Question

Assertion: Voltmeter is much better than a potentiometer for measuring emf of the cell.
Reason: A potentiometer draws no current while measuring the emf of a cell.
(A) Both (A) and (R) are true and (R) is the correct explanation of A
(B) Both (A) and (R) are true but (R) is not the correct explanation of A
(C) (A) is true but (R) is false
(D) (A) is false but (R) is true

Answer 1

Hint:Both voltmeter and potentiometer are used in labs in the
measurement of emf. Both are used widely. The potentiometer is used in circuits where accurate measurement of voltage is required. The terms we often came across are emf and potential difference although both are measured in Volts both are different.

Complete step by step answer:
The voltmeter is a device which is used to measure the end terminal voltage of the circuit. Emf refers to the maximum potential difference which a battery can give in an open circuit. If in the circuit there is battery and no other resistance then emf equals the potential
difference. Voltmeter does not measure emf but it measures potential difference. So, the assertion is incorrect.The second statement says that potentiometer draws no current, an electronic device may draw any current while it is attached in the circuit. That would be an ideal case and there is no ideal Voltmeter. But the potentiometer can be adjusted so that it does not draw any current, hence, the reason is true.
So, the correct option is (D).

Note: The potentiometer accurately measures the emf because it has zero internal resistance while voltmeter has an appreciable magnitude of resistance and when current moves through it there is a potential drop which the voltmeter measures accurately.