Rankers Physics
Topic: Current Electricity
Subtopic: Power of Electrical Circuit

If two bulbs of wattage 60 W and 100 W respectively each rated at 110 V are connected in series with the supply of 220 V, which bulb will fuse ?
60 W bulb
100 W bulb
Both the bulbs
Bulbs will not fuse

Solution:

Step 1: Resistance of the bulbs

R1=110260202Ω,R2=1102100=121Ω.R_1 = \frac{110^2}{60} \approx 202 \, \Omega, \quad R_2 = \frac{110^2}{100} = 121 \, \Omega.

Step 2: Total current in the circuit

In series, the same current flows through both bulbs:

I=VtotalR1+R2=220202+1210.682A.I = \frac{V_{\text{total}}}{R_1 + R_2} = \frac{220}{202 + 121} \approx 0.682 \, \text{A}.

Step 3: Voltage across each bulb

  • Voltage across the 60 W bulb:

V1=IR1=0.682202137.6V.V_1 = I \cdot R_1 = 0.682 \cdot 202 \approx 137.6 \, \text{V}.

  • Voltage across the 100 W bulb:

V2=IR2=0.68212182.5V.V_2 = I \cdot R_2 = 0.682 \cdot 121 \approx 82.5 \, \text{V}.

Step 4: Compare with rated voltage

  • The 60 W bulb experiences 137.6V137.6 \, \text{V}, exceeding its 110V110 \, \text{V} rating, so it fuses.
  • The 100 W bulb experiences 82.5V82.5 \, \text{V}, within its 110V110 \, \text{V} rating.

Conclusion

The 60 W bulb will fuse.\boxed{\text{Answer: 60 W bulb.}}

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