Question

Two electric bulbs marked $25W-220V$ and $100W-220V$ are connected in series to a $440V$ supply. Which of the bulbs will fuse?

• A. Both
• B. $100W$
• C. $25W$
• D. None of these

Answer 1

Answer: (C)

Here we will use the formula of power $P=\frac{V^2}{R}$ . So resistance $R=\frac{V^2}{P}$ . Resistance of $25W$ bulb $R_1=\frac{{\left(220\right)}^2}{25}=1936ohm$ and resistance of $100W$ bulb $R_2=\frac{{\left(220\right)}^2}{100}=484ohm$ . When they are connected in series with a supply of $440V$ , the resultant current will be $I=\frac{440}{\left(1936+484\right)}=0.1819A$
Now the voltage drop across $25W$ bulb will be $V_1=IR=0.1819\times 1936=352V$ and the voltage drop across $100W$ bulb will be $V_2=0.1819\times 484=88V$ . Here because of the excessive voltage (much higher than rated value) across $25W$ bulb, it will fuse.