Electromagnetic induction - emf induced in temp. and perm. magnets

Question 1

Faraday’s law states that …

Answer 1

… the rate of change of flux linkage is directly proportional to the emf induced through the wire

Question 2

If there is no change in the magnetic field, magnetic field strength, or the magnet isn’t moved then what isn’t induced? Why?

Answer 2

… then there is NO EMF INDUCED.

Because the rate of change of flux linkage is 0.

Question 3

When is energy supplied to electrons? Why?

Answer 3

When an emf is induced.

Due to there being a rate of change of flux linkage

Question 4

A piece of copper, 30 cm long, is horizontal and is at 90° to a horizontal magnetic field of flux density 1.8×10^-5 T. The piece of copper is allowed to fall freely (from rest) through this field. Calculate the emf induced in it at the instant when it is 5 m below its starting point.

Answer 4

Emf = -2.67 x 10^-5 V

Question 5

How do you increase the magnetic field that a wire is producing?

Answer 5

By coiling the wire.

Question 6

Describe what happens when current starts to flow in the wire

Answer 6

• A current flows so the wire induces a magnetic field

- This magnetic field is perpendicular to the direction of current

Question 7

Explain how current flows in a coil of wire when it nears to a wire carrying a current

Answer 7

• A wire carrying current induces a magnetic field, perpendicular to the direction of current
• The coil of wire experiences the magnetic field
• The coil of wire therefore experiences a rate of change of flux linkage
• The coil of wire induces an emf
• This means the electrons in the wire are provided with energy
• The electrons flow and cause a short burst of current

Question 8

Why does current only flow for a short time in the coil of wire when it is drawn close to a magnet/current carrying wire

Answer 8

• When the coil of wire is being moved in the magnetic field created, there is a rate of change of flux linkage
• This results in an emf during the rate of change of the flux linkage
• The current flows only during this time
• When the coil is stopped the rate of change returns to 0
• Emf returns to 0 and current stops flowing

Question 9

Why can you not measure the current being carried in the wire of a domestic appliance using an amp clamp?

Answer 9

• There are two wires which bring the current into the appliance and back out of the appliance in the main wire (connected to the mains supply)
• These two wires carrying current in different directions induce magnetic fields in the opposite directions
• These cancel out and there is no overall rate of change of flux linkage
• No emf so no current flows in the coil, showing no reading

Question 10

Describe how a transformer works

Answer 10

• Alternating current is flowing in the first coil of wire
• This current carrying wire induces a magnetic field.
• Due to the current alternating, the magnetic field is also alternating.
• This alternating magnetic field induces the core and the second coil of wire in the transformer
• This alternating magnetic field results in the flux linkage through the second coil to vary continuously
• This induces a variable emf and so the current in the second coil flows

NOTE: more coil turn, means higher flux linkage (N fie) so higher P.d in the side with more coils