Given below are two statements : One is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : For measuring the potential difference across a resistance of 600 Ω, the voltmeter with resistance 1000 Ω will be preferred over voltmeter with resistance 4000 Ω.
Reason R : Voltmeter with higher resistance will draw smaller current than voltmeter with lower resistance.
In the light of the above statements, choose the most appropriate answer from the options given below.

Solution

Physics sir · 3 steps

A voltmeter sits in parallel, so the higher its resistance, the less it disturbs what it is measuring.

1. Voltmeter in parallel. A voltmeter is connected across the 600 Ω resistor, so it forms a parallel combination with it. This combination has a lower resistance than 600 Ω, which changes the very voltage you are trying to measure.

   Voltmeter resistance	Parallel with 600 Ω	Change from 600 Ω
   1000 Ω	(600 × 1000)/1600 = 375 Ω	drops by 225 Ω
   4000 Ω	(600 × 4000)/4600 ≈ 522 Ω	drops by only about 78 Ω

2. Check A and R. The 4000 Ω voltmeter disturbs the circuit far less, so it is the better choice and A is wrong. Both voltmeters have the same voltage across them, so by I = V/R the higher-resistance one draws less current. R is correct.
3. So. A is not correct but R is correct, which is option (a). R actually explains why the 4000 Ω voltmeter should be preferred.

The idea to keep. An ideal voltmeter has infinite resistance and draws no current. So between two real voltmeters, always pick the one with the higher resistance. It draws less current and changes the circuit less.

Answer from the NTA final answer key. Solution by PrepWiser.