(a)
The following conditions are required to obtain permanent electromagnet when a current carrying solenoid is used:
Rod inside the solenoid should be made of magnetic material like steel which should retain magnetic properties for a long time after magnetization.
The current through the solenoid should be direct current.
The number of turns in the solenoid should be large and closely packed, so that a strong uniform magnetic field inside it is produced.
(b)
For earth wire, green or yellow colour insulation is used. The earth wire provides the low resistance conducting path for the current and maintains the potential of appliances body with that of the earth. So, earth wire is used as a safety measure. Any leakage of current to the metallic parts is safely conducted to the ground, thereby reducing chances of severe shock.
