Question

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

Answer 1

To answer this question we must know what a voltmeter and a potentiometer is. We must also know their uses and their features. A voltmeter measures the potential difference across any component of a circuit. A potentiometer measures the exact value of the emf of a cell.

Complete answer:
Let us first discuss a voltmeter and a potentiometer in brief. Then we can decide which of the given options are correct.A voltmeter is a device, which is used to measure the voltage across any component of the circuit. In other words, it measures the potential difference across any component of a circuit.

An ideal voltmeter does not involve the circuit or it does not draw any current. Meaning, when the voltmeter is connected in the circuit, no current passes through it.However, a real voltmeter possesses some resistance. Therefore, it draws some current into it and thus the reading is not 100% accurate.

A potentiometer is an instrument used to measure the emf of an unknown cell. It consists of a main emf source and a resistive wire. The emf of the unknown cell is measured by balancing the emf with voltage across a specific length of the wire. The emf of the cell is given as $E=V\dfrac{x}{L}$ ….. (i),
where E is the emf of the unknown cell, V is the emf of the main source, L is the total length of the wire and x is the balancing length of the wire.

The measured emf of the cell is not affected by the current flowing in the potentiometer.
From the above data, we know that voltmeter will measure the potential difference across the cell and not its emf. Therefore, we cannot say that a voltmeter is better than a potentiometer for measuring the emf of a cell.However, we know that a potentiometer does not draw any current while measuring the emf.

Therefore, the given assertion is incorrect but the reason is correct.Hence, the correct option is D.

Note: Note that the emf of the main voltage source in a potentiometer must be greater than the emf of the unknown cell. Otherwise, we will not obtain a balancing length. Which means that we will not be able to measure the emf of the cell. This can be understood from equation (i).