USES OF POTENTIOMETER
Uses of potentiometer –
- Measurement of emf of cell-
Let ,
E is the emf of driver cell and , E1 is the emf of cell to be measured
Here potential gradient K= [ {E / (R+r)} r]/l .
Let l1 is the length of the wire from zero end where galvanometer shows no deflection
Then E1 = K l1
E1 = [ {E / (R+r)} r]/l X l1
- Comparison of EMFs of two cells
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To find the emf of two cells we connect the two cells with the potentiometer as shown in figure having emfs E1 and E2 .
When 1 and 2 is connected together let null point is at length l1 from zero end,
Then E1 = K l1 …….(i)
Again when 2 and 3 are connected , null point is at length l2 from zero end ,
Then E2 = Kl2 ……..(ii)
By dividing (i) and (ii) we get E1 /E2 = l1 / l2 ………..(req. equation)
To find the internal resistance of a cell –
Let,
E – is the emf of the cell of which we have to find the internal resistance
R – is the external resistance connected with the cell
R – internal resistance of the cell
Let initially switch K’ is off and l1 is the length of the null point from the zero end .
then E = K l1 ……….(i)
After that switch K’ is switched on then the null point is at l2 from zero end
then ( potential across R ) V = K l2 ……… (ii)
E/V = l1 /l2 …………..(iii)
But, internal resistance r = [(E/V)-1] x R ……………..(iv)
So, from equation (iii) and (iv)
r = [(l1 / l2 )-1] x R …….(req. equation)
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