A p.d. of $10 \mathrm{~V}$ is needed to make a current of 0.02 A flow through a wire. What p.d. is needed to make a current of 250 mA flow through the same wire?

Given: A p.d. of $10 \mathrm{~V}$ is needed to make a current of 0.02 A flow through a wire.

To find: p.d. needed to make a current of 250 mA flow through the same wire

Solution:

Case {i}

$I=0.02 \mathrm{amp}, \mathrm{V}=10$ volt

$\mathrm{V}=\mathrm{IR}$

$10=0.02 \times \mathrm{R}$

$\mathrm{R}=10 / 0.02=500 \mathrm{ohm}$

Case {ii}

$\mathrm{I}=250 \times 10^{-3} \mathrm{amp}, \mathrm{R}=500 \mathrm{ohm}$

$\mathrm{V}=\mathrm{IR}$

$\mathrm{V}=250 \times 10^{-3} \times 500$

$\mathrm{v}=125$ volt