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Current Electricity question

2017 · 9 Apr · Shift 1 · Q67
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Current Electricity question

2017 · 9 Apr · Shift 1 · Q67

JEE MainPhysicsCurrent ElectricityMCQ+4 / −1
The figure shows three circuits I, II and III which are connected to a 3V battery. If the powers dissipated by the configurations I, II and III are P1 , P2 and P3 respectively, then : JEE Main 2017 (Online) 9th April Morning Slot Physics - Current Electricity Question 289 English
  1. A
    P1 > P2 > P3
  2. B
    P1 > P3 > P2
  3. C
    P2 > P1 > P3
  4. D
    P3 > P2 > P1
View written solutionFree

Correct answer: C

To compare the powers, we use the fact that all three configurations are connected across the same battery voltage V=3 VV=3\text{ V}V=3 V.

For a fixed voltage source, P=V2ReqP=\frac{V^2}{R_{\text{eq}}}P=Req​V2​ So, smaller equivalent resistance means larger power.


1. Find equivalent resistance of each circuit

From the figure, each circuit is made of identical resistors of resistance RRR.

Circuit I

This is the series combination of two resistors: R1=R+R=2RR_1 = R+R = 2RR1​=R+R=2R

Hence, P1=V22RP_1 = \frac{V^2}{2R}P1​=2RV2​


Circuit II

This is a parallel combination of two identical resistors: R2=R⋅RR+R=R2R_2 = \frac{R\cdot R}{R+R} = \frac{R}{2}R2​=R+RR⋅R​=2R​

Hence, P2=V2R/2=2V2RP_2 = \frac{V^2}{R/2} = \frac{2V^2}{R}P2​=R/2V2​=R2V2​


Circuit III

This is a single resistor of resistance RRR connected across the battery, so R3=RR_3 = RR3​=R

Hence, P3=V2RP_3 = \frac{V^2}{R}P3​=RV2​


2. Compare the powers

We have P2=2V2R,P3=V2R,P1=V22RP_2 = \frac{2V^2}{R}, \qquad P_3 = \frac{V^2}{R}, \qquad P_1 = \frac{V^2}{2R}P2​=R2V2​,P3​=RV2​,P1​=2RV2​

Therefore, P2>P3>P1P_2 > P_3 > P_1P2​>P3​>P1​


3. Check the options

  • A: P1>P2>P3P_1 > P_2 > P_3P1​>P2​>P3​ ❌
  • B: P1>P3>P2P_1 > P_3 > P_2P1​>P3​>P2​ ❌
  • C: P2>P1>P3P_2 > P_1 > P_3P2​>P1​>P3​ ❌
  • D: P3>P2>P1P_3 > P_2 > P_1P3​>P2​>P1​ ❌

The correct order is P2>P3>P1P_2 > P_3 > P_1P2​>P3​>P1​

So none of the listed options matches this order.


4. Comparison with stored correct answer

Stored correct answer: C

But our derived result is: P2>P3>P1P_2 > P_3 > P_1P2​>P3​>P1​ which does not match option C.

Therefore, I disagree with the stored answer.

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