Circuit with Ammeter and Voltmeter – Rankers Physics

Measuring Devices ( Galvanometer, Voltmeter and Ammeter & Meter Bridge ): Practice Problem & Solution

Assertion (A): In the given circuit, when switch \(S\) is ON reading of ammeter and voltmeter will increase. Reason (R): In parallel combination net resistance will reduce.  
(1) Both (A) & (R) are true and the (R) is the correct explanation of the (A)
(2) Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
(3) (A) is true but (R) is false
(4) Both (A) and (R) are false

Solution Explained:

To solve this problem, we apply the core principles of Measuring Devices ( Galvanometer, Voltmeter and Ammeter & Meter Bridge ). Understanding the underlying formula is key to arriving at the correct answer below:

When switch \(S\) is ON, another resistor \(R\) is connected in parallel. The equivalent resistance of the parallel combination is \(R/2\), which is less than \(R\). Thus, the net resistance reduces (Reason R is true). The total current (ammeter reading) will increase. However, assuming an ideal voltage source, the voltage across the parallel combination (voltmeter reading) will remain constant (equal to the source EMF). Therefore, Assertion (A) is false as the voltmeter reading does not increase.

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