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A bulb B1 100 W, bulb B2 60 W and bulb B3 40 W are connected as shown in the figure. The output powers of these would be :

A bulb B1 100 W, bulb B2 60 W and bulb B3 40 W are connected as shown in the figure. The output powers of these would be. Current Electricity, Concise Physics Solutions ICSE Class 10.
  1. W1 < W2 < W3
  2. W1 = W2 > W3
  3. W1 > W2 > W3
  4. W1 > W2 = W3

Current Electricity

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Answer

W1 < W2 < W3

Reason — The bulbs B1 and B2 are connected in series in the upper branch, while bulb B3 is connected in a parallel branch directly across the 240 V supply.

For bulbs of the same voltage rating,

R=V2P\text R=\dfrac{\text V^2}{\text P}

So, resistance is inversely proportional to power.

Thus,

  • 100 W bulb has the least resistance
  • 60 W bulb has greater resistance
  • 40 W bulb has the greatest resistance

Hence,

R1 < R2 < R3

Since B1 and B2 are in series, the same current flows through both. Power in a series circuit is given by:

P=I2R

Therefore, the bulb with greater resistance in that branch will consume more power.

So,

W2 > W1

Bulb B3 is connected directly across the mains, so it gets the full 240 V and works at its rated power, i.e. 40 W.

But B1 and B2, being in series, share the voltage, so each consumes less power than B3.

Therefore, the output powers are in the order : W1 < W2 < W3.

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