Question

Determination of internal resistance of a cell using potentiometer​

Answer 1

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Potentiometer is a device used to measure the internal resistance of a cell, to compare the e.m.f. of two cells and potential difference across a resistor. It consists of a long wire of uniform cross sectional area and of 10 m in length.

Answer 2

Answer:Aim

To determine the internal resistance of a given primary cell using a potentiometer.

Materials Required

1 galvanometer

A battery

1 potentiometer

1 rheostat of low resistance

A fractional resistance box

1 ammeter

2 numbers keys (one-way)

Connecting wires

A jockey

1 high resistance box

Sandpaper

1 Leclanche cell

1 set square

1 voltmeter

Theory

The potentiometer is a device used to measure the internal resistance of a cell and is used to compare the e.m.f. of two cells and potential difference across a resistor. The relation between potential difference, emf, and internal resistance of a cell is given by

Determine The Internal Resistance Of A Given Primary Cell Using A Potentiometer-1

Hence V=IR = E-Ir

This indicates the value of V is less than E by an amount equal to the fall of potential inside the cell due to its internal resistance.

From the above equation

Determine The Internal Resistance Of A Given Primary Cell Using A Potentiometer-2

The internal resistance of the cell is given by

Determine The Internal Resistance Of A Given Primary Cell Using A Potentiometer-3

Circuit Diagram

Determine The Internal Resistance Of A Given Primary Cell Using A Potentiometer-4

Here the internal resistance of the cell is given by

Determine The Internal Resistance Of A Given Primary Cell Using A Potentiometer-5

Where l1 and l2 are balancing lengths without shunt or with the shunt. R is the shunt resistance in parallel with the given cell.

Procedure

The connections should be according to the diagram shown above.

Using sandpaper clean the ends of the connecting wires and make sure that the connections are tight.

The plugs in the resistance box should be tight.

The e.m.f of the cell and battery is more than that of the cell. If it is not then the null point won’t be obtained.

Rheostat resistance can be made minimum by taking maximum current from the battery.

To check if the circuit connections are correct, the galvanometer deflections should be in the opposite direction. This is done by inserting the key K1 and making note of ammeter reading.

To obtain the null point on the fourth wire, the rheostat should be adjusted without inserting the key K2.

Take the small resistance between 1-5 ohm from resistance box R connected in parallel with the cell.

Slide the jockey and obtain the null point

Record your observation

Observations

The least count of voltmeter = ………

Range of voltmeters = ……..

E.M.F of cell = ………

E.M.F of battery = ……

Table for lengths

Sl.no Corrected ammeter reading (A) Balance point when E1 (Leclanche cell) in the circuit

l1 (cm)

Balance point when E2 (Daniel cell) in the circuit

l2 (cm)

(E1/E2) = (l1/l2)

Calculations

For each set of observation find mean and l2

Calculate the value of r for each set.

Take the mean of values of r.

Result

The internal resistance of the given cell is found to be………

Explanation: