Class 12 Physics practical reading To determine the internal resistance of a given primary cell using a potentiometer.Physics,

Aim

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

Apparatus Required

Potentiometer, the given primary cell, an ammeter, a battery, a resistance

box, two one way plug keys, a galvanometer, a rheostat, jockey & connecting wires.

Theory

In the circuit shown in fig. E is a cell of internal resistance r & e.m.f E. When the key is closed, E causes a current I in the circuit. 

       E= I (R+r) 

         = IR + Ir

         = V + Ir

where V is potential difference across resistance R. 

    r = (E - V) /I = ( E/V - 1) R

If l1 &  l2 are the balancing lengths of potentiometer wire for E & V respectively.
       r = ( l1 / l2 - 1) R
       r = ( l1 - l2 ) R/l2

Procedure 

 1. Clean the ends of the connecting wires using sand-paper. 

2. Complete the circuit as shown in Fig. keeping keys K1, and K2, open. 

3. Close key K1. Place the jockey first near the end A and then near the end B of the potentiometer wire and make sure that the galvanometer shows deflection in opposite directions. If it doesn't, adjust the rheostat. 

4. Obtain the balancing point by placing the jockey J at different positions on the potentiometer wire. Determine the balancing length l1 , from the end A.
5. Close the K2. Introduce a resistance R from the resistance box by removing appropriate key and repeat steps 3 and 4 to get the balancing length l1, from the end A.
6. Repeat the above process for five different values of R for open and closed circuit of the primary cell.

Circuit diagram

Observations

Precautions

1. The ends of the connecting wires should be cleaned with a sand-paper before making connections. 

2. The connections should be tight. 

3. The positive terminals of the battery as well as primary cell should be connected to the same end of the potentiometer wire. 

4. The e.m.f. of battery should be greater than the e.m.f. of primary cell 

5. The jockey should not be slided on the potentiometer wire as it can change the uniformity of the wire. It should be put gently and momentarily on the wire at different positions to find the balance points. 

6. The current should be passed for minimum possible time to prevent heating of the wire 

Sources of Error

1. Connections are not tight 

2. E.m.f. of the battery is less than that of the primary cell. 

3. Keys are always inserted resulting in the heating of the wire