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.

Key concepts

Voltmeter

A voltmeter is an instrument used to measure the voltage, or potential difference, between two points in an electrical circuit. It is typically connected in parallel to the component across which the voltage is being measured. Unlike a potentiometer, a voltmeter operates by drawing some current from the circuit.
When a voltage source is connected to a voltmeter, it introduces a load on the circuit, which can potentially alter the original voltage being measured. This is because the voltmeter itself has a finite resistance, causing a small current to flow through it and the voltage source.
For accurate voltage readings, voltmeters with high internal resistance are preferred, as they minimize current draw and thus the impact on the circuit. However, despite this, voltmeters are not ideal for measuring electromotive force (EMF) due to the loading effect.

Electromotive Force (EMF)

Electromotive Force, commonly abbreviated as EMF, is a measure of the energy provided by a cell or battery per unit charge to move electrons through a circuit. It is essentially the voltage across the terminals of a source when no current is flowing through it, meaning it's the potential difference in an open circuit.
EMF is a crucial concept in understanding how batteries and generators work. It gives an idea of how much work the source can do by pushing electrons through a circuit. Typically denoted by the symbol \( \mathcal{E} \) and measured in volts, EMF includes internal resistances and potential variations within the source.
When a circuit is closed, the terminal voltage (the actual measured voltage) is less than the EMF because of the internal resistance of the source.

Open-Circuit Measurement

Open-circuit measurement refers to measuring the voltage with the circuit in an open state, ensuring no current flows through it. This method provides a true representation of the source's EMF without the added load or influence of the measuring device. This is a distinct advantage of using a potentiometer over a voltmeter.
In practical terms, open-circuit means the measurement is performed when the device being measured is not active or not drawing power. Hence, it reflects the maximum potential difference available from the power source.
Potentiometers are ideal for making this kind of measurement. They balance the unknown voltage against a known reference voltage without drawing current, thus presenting no load to the source.

Voltage Measurement

Voltage measurement is the process of determining the potential difference between two points in an electrical circuit. This can be done using instruments like voltmeters, potentiometers, or oscilloscopes, depending on the precision required and the nature of the measurement.
A voltmeter, as mentioned earlier, measures voltage by allowing current to pass through it, which can sometimes alter readings. For precise measurements, especially when considering the open-circuit EMF, a potentiometer is more suitable.
This is because potentiometers do not draw any current from the source when taking measurements, thereby preventing any circuit load and providing a more accurate reading of the terminal potential or EMF.