BMAT 2020 Q27 - Induced emf (Physics)

Heys guys,

does anyone please know why is 3 incorrect?

Question 27

A bar magnet is rotating at a constant rate of 120 revolutions per minute inside a circular coil of wire, causing an induced voltage across the coil.
Which of the following statements is/are correct?

1 Increasing the number of turns of wire on the coil would increase both the maximum
value and the frequency of the induced voltage.
2 The direction of the induced voltage in this coil reverses every 0.25 seconds.
3 There is always an induced current in a conductor which is experiencing a change in
magnetic field.

A none of them
B 1 only
C 2 only
D 3 only
E 1 and 2 only
F 1 and 3 only
G 2 and 3 only
H 1, 2 and 3

The reason why option 3 is not correct because that’s not always the case, If we plot out what’s happening for current and voltage we get an alternation graph with series of crests and troughs and clearly there are some points in time where we are crossing the X-axis in other words at those points the induced current is zero and it’s a dynamic quality.
So, there isn’t always a induced current when a conductor is sort of experiencing a change in magnetic field which makes it incorrect.
Hope this help:)


Hi Ujjwal is this because the current is alternating? Is direct current possible when induced in a magnetic field?

Hi Callearaya
I think it would not as DC current doesn’t vary over time and is static the electric and magnetic field from DC doesn’t induce current or voltage although DC produced electric and magnetic fields but it’s different from what produced by the AC.
Hope it helps!

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Yes, that makes a lot of sense! Thank you so much :))

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Hi! Yeah this was tricky since the question has a very specific set up, But I think (3) might be added to check our basic understanding whereas (1) and (2) are more related to the question itself (just my take on it) . I worked it out by starting from what I know about conductors, conductors have stationary charges that can begin to “move” or are free to move under an electric field causing a current, but magnetic fields can also induce a current since they push the electrons and ultimately have the some effect as the electric field,with the condition that the mangnetic field continues to vary( change in magnetic field due to the revolutions). Hope this makes sense. :sparkles:

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