Give two ways you would induce P.D
- Moving a magnet in a coil of wire.
- Moving an electrical conductor in a magnetic field.
How do you make an A.C using induction?
1) Turn a magnet end to end in a coil - the magnetic field will change - this induces P.D - current will flow in the wire.
2) When you’ve turned the magnet through 1/2 turn, the direction of the magnetic field through the coil reverses and so current will flow in the opposite direction around the wire.
3) Keep turning the magnet in the same direction = AC current
What is the relationship between magnets and potential difference?
If you move a magnet in the opposite direction/reverse polarity = potential difference/current = reversed.
What is the function of a dynamo?
Used on top of bikes to turn on lights
How does a dynamo work to power its lights?
- Cog wheel at the top positioned to touch one of the wheels.
- As wheel turns, turns cog wheel which is attached to a magnet.
- Creates an AC to power the lights.
Define the motor effect
The force experienced by a current carrying wire in a magnetic field.
How is the motor effect force made bigger?
- Stronger magnetic field
- More current
When is full force experienced in a motor?
When the wire is 90 degrees to the magnetic field.
When is no force experienced in a motor?
When the wire is parallel to the magnetic field.
When is some force experienced in a motor?
When the wire is at an angle other than 90 degrees to the magnetic field.
What is the relationship between the direction of force and the magnetic field?
The direction of force is reversed if the magnetic field is reversed.
What does a split-ring commutator do?
Swaps the contracts every 1/2 turn to keep the motor rotating in the same direction.
How is the motor direction reversed?
- Swapping the polarity of the DC
- Swapping the magnetic poles over
Where are motors used?
- Fans
- CD players
- Food mixers
- Hair dryers
What is induction?
The creation of a potential difference across the ends of an electrical conductor which is experiencing a change in magnetic field.
What are transformers made up of?
- Magnetically “soft” iron core
- Primary coil covered in plastic
- Secondary coil covered in plastic
What does a step up transformer do and how do its features show this?
It steps the voltage up and has more secondary coil then primary coil.
What does a step down transformer do and how do its features show this?
It steps the voltage down and has more primary coils than secondary.
How does a transformer work? (Think induction)
- AC in primary coil means magnetic field in iron core constantly changing direction.
- Rapidly changing magnetic field felt by secondary coil.
- Changing field induced an alternating P.D across secondary coil.
- Relative number of turns on 2 coils determines whether greater or less.
Why do transformers only work in A.C?
- There would still be a magnetic field in the iron core but it wouldn’t be constantly changing.
- So no induction in the secondary coil as you need a changing field to induce P.D
What frequencies do switch mode transformers work at?
50KHz - 200KHz
What are the advantages of switch mode transformers over traditional transformers? (2)
- They work at higher frequencies so they are lighter and smaller (more useful for mobile chargers etc)
- More efficient as they use little power when switched on but no load is applied.
What is the iron core for in a transformer?
It transfers the changing magnetic field from the primary coil to the secondary coil.
It carries NO current.
What is a magnetic field?
A region where magnetic materials and wires carrying currents experience a force.
