to study effect of temperature on internal r of a cell using potentiometer
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aim :-
To determine the internal resistance of given primary cell using potentiometer.
Apparatus
A potentiometer, a battery (or battery eliminator), two one-way keys, a rheostat of low resistance, a galvanometer, a high resistance box, a fractional resistance box, an ammeter, a voltmeter, a cell (say Leclanche cell), a jockey, a set square, connecting wires and a piece of sand paper.
Theory
The internal resistance of a cell is given by
to-determine-the-internal-resistance-of-a-given-primary-cell-using-potentiometer-1
where and l2 are the balancing lengths without shunt and with shunt, respectively, and R is the shunt resistance in parallel with the given cell.
Circuit diagram
to-determine-the-internal-resistance-of-a-given-primary-cell-using-potentiometer2
Procedure
Make the connections accordingly, as shown in circuit diagram.
Clean the ends of the connecting wires with sand paper and make tight connections according to the circuit diagram.
Tight the plugs of the resistance box.
Check the e.m.f. of the battery and cell and see that e.m.f. of the battery is more than that of the given cell, otherwise null or balance point will not be obtained (E’ > E).
Take maximum current from the battery, making rheostat resistance small.
To test the correctness of the connections. (Insert the plug in the key K1 and note the ammeter reading. Take out 2000 Ω resistance plug from the resistance box. Place the jockey first at the end P of the wire and then at the end Q. If the galvanometer shows deflection in opposite directions in the two cases, the connections are correct).
Without inserting the plug in the key K2 adjust the rheostat so that a null point is obtained on the fourth wire of potentiometer.
Insert the 2000 ohm plug back in its position in resistance box and by slightly adjusting the jockey near the previously obtained position of null point, obtain the null point position accurately, using a set square.
Measure the balancing length l1 between this point and the end P of the wire.
Take out the 2000 ohms plug again from the resistance box R.B. Introduce the plugs in key K1, as well as in key K2. Take out a small resistance (1-5 Ω) from the resistance box R connected in parallel with the cell.
Slide the jockey along the potentiometer wire and obtain null point.
Insert 2000 ohms plug back in its position in R.B. and if necessary make further adjustment for sharp null point.
Measure the balancing length l1 from end P.
Remove the plug keys at K1 and K2. Wait for sometime and for the same value of current (as shown by the ammeter), repeat the steps 7 to 13.
Repeat the observations for different values of R repeating each observation twice.
Record your observations as given below.
Observations
Range of voltmeter =………
Least count of voltmeter = ………
E.M.F. of battery (or battery eleminator) = ………
E.M.F. of cell = ………
Table for Lengths
to-determine-the-internal-resistance-of-a-given-primary-cell-using-potentiometer-1 (2)
Calculations
For each set of observation find mean and l2 and write in column 3c and 3f.
Calculate value of r for each set and write it in column 5.
Take mean of values of r recorded in column 5.
Result
The internal resistance of the given cell is………
Precautions
Same as in Experiment 4 and other precautions are as:
The e.m.f. of the battery should be greater than that of the cell.
For one set of observation the ammeter reading should remain constant.
Current should be passed for short time only, while finding the null point.
Rheostat should be adjusted so that initial null point lies on last wire of the potentiometer.
Cell should not be disturbed during experiment.
Jockey should not be rubbed against the potentiometer wire.
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