Evan's Space

Wonders of Physics

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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.

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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.

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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.