Question

Three $60\text{W}$ bulbs are mistakenly wired in series and connected to a $120\text{V}$ power supply. Assume that, the bulbs are rated for single connection to $120\text{V}$ with the mistaken connection, the power dissipated across each bulb is

• A. $6.7\text{W}$
• B. $13.3\text{W}$
• C. $20\text{W}$
• D. $40\text{W}$

Answer 1

The correct option is A $6.7\text{W}$

Let the resistance of each bulb be $R$,

$P=\frac{V^2}{R}\Rightarrow R=\frac{{\left(120\right)}^2}{60}=240\Omega$

$\because$ All three are identical bulbs and connected in series. Thus, potential drop across them is the same.

$\therefore$ Potential drop across each bulb be,

$V^{{}^{\prime }}=\frac{120}{3}=40\text{V}$


So, power dissipated across each bulb is

$P^{\prime }=\frac{{\left(40\right)}^2}{240}=6.7\text{W}$

Hence, option $\left(a\right)$ is correct.