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Finding the internal resistance of a power supply.

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Fiona Shallis 12JPI        PLAN        Physics Practise Investigation

PHYSICS INVESTIGATION : Finding the internal resistance of a power supply.

A range of resistors were used -- different ones were used in the actual experiment than planned -- doesn’t matter though as still good range obtained. --


        The aim of this investigation is to find the internal resistance of a solar cell. This will not be measured directly but must be obtained by calculation from values of current and voltage in a circuit incorporating a solar cell.


  • Solar cell (emf about 2V)
  • Lamp (and power pack)
  • Several resistors - 100Ω, 70Ω, 50Ω, 25Ω, 10Ω, 5Ω and 1Ω.
  • Voltmeter
  • Ammeter
  • Clips and connecting leads

The equipment will be set up as shown in the diagram. Multimeters will be used as the voltmeter and ammeter. The lamp will be used to provide light for the solar cell. Various resistors will be used to provide a load resistance, values as above. The experiment will take place in a darkened room.


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        Once the values for each resistor are recorded, they should be added together and divided by 3 to give an average value each for current and voltage. These can then be applied to the following equation to work out the internal resistance of the solar cell.


E= I (R+r) and V = I R rearranged to give

V = -rI +E

This corresponds to the equation y = mx + C.

When V is plotted against I and the corresponding equations y = mx + C and

V = -rI +E, then gradient m = - r and C = E.


The equipment used is not inherently dangerous but care should be taken not to over heat the circuit. The work area should be clear.

Fair Test & Reliability:

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        Precision in this experiment is improved by using a wide range of values of resistance. This means that the dependant variable changes significantly so that any trends are more obvious and can be identified easily. Precision will also be improved as the uncertainty value will be calculated. This can be used to gauge the reliability of the results. Repeating the experiment and taking averages increases the likelihood of having results near to the ‘true’ value of what is being measured.

        If all these points are considered and the results show a pattern or trend as expected, it can be assumed that the results are quite reliable. Any obviously anomalous results would decrease the reliability as they are not expected and probably the result of an error in the method or equipment.

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