Define magnetic equator.

Differentiate between a temporary magnet and a permanent magnet.

The north and south poles of a bar magnet are situated $0.4 \mathrm{cm}$ inside the geometric ends of the magnet. If the geometric length of the magnet is $6 \mathrm{cm}$, then its effective length is ______$\mathrm{mm}$ .

The effective length of the first magnet is two times that of the second magnet. The effective length of the third magnet is one-third of the first magnet. If the effective length of the second magnet is $7.5 \mathrm{cm}$, the effective length of the third magnet is _____.

Calculate the effective length of magnet 'R' if the effective length of magnet 'P' is twice of magnet 'Q' and the effective length of 'R' is thrice of 'P' and the effective length of 'Q' is $10 \mathrm{cm}$.