Question #053aa
1 Answer
See below
Explanation:
We know that the terminal voltage of a cell is the potential difference between its electrodes.
A voltmeter should not be used to measure the emf of a cell as a voltmeter draws some current from the cell during measurement. When voltmeter is connected across the cell it amounts to closed circuit measurements and there is inner potential drop due to internal resistance of the cell. Therefore, the measurement of cell's emf will not be accurate
To measure a cell's emf, use of a potentiometer is preferred since in potentiometer-measurement no current flows through the cell. It is an open circuit measurements. The emf of cell will be measured accurately.
Other advantages of using a potentiometer are
- Sensitivity of potentiometer is high as compared to that of voltmeter.
- The resistance of potentiometer becomes infinite at the time of measurement. Whereas in case of voltmeter the resistance is very high but still a measurable number.
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Look at the figure below
A potentiometer is a circuit that is used to measure EMF of a cell. the circuit is as shown: A battery of known emf
The ends of a resistance
Step 1. Close key K. Adjust the resistance
Suppose
(100cm for a lab model Meter potentiometer and Length of the potentiometer wire varies from 5 to 10m).
Current through the potentiometer
Potential drop across potentiometer wire
Step 2. Close key
Step 3. Adjust the position of jockey on potentiometer wire such that there is no deflection seen on the galvanometer. Indicates a balanced circuit and that the potentiometer is another null- measurement device.
Let this position of jockey be is J.
Let length AJ of potentiometer wire be
Let
With Galvanometer showing no deflection, EMF of the cell is equal to the potential difference
Inserting value of