Question

Three 60 W, 120 V light bulbs are connected across a 120 V power line as shown in Fig. The total power is $10\times x\left(W\right)$ dissipated in the three bulbs. find 'x'?

Answer 1

Resistance of each bulb, $R=\frac{V^2}{P}=\frac{{120}^2}{60}=240\Omega$
Equivalent resistance for the parallel combination of resistor B and C, $R_{bc}=\frac{240}{2}\Omega =120\Omega$
Total equivalent resistance, $R_{eq}=R_a+R_{bc}=240+120\Omega =360\Omega$
Total power dissipated is given by, $P=\frac{V^2}{R_{eq}}=\frac{{120}^2}{360}=40W$