To Compare EMF Of Two Given Primary Cells Using Potentiometer - Practice Questions & MCQ

Aim-
To compare the EMF of two given primary cells using the potentiometer.

Apparatus-
Potentiometer, a Leclanche cell, a Daniel cell, an ammeter, a voltmeter, a galvanometer, a battery (or battery
eliminator), a rheostat of low resistance, a resistance box, a one-way key, a two-way key, a jockey, a set
square, connecting wires and a piece of sandpaper.

Theory-

$
\frac{E_1}{E_2}=\frac{l_1}{l_2}
$

where, $\mathrm{E}_1$ and $\mathrm{E}_2$ are the e.m.f. of two given cells and $\mathrm{l}_1$ and $\mathrm{l}_2$ are the corresponding balancing lengths on potentiometer wire.

Circuit Diagram-

 Procedure: 

1. Arrange the apparatus as shown in the circuit diagram figure.
2. Remove the insulation from the ends of the connecting copper wires with sandpaper.

3. Measure the e.m.f. (E) of the battery and the e.m.fs. (E₁, and  E₂ ) of the cells. See that
 E>E₁ and also  E>E₂ .
4. Connect the positive pole of the battery (a battery of constant e.m.f.) to the zero end (P) of the potentiometer and the negative pole through a one-way key, an ammeter, and a low resistance rheostat to the other end (Q) of the potentiometer.

5. Connect the positive poles of the cells E₁ and  E₂  to the terminal at the zero end (P)
and the negative poles to the terminals a and b of the two way key.
6. Connect the common terminal c of the two-way key through a galvanometer (G) and
a resistance box (R.B.) to the jockey J.

7. Take maximum current from the battery making rheostat resistance zero.

8. Insert the plug in the one-way key (K) in the circuit and also in between the terminals a
and c of the two-way.
9. Take out a 2000 ohms plug from the resistance box (R.B).
10. Press the jockey at the zero end and note the direction of deflection in the galvanometer.

11. Press the jockey at the other end of the potentiometer wire. If the direction of
deflection is opposite to that in the first case, the connections are correct. (If the
deflection is in the same direction then either connection are wrong or e.m.f. of the
auxiliary battery is less).
12. Slide the jockey gently over the potentiometer wires till you obtain a point where
the galvanometer shows no deflection.

13. Put the 2000 ohms plug back in the resistance box and obtain the null point position
accurately, using a set square.
14. Note the length   of the wire for the cell  E₁  Also note the current as indicated by the
ammeter.
15. Disconnect the cell E₁  by removing the plug from gap ac of two-way key and
connect the cell  E₂ by inserting plug into gap be of two-way key.

16. Take out a 2000 ohms plug from resistance box R.B. and slide the jockey along
potentiometer wire so as to obtain no deflection position.
17. Put the 2000 ohms plug back in the resistance box and obtain an accurate position of
null point for the second cell E₂.
18. Note the length    of wire in this position for the cell E₂ . However, make sure that
ammeter reading is the same as in step 14.

19. Repeat the observations alternately for each cell again for the same value of current.
20. Increase the current by adjusting the rheostat and obtain at least three sets of
observations in a similar way.
21. Record your observations and on the basis of observations  compare emf of two given primary cells

Observations
1. Range of voltmeter $=\ldots$.

Least count of voltmeter $=\ldots$.
E.M.F. of battery (or battery eliminator),
E.M.F. of Leclanche cell,

$
\begin{aligned}
& E=\ldots \ldots \\
& E_1=\ldots \ldots \\
& E_2=\ldots \ldots
\end{aligned}
$

E.M.F. of Daniel cell,

Calculations
1. For each observation find mean $l_1$ and mean $l_2$

$
\begin{aligned}
& \frac{E_1}{E_2} \text { for eac } \\
& \frac{E_1}{E_2}
\end{aligned}
$

Result-
The ratio of E.M.Fs, $\frac{E_1}{E_2} \cong \ldots \ldots$