Question

Three light bulbs $(60\text{W},120\text{V})$ are connected across a $120\text{V}$ power source. If the resistance of each bulb does not change with current, then find out total power delivered to the three bulbs.

• A. $80\text{W}$
• B. $60\text{W}$
• C. $40\text{W}$
• D. $20\text{W}$

Answer 1

The correct option is C $40\text{W}$

Resistance of each bulb,
$R=\frac{V^2}{P}=\frac{{120}^2}{60}=240\Omega$

On assuming each bulb as a resistor of $240\Omega$ and redrawing the circuit, we get,

Equivalent resistance,
$R_{\text{eq}}=\frac{240\times 240}{240+240}+240=360\Omega$

So, total power delivered,

$P=\frac{V^2}{R_{\text{eq}}}=\frac{{120}^2}{360}=40\text{W}$