Question

A sheet of copper is placed between the poles of an electromagnet with the magnetic field perpendicular to the sheet. When the sheet is pulled out, a considerable force is required, and the force required increases with speed. Explain. Is a force required also when the sheet is inserted between the poles? Explain.

Short answer

Yes, a force is also required when the sheet is inserted between the poles.

Step-by-step solution

Concept 

Lens Law

The direction of any magnetic induction effect is such as to oppose the cause of the effect.

Faraday Law of EMF

Faraday Law of EMF states that the induced emf in a loop equals the negative of the time rate of change of magnetic flux through the loop.

Mathematically it can be given by

$$\begin{gathered} \mathrm{e}=-\frac{\mathrm{d\varphi }}{\mathrm{dt}} \\ =\frac{\mathrm{Bd}\left(\mathrm{A}\right)}{\mathrm{dt}} \end{gathered}$$

Where, eis the EMF,$\mathrm{\varphi }$ is the magnetic flux, B is the applied magnetic field and A is the surface area.

Explanation

On pulling out the sheet from the magnetic field there is a decrease in magnetic flux occurs (due to a decrease in exposed area in the magnetic field ) which results in the production of induced current (due to production of induced EMF) in such a direction that opposes the decrease in magnetic flux.

Therefore, a considerable force is required to pull out the sheet from the magnetic field.

If there is an increase in the speed of pulling out the sheet from the magnetic field then the resulting induced current will also be larger.

Hence, more force is required with an increase in the speed of pulling out the sheet.

Similarly, on inserting the copper sheet there is an increase in magnetic flux which results in an induced current that flows in such a way to oppose magnetic flux.

Hence, a considerable force is required to insert the metal between the poles. Therefore, considerable force is required to insert the copper sheet between poles.