Question

Two bulbs $40W$ and $60W$ and rated voltage $240V$ are connected in series across a potential difference of $420V$. Which bulb will work at above its rated voltage?

• A. $40W$ bulb
• B. $60W$ bulb
• C. Both will work
• D. None of these

Answer 1

Answer: (B)

$60W$ bulb

Maximum current flowing through a bulb $I=\frac{P}{V}$

For bulb of 40 W :

Maximum current $I_1=\frac{40}{240}=0.167$ A

Resistance of bulb, $R_1=\frac{(240{)}^2}{40}=1440\Omega$

For bulb of 60 W :

Maximum current $I_2=\frac{60}{240}=0.25$ A

Resistance of bulb, $R_2=\frac{(240{)}^2}{60}=960\Omega$

Now both the bulbs are connected in series :

Total resistance of the circuit $R=960+1440=2400\Omega$

Thus current flowing through the circuit $I=\frac{420}{2400}=0.175$ A

As current flowing through the circuit is greater than the maximum current that flows through bulb of 40 W, thus this bulb will not work but bulb of 60 W does.