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Question

Three wires each of length \(L\), cross-sectional area \(A\) and resistivity \(\rho\) are connected as shown in the figure.
These are to be replaced by another wire of same resistivity such that the resistance between points \(X\) and \(Z\) does not change. If \(L_1\) is the length and \(A_1\) is the cross-sectional area of the new wire, then which one among the following is correct?

This question was previously asked in
NDA 2 2026 GAT Question Paper (13-Sep-2026)
The correct answer is
\(L_1 = 3L\) and \(A_1 = 2A\)

To find the equivalent wire that has the same resistance between points \(X\) and \(Z\), we need to analyze the given setup and calculate the total resistance.

The three wires in the setup are arranged in such a way that two wires, each with resistance \(R\), are in parallel, and this combination is in series with another wire of resistance \(R\).

Step-by-step Calculation:

  1. Calculate the resistance of each wire:
    • The resistance of a wire is given by the formula: \(R = \frac{\rho L}{A}\).
    • For each wire, \(R = \frac{\rho L}{A}\).
  2. Calculate the equivalent resistance of the two parallel resistors:
    • The formula for two resistors, \(R\) in parallel is: \(\frac{1}{R_{\text{parallel}}} = \frac{1}{R} + \frac{1}{R}\).
    • Thus, \(R_{\text{parallel}} = \frac{R}{2} = \frac{\rho L}{2A}\).
  3. Calculate the overall series resistance:
    • Add the series resistance: \(R_{\text{total}} = R_{\text{parallel}} + R\).
    • Substituting values: \(R_{\text{total}} = \frac{\rho L}{2A} + \frac{\rho L}{A} = \frac{3\rho L}{2A}\).
  4. For the equivalent single wire with resistance \(R_{\text{new}}\), we have:
    • \(R_{\text{new}} = \frac{\rho L_1}{A_1}\) should equal \(R_{\text{total}} = \frac{3\rho L}{2A}\).
    • Equating the resistances: \(\frac{\rho L_1}{A_1} = \frac{3\rho L}{2A}\).
    • This simplifies to: \(\frac{L_1}{A_1} = \frac{3L}{2A}\).
    • One possibility is \(L_1 = 3L\) and \(A_1 = 2A\).

Conclusion:

Thus, the correct option is:

\(L_1 = 3L\) and \(A_1 = 2A\)

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