Question

An electric motor is plugged into a standard wall socket and is running at normal speed. Suddenly, some dirt prevents the shaft of the motor from turning quite so rapidly. What happens to the back emf of the motor, and what happens to the current that the motor draws from the wall socket?

a.) The back emf decreases, and the current drawn from the socket decreases.
b.) The back emf increases, and the current drawn from the socket decreases.
c.) The back emf decreases, and the current drawn from the socket increases.
d.) The back emf increases, and the current drawn from the socket increases.

Answer 1

The back emf of the motor decreases due to reduced speed caused by dirt in the shaft, which, in turn, causes the current drawn from the wall socket to increase.

Step-by-step explanation:

When dirt prevents the shaft of an electric motor from turning rapidly, the back emf produced by the motor decreases. Since the back emf is proportional to the motor's angular velocity, any decrease in speed results in a decrease in back emf. Because the back emf opposes the applied voltage from the wall socket, a decrease in back emf implies that the voltage across the motor's coil will increase, causing the motor to draw a larger current from the socket. Therefore, the current that the motor draws from the wall socket will increase. The correct answer to the question is (c) The back emf decreases, and the current drawn from the socket increases.

Answer 2

Option C is correct answer.

Step-by-step explanation:

The EMF(
`E`) produced by a coil is given by

`E= N * A * B * \omega`

Here,
`N` is number of turns ,
`A` is area
`B` is Magnetic field and
`\omega` is rate at which coil turns.

If dirt prevents the shaft from turning that means
`\omega` is decreasing (As rate is decreasing). That simply means Induced EMF decreases.

But according to formula of Self Inductance

`E=-L(dI)/(dt)`

( Here
`L` is self inductance and
`(dI)/(dt)` is rate of change of current).

We can say that the EMF is depends as negative of rate of current growth.

So, from this formula we can say that the EMF is decreases and the current drawn from the socket increases.

The correct option is C.