Question

What will happen when a 40 W, 220 V lamp and 100 W, 220 V lamp are connected in series across 40 V supply?

• A. 100 W lamp will fuse
• B. 40 W lamp will fuse
• C. Both lamps will fuse
• D. Neither lamp will fuse

Answer 1

Answer: (D)

When current $i$ flows, across potential V, then power $v_i,$ and $V=iE$ (Ohms law)
The currents required by the two lamps for their normal brightness are $i_1=\frac{p_1}{V_1}=\frac{40}{220}$ $=\frac{2}{11}=0.18A$ $i=\frac{V}{R_1+R_2}$ $=\frac{40}{1694}=0.236$ $i_2=\frac{p_2}{V_2}=\frac{100}{200}=0.45A$ The resistance of the filaments are $R_1-\frac{V}{i_1}=\frac{220\times 11}{2}=121\Omega$