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Current Electricity question

2002 · Shift 0 · Q118
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Current Electricity question

2002 · Shift 0 · Q118

JEE MainPhysicsCurrent ElectricityMCQ+4 / −1
A wire when connected to 220V220V220V mains supply has power dissipation P1.{P_1}.P1​. Now the wire is cut into two equal pieces which are connected in parallel to the same supply. Power dissipation in this case is P2.{P_2}.P2​. Then P2:P1{P_2}:{P_1}P2​:P1​ is
  1. A
    111
  2. B
    444
  3. C
    222
  4. D
    333
View written solutionFree

Correct answer: B

  1. Initial power dissipation

Let the resistance of the original wire be RRR.

When connected across a potential difference V=220 VV = 220\,\text{V}V=220V, the power dissipated is

P1=V2RP_1 = \frac{V^2}{R}P1​=RV2​

  1. Resistance after cutting the wire into two equal pieces

Resistance is proportional to length, so when the wire is cut into two equal parts, each piece has resistance

R2\frac{R}{2}2R​

  1. Equivalent resistance of two equal pieces in parallel

Each piece has resistance R2\frac{R}{2}2R​, and they are connected in parallel:

Req=R22=R4R_{\text{eq}} = \frac{\frac{R}{2}}{2} = \frac{R}{4}Req​=22R​​=4R​

(Equivalently, for two equal resistors rrr in parallel, Req=r2R_{\text{eq}} = \frac{r}{2}Req​=2r​.)

  1. New power dissipation

With the same supply voltage VVV, the new power is

P2=V2Req=V2R/4=4V2RP_2 = \frac{V^2}{R_{\text{eq}}} = \frac{V^2}{R/4} = 4\frac{V^2}{R}P2​=Req​V2​=R/4V2​=4RV2​

So,

P2=4P1P_2 = 4P_1P2​=4P1​

Therefore,

P2P1=4\frac{P_2}{P_1} = 4P1​P2​​=4

Hence,

P2:P1=4:1P_2 : P_1 = 4 : 1P2​:P1​=4:1

  1. Option check
  • A: 111 ❌
  • B: 444 ✅
  • C: 222 ❌
  • D: 333 ❌

So the correct option is B.

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