (electromagnetic induction)
- 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
What is an alternator?
A coil of wire rotating in a magnetic field
Describe how alternators generate an alternating current
- 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
Describe how a sound wave is produced in loud speakers and microphones
- 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
Describe how in both transformers of the National Grid, how an alternating current is caused
- 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
Describe how sound waves are produced in a loud speaker
- 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
Describe how dynamos generate direct current
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
Describe how a solenoid can increase the strength of the magnetic field that the wire produces
- 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
Explain why when there’s a current in the coil, the coil rotates continuously
- 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