Question

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

• A. $180W$
• B. $20W$
• C. $40W$
• D. $60W$

Answer 1

The correct option is C $40W$

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

Total resistance of the bulbs in the circuit is $R_{eq}=R+\frac{RR}{R+R}=\frac{3}{2}R=\frac{3}{2}\times 240=360\Omega$

Total power delivered to three bulbs is: $P_t=\frac{(120{)}^2}{360}=40W$