Experiment to find the emf (E) and the internal resistance
(r) of a cell
Apparatus : 1.5V (approx) cell, Resistance box, Push switch, Ammeter
(0-1A).
Method :
Set R at 10 W . Reduce in steps of 1 W , recording resistance and current. Read the Ammeter as accurately as possible. Release switch after each reading, otherwise the cell will run down during the experiment
- Repeat the readings, increasing R
back up to 10
W.Obtain average values of I
, the current.
- Calculate 1/I .
- Plot a graph of R
against 1/I . Draw the best possible straight
line through the points - they will be quite scattered.
- The gradient of this graph is the emf of the cell.
The negative intercept on the y-axis is the internal resistance.
Theory:
The emf of a cell is the total p.d. it can produce around
the circuit, including any potential wasted in driving current through the
cell itself.
The internal resistance of a cell is simply the resistance from one terminal
of the cell to the other.
then E = I (R+r)
so E/I = R + r
Hence: R = E(1/I) - r
Therefore R plotted against
1/I has gradient E
and intercept -r .