Question

Three $60W$ light bulbs are mistakenly wired in series and connected to a $120V$ power supply. Assume the light bulbs are rated for single connection to $120V$. With the mistaken connection, the power dissipated by each bulb is

• A. $6.7W$
• B. $13.3W$
• C. $20W$
• D. $40W$

Answer 1

The correct option is A $6.7W$

Here, three 60W bulbs are connected in series,
Resistance of each bulb is
$R_1=\frac{(120{)}^2}{60}=\frac{14400}{60}=240\Omega$
In series connection

$R=R_1+R_2+R_3=720\Omega$ .... $($ sincce $R_1=R_2=R_3)$

Hence, current will be

$I=\frac{120}{720}=0.166A$

Power dissipated by each bulb

$P=I^{2}R=(0.166{)}^2\times 240=6.7W$