What is the difference between an electromagnet and a permanent magnet ? How is an electromagnet designed ? State any two factors on which the strength of an electromagnet depends.

Explanation

An efficient way to make a magnet is to place a ferromagnetic rod in a solenoid and pass a current in it. The magnetic field of the solenoid magnetises the ferromagnetic rod. An electromagnet is that which behaves as a magnet so long as current is being passed through the solenoid coil. As soon as current flow is stopped, an electromagnet loses its magnetisation. On the other hand, a permanent magnet, once magnetised by passing an electric current through solenoid coil, retains its magnetic behaviour for a long time at room temperature .

To design an electromagnet we take a soft iron and place it in a solenoid and pass a current . As relative permeability of soft iron is extremely hihg, the presence of soft iron core increases the magnetic field of solenoid coil by a thousand fold. When we switch off the solenoid current, the magnetism is effectively switched off because the soft iron core has a low retentivity.
Strength of an electromagnet depends on (i) the number of turns per unit length in soleniod coil and current flowing through it, (ii) magnetic permeability of soft iron core.