Question

Forty electric bulbs are connected in series across a 220 V supply. After one bulb is fused, the remaining 39 are connected again in series across the same supply. The illumination will be

• A. More with 40 bulbs than with 39
• B. More with 39 bulbs than with 40
• C. Equal in both the cases
• D. In the ratio of 40² : 39²

Answer 1

Answer: (B)

More with 39 bulbs than with 40

Answer 2

Illumination = P_Consumed $= \frac{V^{2}}{R}$. Initially there were 40 bulbs in series so equivalent resistance was 40 R, finally 39 bulbs are in series so equivalent resistance becomes 39 R. Since resistance decreases so illumination increases with 39 bulbs.