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Current Electricity question

2004 · Shift 0 · Q132
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Current Electricity question

2004 · Shift 0 · Q132

JEE MainPhysicsCurrent ElectricityMCQ+4 / −1
An electric current is passed through a circuit containing two wires of the same material, connected in parallel. If the lengths and radii are in the ratio of 43{4 \over 3}34​ and 23{2 \over 3}32​, then the ratio of the current passing through the wires will be
  1. A
    8/98/98/9
  2. B
    1/31/31/3
  3. C
    333
  4. D
    222
View written solutionFree

Correct answer: B

  1. Use resistance formula

For a wire of length LLL and radius rrr, resistance is R=ρLA=ρLπr2R=\rho\frac{L}{A}=\rho\frac{L}{\pi r^2}R=ρAL​=ρπr2L​ where ρ\rhoρ is the resistivity.

So, R∝Lr2R \propto \frac{L}{r^2}R∝r2L​ for wires of the same material.

  1. Given ratios

Let the two wires be 1 and 2.

Length ratio: L1L2=43\frac{L_1}{L_2}=\frac{4}{3}L2​L1​​=34​ Radius ratio: r1r2=23\frac{r_1}{r_2}=\frac{2}{3}r2​r1​​=32​

  1. Find resistance ratio

Since R∝Lr2R \propto \frac{L}{r^2}R∝r2L​ we get R1R2=L1L2⋅r22r12\frac{R_1}{R_2}=\frac{L_1}{L_2}\cdot\frac{r_2^2}{r_1^2}R2​R1​​=L2​L1​​⋅r12​r22​​

Substitute the given ratios: R1R2=43⋅(3)2(2)2\frac{R_1}{R_2}=\frac{4}{3}\cdot\frac{(3)^2}{(2)^2}R2​R1​​=34​⋅(2)2(3)2​ R1R2=43⋅94=3\frac{R_1}{R_2}=\frac{4}{3}\cdot\frac{9}{4}=3R2​R1​​=34​⋅49​=3

Thus, R1:R2=3:1R_1:R_2=3:1R1​:R2​=3:1

  1. Use parallel connection condition

In parallel, both wires have the same potential difference. Hence current is inversely proportional to resistance: I∝1RI \propto \frac{1}{R}I∝R1​

Therefore, I1I2=R2R1=13\frac{I_1}{I_2}=\frac{R_2}{R_1}=\frac{1}{3}I2​I1​​=R1​R2​​=31​

So the ratio of currents is I1:I2=1:3I_1:I_2=1:3I1​:I2​=1:3

  1. Match with options

The correct option is: 13\boxed{\frac{1}{3}}31​​ which is Option B.

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