# Evan's Space

## Induced emf not affected by resistance of wire

The electromotive force (e.m.f.) induced in a conductor moving at right-angles to a magnetic field does not depend upon

A) the length of the conductor.
B) the resistance of the conductor.
C) the speed of the conductor.
D) the strength of the magnetic field.

Solutions: B

The longer the wire, the rate in which the magnetic lines of force cutting the wire is greater. So A is true.

As it is a wire and not a closed circuit, no induced current is formed. Hence induced emf is produced and the resistance of the wire will not affect the in emf induced.

## Electromagnetic Induction Question

Solutions: Option D – (i) and (iii)

(i) is true because as the rod cuts the magnetic field of the magnet, an induced emf is produced, according to Faraday’s Law. But NOT induced current as it is not a closed circuit. The end of the rod nearer is higher potential.

(ii) is false because as mentioned above, an induced emf will be produced when the rod cuts the magnetic field.

(iii) is true as the magnitude (value or size) of the induced emf depends on factors such as using stronger magnet and moving the rod faster. So this statement is true.

But do note that if the statement is phrased such that ‘…..induced emf ONLY depends on the magnetic field…….’, then this statement is false as moving the rod faster is another way, so using stronger magnet is NOT the only way.

## Ring floats due to Lenz’s Law

Solutions: Option C

Alternating current (AC) changes direction many times per second. Hence it is able to create a changing magnetic fluz which cuts the ring and hence opposes / repels the ring due to Lenz’s Law.

When direct current isused, the current initial will increase from min to max, hence creates a one-off changing expanding magnetic flux which still cuts the ring and causes it to float.

But once current stablises, there is no changing magnetic flux, hence ring will descend. As it descends, the fixed magnetic field of the solenoid cuts the ring again. But the magnitude of the induced current is smaller, hence opposing foce of the like poles will be smaller than the weight of ring. Hence ring descends and eventually rests on top of the coil.

## Electromagnetic Induction

Consider these two scenarios.

Scenario 1 A wire moves vertically between the magnets. An induced current is produced when the wire cuts the magnetic lines of force. The direction of the induced current is out of paper and magnets are stationary. Is the wire moving up, A, or downwards, B?

Solutions: Since force is applied to the wire and an induced current is produced, Fleming’s Right Hand Rule (FRHR) is applied here. Using FRHR, you will be able to determine that the direction of the motion of wire (force) is downwards (towards B).

Scenario 2 If now the wire is stationary, but the magnets move vertically instead.  An induced current that flows out of paper is produced as the magnets move. Which direction does the magnets move, upwards (towards A) or downwards (towards B)?

Solutions: From Scenario 1, FRHR is applied to know that wire moves down in order to produce an induced current out of paper. Here, wire is stationary, and magnets move instead. One has to know that the effect of wire moving down (scenario 1) is the same as the magnets moving up (scenario 2). In both cases, the way the wire cuts the magnetic lines of force is the same. Hence, in this scenario, the magnets are moving up (towards A) in order to achieve induced current out of paper.