KVPY Sample Paper KVPY Stream-SX Model Paper-14

A voltmeter with resistance \[500\,\Omega \]is used to measure the emf of a cell of internal resistance \[4\Omega .\] Percentage error in reading of voltmeter is around below

A) 1%

B) 0.8%

C) 0.1%

D) 8%

Correct Answer: B

Solution :

Let E = emf of cell. Current in the circuit is \[i=\frac{E}{R+r}=\frac{E}{500+4}\]

Potential drop across terminals of voltmeter is \[V=iR=500\times \frac{E}{500+4}=\frac{500E}{504}\]

So, voltmeter reads \[\frac{500E}{504}\]instead of E.

Hence, per cent error in reading is

Per cent error \[=\frac{\left( E-\frac{500}{504}E \right)}{\left( \frac{500}{504}E \right)}\times 100\approx 0.85\]