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.
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
since V = IR
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 .