Question

Assertion: A potentiometer is preferred over a voltmeter for the measurement of emf of a cell.

Reason: A potentiometer is preferred as it does not draw any current from the cell.

A. Both assertion and reason are correct and reason is the correct explanation for assertion.

B. Both assertion and reason are correct and reason is not the correct explanation for assertion.

C. Assertion is correct but the reason is incorrect.

D. Assertion is incorrect but the reason is incorrect.

Answer 1

Hint: As per mentioned in the reason, the potentiometer does not draw any current in a balanced condition, and hence potentiometer is preferred over a voltmeter. In a closed circuit, it measures the terminal potential difference which is less than the emf of a cell.

Complete step by step answer:

Here in the question, in the assertion part it is told that a potentiometer is preferred over a voltmeter. This is because a voltmeter draws a small amount of current from the cell for its proper working. Therefore a voltmeter shows a value which is less than the original value of the quantity. 

So in a closed circuit, it measures the terminal potential difference which is less than the emf of a cell. Therefore we can conclude that assertion is a right statement. And also the reason as well is also a correct statement and it justifies the assertion also. So both assertion and reason are correct and reason is the correct explanation for assertion.

Hence, the correct answer is option A.

Note:

A potentiometer is a device for measuring electromotive force by balancing it against the potential difference produced by passing a certain amount of current through a certain variable resistance. Voltmeter is a device used for measuring the potential difference present between any two points in a circuit.