CBSE 2025, Set 1

Question

Four resistors, each of resistance R and a key K are connected as shown in the figure. The equivalent resistance between points A and B when key K is open, will be :

![Circuit diagram with four resistors and key K](https://www.edvaya.com/images/cbse/physics/2025/set1/q3-four-resistors-key-circuit.webp) A). $4R$ B). $\infty$ C). $\dfrac{R}{4}$ D). $\dfrac{4R}{3}$

Edvaya · Accepted Answer

The correct answer is $\dfrac{4R}{3}$. When key K is **open**, no current flows through the branch containing the key, effectively removing it from the circuit. By applying series-parallel reduction to the remaining active branches of the circuit diagram between nodes A and B, the network simplifies to an equivalent resistance of $\dfrac{4R}{3}$. > **Tip:** Always check for bridge configurations. When K is closed, such circuits often form a balanced Wheatstone bridge, which drastically alters the current distribution and the equivalent resistance.

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Physics > Electrostatics And Current Electricity > Current Electricity

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