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AimTo compare the EMF of two given primary cells (Daniel and Leclanche cells) with the help of a potentiometer.
Apparatus RequiredPotentiometer
Daniel Cell
Leclanche Cell
low resistance Rheostat
Ammeter
Voltmeter
Galvanometer
A one-way key
A two-way key
Set Square
Jockey
Resistance Box
Connecting wires
Piece of sandpaper
TheoryA potentiometer is an instrument used to measure the emf of a cell or potential difference between two points in an electric circuit. It works on the principle of comparing unknown emf with a known emf. In this experiment, we use a potentiometer to compare the emfs of two given primary cells.
While a voltmeter can only measure the potential difference between two terminals of a cell, a potentiometer enables us to determine the exact value of the EMF of a given cell. The EMFs of two cells, E1 and E2, can be compared by measuring the balancing lengths l1 and l2 on the potentiometer wire, respectively, and the potential gradient φ along the wire.
The ratio of the two EMFs, E1/E2, can be expressed as the ratio of the balancing lengths l1/l2, which is also equal to the ratio of the potential gradient φ along the potentiometer wire.
ProcedureClean the connecting wires with sandpaper to remove any insulating coating on them.
Set up the potentiometer circuit as per the given diagram.
Connect the first primary cell to the circuit and note down the balancing length of the potentiometer wire.
Replace the first primary cell with the second primary cell and note down the balancing length of the potentiometer wire.
Using the balancing length of the potentiometer wire for both cells, calculate their respective emfs using the formula:
E1 / E2 = l1 / l2
Where E1 and E2 are the emfs of the first and second primary cells, and l1 and l2 are their respective balancing lengths on the potentiometer wire.
Compare the calculated emfs to determine which primary cell has a higher emf.
Repeat the experiment multiple times to ensure accurate results.
Note: It is essential to ensure that the potentiometer wire is uniform throughout its length and that the jockey is in contact with the wire to get accurate balancing length readings.
Observation and ResultObservations:
The balancing length of the potentiometer wire for the first primary cell is noted down as l1.
The balancing length of the potentiometer wire for the second primary cell is noted down as l2.
Results:
The emf of the first primary cell is calculated using the formula:
E1 = E2 * l1 / l2
The emf of the second primary cell is calculated using the formula:
E2 = E1 * l2 / l1
The emfs of both primary cells are compared to determine which cell has a higher emf.
Calculations:
Let E1 be the emf of the first primary cell, E2 be the emf of the second primary cell, l1 be the balancing length of the potentiometer wire for the first cell, and l2 be the balancing length of the potentiometer wire for the second cell.
Then, using the formula:
E1 / E2 = l1 / l2
We can rearrange to obtain:
E1 = E2 * l1 / l2
And:
E2 = E1 * l2 / l1
For example, if the balancing length of the potentiometer wire for the first cell is 30 cm and for the second cell is 40 cm, and the known emf of the first cell is 2 V, we can calculate the emf of the second cell as:
E2 = 2 V * 40 cm / 30 cm = 2.67 V
Similarly, we can calculate the emf of the first cell using the balancing length of the potentiometer wire for the second cell and the known emf of the second cell.