Page 121 - Physics - XII

(iii) Concentration of electrolyte: Internal resistance (r) of a cell is inversely proportional to the ionic
concentration (c) of the electrolyte used in the cell, i.e.
1
r ∝
c
In addition to the above factors, the internal resistance of a cell depends upon the nature of the
electrolyte used, its temperature, and amount of current drawn from the cell.
When a resistance R is connected across a cell of e.m.f. E and internal resistance r, then the current I in the
circuit is,
E
I =
Rr+
The potential diff erence V across the two terminals of the cell is,
E
V = IR R
Rr

E
⇒ r = 1 R …(1)
V

In Fig. 1.1, if l and l are the distances of the balance null point from end B of the potentiometer for an open
0
and a closed circuit respectively, then,
E l
= 0 …(2)
V l

From equations (1) and (2), the internal resistance of the cell is, r R l 0 l

l
Procedure
1. First of all prepare a voltaic cell fi lled with an electrolyte of given concentration.
2. Look at the circuit diagram as shown in Fig. 1.1 and arrange the circuit. Check connections and close key
K .
1
3. Keep keys K and K open, introduce a protective high resistance P from resistance box 2 (RB2).
3
2
4. Through sliding the jockey over the potentiometer wire, fi nd the position of balance point.
5. Now, short circuit the resistance P by closing the Key K and fi nd the balance length l .
0
3
6. Take R = 10 Ω (from resistance box 1 (RB1); close key K and quickly measure l. Open the key K as
2
2
soon as you measure l.
7. Change values for R in small reductions and fi nd the values for l for each R.
8. Find the internal resistance of a cell by changing the depth up to which the two plates are immersed in

the electrolytic solution and keep other factors constant.
9. Find the internal resistance of a cell by varying the distance between the two plates and keep other
factors constant.
10. Find the internal resistance of a cell by changing the concentration of electrolyte and keep other factors
constant.
11. Note your observation.

119