5-2 Internal Resistance

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					                    International School of Amsterdam
                            IB Diploma Physics
             Experiment 5.2                       Internal Resistance of a Cell

Outline: Set up the following circuit:


                                             R                        

Use an old cell, which will have a reasonably high internal resistance. For various positions of
the rheostat, measure the current that flows in the circuit. Measure the resistance of the
rheostat each time using a multi-meter. (The rheostat must be disconnected from the circuit
while this is done.)

Analysis: In this circuit, Ohm’s Law tells us that E = I ( R + r ).

       Rearranging this gives I = E / ( R + r )     or    1/I = r / E + R / E

Plotting a graph of 1/I against R should give a straight line. The intercept on the resistance
axis will be –r. The gradient will be 1 / E, and to confirm this, the intercept on the current axis
should be r / E.

                              1/ I



                        r                                    R

Don’t forget to consider the degree of accuracy of your measurements, and make an estimate
of the values and uncertainties of the Cell EMF and Internal Resistance.

Assessment: Data Collection and Processing

       J Charters       revised 11 August 2007