Physics 151 Lab 5

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 :
  1. 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
  1. Repeat the readings, increasing R back up to 10 W.Obtain average values of I , the current.
  2. Calculate 1/I .
  3. Plot a graph of R against 1/I . Draw the best possible straight line through the points - they will be quite scattered.
  4. The gradient of this graph is the emf of the cell. The negative intercept on the y-axis is the internal resistance.


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.

since V = IR

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 .