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Current Electricity question

2021 · 1 Sep · Shift 2 · Q71
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Current Electricity question

2021 · 1 Sep · Shift 2 · Q71

JEE MainPhysicsCurrent ElectricityNumerical+4 / −1
A uniform heating wire of resistance 36 Ω\OmegaΩ is connected across a potential difference of 240 V. The wire is then cut into half and potential difference of 240V is applied across each half separately. The ratio of power dissipation in first case to the total power dissipation in the second case would be 1 : x, where x is ‾\underline{\hspace{2cm}}​
Numerical answer
View written solutionFree

Correct answer: 4

  1. Power dissipation in the original wire

Given:

  • Resistance of wire, R=36 ΩR = 36\,\OmegaR=36Ω
  • Applied voltage, V=240 VV = 240\,\text{V}V=240V

Power dissipated is P=V2RP = \frac{V^2}{R}P=RV2​

So, P1=240236P_1 = \frac{240^2}{36}P1​=362402​

  1. Resistance of each half after cutting

For a uniform wire, resistance is proportional to length. When the wire is cut into two equal halves, resistance of each half becomes R′=R2=362=18 ΩR' = \frac{R}{2} = \frac{36}{2} = 18\,\OmegaR′=2R​=236​=18Ω

  1. Power dissipated by each half

Each half is connected separately across the same potential difference 240 V240\,\text{V}240V.

So power in each half is P′=240218P' = \frac{240^2}{18}P′=182402​

  1. Total power dissipated in the second case

There are two halves, each dissipating 240218\frac{240^2}{18}182402​. Thus, P2=2×240218P_2 = 2\times \frac{240^2}{18}P2​=2×182402​

  1. Find the ratio

We need P1:P2=240236:2×240218P_1 : P_2 = \frac{240^2}{36} : 2\times \frac{240^2}{18}P1​:P2​=362402​:2×182402​

Cancel 2402240^22402: =136:218= \frac{1}{36} : \frac{2}{18}=361​:182​ =136:19= \frac{1}{36} : \frac{1}{9}=361​:91​

Now, 1/91/36=4\frac{1/9}{1/36} = 41/361/9​=4

Hence, P1:P2=1:4P_1 : P_2 = 1 : 4P1​:P2​=1:4

So, x=4x = 4x=4.

  1. Comparison with stored answer

Stored correct answer = 444

This matches our derived answer.

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