Question

A potentiometer is preferred over a voltmeter to measure the emf of a cell because... (A) The material of the potentiometer wire has a low temperature coefficient of resistance. (B) emf measured by potentiometer is more accurate because the cell is open- circuit. (C) potential gradients can be varied in a potentiometer using a rheostat. (D) Potentiometer is more sensitive than voltmeter.

Short answer

A potentiometer is preferred over a voltmeter to measure the emf of a cell because emf measured by potentiometer is more accurate since the cell is in open-circuit, meaning no current is drawn from the cell during the measurement. This results in a more accurate and unaffected reading of the emf.

Step-by-step solution

Understand potentiometers and voltmeters

A potentiometer is an instrument that measures voltage by comparing an unknown voltage to a known voltage. It has a long wire of uniform area and length of which a known potential difference is applied across. The voltage is measured without drawing any current from the cell, making it an open-circuit measurement. A voltmeter, on the other hand, measures voltage by passing current through itself when it is connected across a voltage source. This current passing through the voltmeter introduces a load on the voltage source, which can alter the value being measured. Step 2: Evaluate the given reasons from options (A) to (D)

Evaluate the options

(A) The material of the potentiometer wire having a low temperature coefficient of resistance is essential to maintain accuracy, but it's not the main reason it's preferred for emf measurements. (B) The fact that emf measured by potentiometer is more accurate because the cell is in open-circuit means there will be no current drawn from the cell, which is the main advantage of using potentiometers to measure the emf. This option seems plausible. (C) The ability to vary potential gradients in a potentiometer using a rheostat is a useful feature and adds versatility, but it doesn't explain why a potentiometer is preferred over a voltmeter in this specific case. (D) Potentiometers tend to be more sensitive than voltmeters, but this is not the main reason they are used for measuring cell emf specifically. Step 3: Choose the correct option

Select the correct answer

Based on the analysis in the previous steps, the most accurate reason as to why a potentiometer is preferred over a voltmeter to measure the emf of a cell is because: (B) emf measured by potentiometer is more accurate because the cell is open-circuit.