Chapter 29, Problem Q29.1DQ

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.

To determine

The reason why a force is required to pull or insert a copper sheet placed in between the poles of an electromagnet.

Explanation of Solution

A force is required to pull or insert a copper sheet between the electromagnet poles as per Lenz law due to an induced current formed due to the displacement of the sheet.

As per Lenz law, an induced emf gives rise to a current whose magnetic field opposes the cause inducing a change. This can be applied to the given situation.

As per the given data, when the copper sheet is pulled from the poles of electromagnet there will be a change in the magnetic flux. This magnetic flux change induces currents in the copper sheet.

This induced current opposes the change which induces it to flow. That is as per Lenz law, induced current opposes the moving copper sheet which is the primary cause for these currents. Hence there is a difficulty in pulling the copper sheet and thereby requires a force to pull the copper sheet.

A similar sequence arises when we insert a copper sheet between the electromagnet poles. This means there is an induced current due to magnetic flux which in turn is due to this moving copper sheet. Hence, this current opposes the moving of this copper sheet, both insertion and exertion, between two electromagnet poles.

Conclusion:

Thus, a force is required to pull or insert a copper sheet between the electromagnet poles as per Lenz law.