question_answer

If a 30 V, 90 W bulb is to be worked on a 120 V line, a resistance of how many ohms should be connected in series with the bulb [MP PMT 2002; KCET 2003]

A)            10 ohm                                   

B)            20 ohm

C)            30 ohm                                   

D)            40 ohm

Correct Answer: C

Solution :

                   Suppose resistance R is corrected in series with bulb. Current through the bulb \[i=\frac{90}{30}=3\,A\] Hence for resistance \[V=i\,R\]Þ \[90=3\times R\Rightarrow R=30\,\Omega \]