Question

A lamp of 6 V and 30 W is used in a laboratory but the supply is of 120 V. what will be done to make use of the lamp?
(1) A resistance may be used
(2) A resistance may be used in series with lamp.
(3) The resistance should be of 18 $\Omega$.

• A. 1, 2 and 3 are correct
• B. 1 and 2 are correct
• C. 1 and 3 are correct
• D. 2 and 3 are correct

Answer 1

The correct option is B 1 and 2 are correct

Here, the resistance should be used in series with lamp in order to divide the voltage or to reduce potential drop across the lamp.

Lamp can withstand a maximum of 6 V , and a current of $\frac{6}{R_{lamp}}$ where $R_{lamp}$ is $\frac{6^2}{30}=1.2\Omega$

hence maximum current that lamp can with stand is $\frac{6}{1.2}=5A$

so the resistance which is to be connected in series would also get same current of 5 A hence if it will be of $18\Omega$ then voltage drop accros it will be 90 V and maximum of 6 V across lamp but both voltage drop does not make 120 V . hence the resistance value should be more than $18\Omega$. So 1 and 2 is correct but 3 is incorrect.