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Potentiometer Experiment


Potentiometer Experiment with Readings (Class 12)


POTENTIOMETER EXPERIMENT - 1


Aim:


To compare the e.m.fs of two primary cells (Daniel and Leclanche cells).


Apparatus:


Potentiometer, Accumulator (E), The given primary cells (E1 and E2), Rheostat (Rh), Key (K), Two way key (K1), Connecting wires etc.


Principle:


When a steady current flows through the potentiometer wire, the e m.f of the cell in the secondary circuit is proportional to its balancing length.


Let E1 and E2 be the e.m.fs of a Daniel cell and a Leclanche cell and l1 and l2 be their balancing lengths respectively, then


E1 l1 and

E2 l2

Therefore, E1/E2 = l1/l2


Procedure


The terminals A and B of the potentiometer are connected to an accumulator (E) through a rheostat (Rh) and a key (K). This forms the primary circuit. The positive terminal of the accumulator is connected to A. The Leclanche cell (E1) and the Daniel cell (E2) are included in the circuit using a two way key (K1) along with a galvanometer (G) and a high resistance (HR) through a jockey (J). This forms the secondary circuit.


The primary circuit is closed and the rheostat is adjusted for a suitable current. The Leclanche cell (E1) is included in the circuit. The jockey is pressed at the ends of the potentiometer wire (A and B). If the deflections obtained at the two ends are in opposite directions connections are correct. The jockey (J) is moved along the wire from A to B till the galvanometer shows zero deflection. The high resistance (HR) is then cut off and the exact balancing point is determined. The balancing length (l1) of the wire (distance from A to the jockey) is measured. The Daniel cell (E2) is then included in the circuit and the balancing length l2 is measured. The ratio E1/E2 = l1/l2 is calculated. The experiment is repeated by adjusting the rheostat (Rh) for different currents and the mean value of E1/E2 is determined.


Circuit Diagram

Observations and Readings


Trial

Balance length

E1/E2

Cell of emf E1, l1 cm

Cell of emf E2, l2 cm

1

150

374

0.40

2

141

364

0.38

3

139

365

0.38

4

190

392

0.48

5

177

399

0.44

 

Mean, E1/E2 = 0.416


Result:


The ratio of e.m.f’s = 0.416

 

POTENTIOMETER EXPERIMENT - 2


Aim:


To determine the resistance of the given cell using a potentiometer


Apparatus:


A potentiometer, accumulator (storage cell), the given cell (ex: Daniel or Leclanche cell), resistance box, keys, galvanometer, high resistance etc.


Theory:


When the key K is open, E l  ------------- (i)

When the key K is closed, ER/(R+r) 1’ ------------(ii)

Solving equation (i) and (ii), we get the internal resistance, r = R(l — l’)/l’

 

Procedure:


Connections are done as shown in the figure. The storage cell, the rheostat and a key are connected in series between the terminals A and B of the potentiometer wire. The positive terminal of the cell should be connected to A. The given cell, the galvanometer and a high resistance (HR) are connected in series between the terminal A and the jockey J of the potentiometer. See that the positive terminal of the battery is connected to the terminal A. A resistance box R is connected in parallel to the cell through a key K.


The primary circuit is closed. The key K is opened and the balancing length l is determined. (The rheostat is adjusted to get l as large as possible. Thereafter the rheostat should be adjusted).


The key K is closed. A suitable resistance R, say 5Ω, is taken from R. The balancing length I’ is determined. The internal resistance r of the cell is calculated using the relation,


r = R(l – l’)/l’ 


The experiment is repeated for different values of R in closed circuit. In each case, the internal resistance r of the cell is calculated. [The balancing  length l with R in open circuit need to be determined only at the beginning and at the end of the experiment. The average value l is taken].


Circuit Diagram


Observations and Readings


Balancing length with R in open circuit (l)

(1) 373 cm

(2) 369 cm

Mean l = 371 cm


Balancing length with R in closed circuit (l’)


Trial

R(Ω)

Balancing length, l’ cm

r = R(l – l’)/l’

1

1

81

3.58

2

2

134

3.53

3

3

152

4.32

4

4

163

5.10

5

5

207

3.96

 

Mean r = 4.098

 

Result:


Internal resistance of the cell is determined for different values of the external R.

 

Viva Questions and Answers


1. What is a cell?


A cell is a device by which electric energy is generated due to chemical action taking place inside it.


2. Define primary cell.


Primary cell is a cell in which gives electrical energy from chemical energy. This cell cannot be recharged.

Eg: Dry cell, Daniel cell, Leclanche cell


3. Define secondary cell.


Cell which stores electrical energy as chemical energy and returns it back as electrical energy, is called a secondary cell. This cell can be recharged.

Eg: Lead (acid) accumulator


4. Why is potentiometer called so?


Potentiometer works on the principle that for a constant current, fall of potential along a uniform wire is directly proportional to its length.


5. Define e.m.f. of a cell.


e.m.f. of a cell is defined as the p.d between the terminals of the cell when the cell is in an open circuit.


6. Why we prefer potentiometer rather than a voltmeter to measure e.m.f. of a cell?


Potentiometer method is a null deflection method. It does not draw any current from the source whose e.m.f. is to be measured. A voltmeter always draws some current.


7. The e.m.f. of the cell used in the primary circuit of the potentiometer should be more than the potential difference to be measured. Why?


If it is not so, the balance point will not attain on the potentiometer wire.


8 What is the principle of a potentiometer?


For a constant current, the potential drop across a wire is directly proportional to its length.


9. Which materials are suitable for potentiometer wire?


Manganin, constantan


10. What do you mean by sensitivity of a potentiometer?


Sensitivity of a potentiometer is the smallest potential difference that it can measure.

 

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