JEE Main 2024 · 1 February · morning · Single correct · +4 / −1
The reading in the ideal voltmeter (V) shown in the given circuit diagram is :
- (a) 3 V
- (b) 10 V
- (c) 5 V
- (d) 0 V
Answer and solution
Answer (d) 0 V
Solution
Physics sir · 3 steps
Eight identical cells in one loop: find the current, then the drop inside each cell eats its whole EMF.
- Count the loop. There are 8 identical cells, 3 on top, 3 at the bottom, 1 on the left and 1 on the right. Each is 5 V with internal resistance 0.2 Ω, all in series and aiding each other. The ideal voltmeter draws no current, so it does not change the loop.
- Loop current. Total EMF is 8 × 5 = 40 V and total resistance is 8 × 0.2 = 1.6 Ω. So I = 40/1.6 = 25 A.
V = E - Ir = 5 - 25 × 0.2 = 0 V
- So. The voltmeter sits across the right-hand cell, and that cell's terminal voltage is zero. The reading is 0 V, option (d).
The idea to keep. A voltmeter across a cell reads the terminal voltage E - Ir, not the EMF. When identical cells are shorted in a ring, Ir = E for every cell, so every terminal voltage is zero.
Answer from the NTA final answer key. Solution by PrepWiser.