NVAEasyJEE 2023Kirchhoff's Laws & Circuits

JEE Physics 2023 Question with Solution

For the given circuit, in the steady state, VBVD=|V_B - V_D| = ......... V\text{V}.

Circuit with a 6 V battery across nodes A and C, diamond network containing resistors 2 ohm, 10 ohm, 2 ohm, and a capacitor branch between B and C.

Answer

Correct answer:1

Step-by-step solution

Standard Method

Given: In the steady state, the capacitor behaves as an open circuit.

Find: The value of VBVD|V_B - V_D|.

So the circuit simplifies to:

Equivalent circuit after removing capacitor branch in steady state, showing top branch 2 ohm and 1 ohm through B, and bottom branch 10 ohm and 2 ohm through D, across 6 V battery.

From the simplified circuit,

iAB=63=2A,iAD=612=0.5Ai_{AB}=\frac{6}{3}=2\,\text{A}, \qquad i_{AD}=\frac{6}{12}=0.5\,\text{A}

Using the potential relation shown in the solution,

VB+2×210×0.5=VDV_B + 2 \times 2 - 10 \times 0.5 = V_D

Hence,

VBVD=45=1V|V_B - V_D| = |4 - 5| = 1\,\text{V}

Therefore, the required voltage difference is 11.

Open-Circuit Capacitor View

Given: The circuit contains a capacitor and a 6V6\,\text{V} source.

Find: The steady-state voltage difference between points BB and DD.

In steady state, the capacitor behaves like an open circuit, so no current flows through the capacitor branch. The remaining network has two branches between the source terminals.

Top branch resistance:

2+1=3Ω2 + 1 = 3\,\Omega

Bottom branch resistance:

10+2=12Ω10 + 2 = 12\,\Omega

So the branch currents are

Itop=63=2AI_{top}=\frac{6}{3}=2\,\text{A} Ibottom=612=0.5AI_{bottom}=\frac{6}{12}=0.5\,\text{A}

Potential drop from AA to BB is

2×2=4V2 \times 2 = 4\,\text{V}

Potential drop from AA to DD is

10×0.5=5V10 \times 0.5 = 5\,\text{V}

Therefore the magnitude of the potential difference is

VBVD=45=1V|V_B - V_D| = |4 - 5| = 1\,\text{V}

So, the correct answer is 11.

Common mistakes

  • Treating the capacitor as a short circuit in steady state is incorrect because for DC steady state it behaves as an open circuit. First remove the capacitor branch, then analyze the resistor network.

  • Adding all resistors into one series combination is wrong because the top and bottom paths are separate branches across the same 6V6\,\text{V} source. Compute current in each branch independently.

  • Using signed difference instead of magnitude is incorrect because the question asks for VBVD|V_B - V_D|. After finding the two node potentials, take the absolute value.

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