Electromagnetic Induction in a disc

[Yashbhatt]

Homework Statement

Please see the attached file.

Homework Equations

$$\frac{dΦ}{dt} = ε$$

The Attempt at a Solution

The only way I see is to apply some conservation of energy. But I don't know how. The potential energy is being converted to rotational energy. But how do I find the potential energy?

 

[mfb]

Switching off the external field releases energy you cannot control here.

A time-dependent magnetic field leads to an electric field, which then leads to a torque. Angular momentum allows to calculate everything else then.

 

[Yashbhatt]

Switching off the external field releases energy you cannot control here.

A time-dependent magnetic field leads to an electric field, which then leads to a torque. Angular momentum allows to calculate everything else then.

The only problem I face here is I don't know in what time is the magnetic field switched off. It says it is switched off instantaneously.

 

[mfb]

It doesn't matter, a faster switching gives a larger torque for a shorter timescale. For calculations, you can assume that it gets switched off linearly during time T. This parameter (and also the assumption of linearity) will drop out of the calculations later.