explaining induction

Question 1

What is the simple structure of electromagnetic induction?

Answer 1

Induction –> external B field –> change in flux –> induced emf –> current/eddy currents –> induced magnetic field –> interaction between two fields that ultimately causes induction

Question 2

What is the first mark in explaining how a magnet falls down a copper tube?

1 mark - _____-

Answer 2

When the magnet leaves the tube, there is an increasing upward magnetic flux through the tube. This induces an emf proportional to the rate of change of mag. flux by Faraday’s law.

1 mark for referencing and explaining Faraday

Question 3

What does it do after there is emf?

eddy currents

Answer 3

This induces horizontal, circ. eddy currents, whose direction is such that the magnetic field due to the current opp. the change in magnetic flux that induced the current by Lenz’s law.

Question 4

How will the current flow?

Answer 4

The current will flow clockwise as seen from above, consistent with an induced magnetic field downward to oppose the flux change, and inducing a S pole at the end of the tube closest to the magnet, repelling the magnet and opposing the flux change as per Lenz’s law.

Question 5

What happens when the magnet is at the centre?

Answer 5

When the centre of the magnet corresponds to the centre, there is not change in magnetic flux bc, the amount of upward flux added to the top half of the tube is equal to amount of upward flux removed from the bottom half of the tube.

Question 6

What happens past the centre of the tube?

Answer 6

• Decreasing upward flux
• induced horizontal eddy currents act anticlockwise as seen from above, to induce an upward B field to oppose the flux change, and induce a south pole at the end of the tube closest to the magnet to attract the magnet and oppose the motion.

Question 7

What would happen if a magnet went across a slotted cylinder?

Answer 7

Incomplete circuit, eddy currents cannot flow, hence the magnet is under the influence of gravity.

Question 8

Overall structure of a Lenz’s law question

Answer 8

1. Change in B external field - as we drop magnet
2. Change in flux
3. Induces an Emf as per the definition
4. Induces a current - eddy current or cur.
5. B field induced
6. effect - force of two fields interacting - moving the conductor