Investigating Internal Resistance

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Investigating the internal resistance of a specific lab power pack.

Luke Parker



        Investigating the internal resistance of a specific lab power pack.


        To discover the internal resistance of a lab power pack as the EMF and current are altered.


  • Lab power pack
  • Wires
  • Variable resistor
  • Voltmeter
  • Ammeter
  • Ruler
  • Pencil

Diagram (fig.1)

Safety Precautions:

        This experiment in general does not have many risks involved however safety precautions need to be taken. The experiment involves electricity; therefore water should be kept well clear. The power should only be supplied for the minimum time possible for results to be taken accurately. This is to prevent the apparatus getting hot and reduce time when the electrics are in use.


        The possible variables within this experiment are as follows:

  • Voltage
  • Current
  • External resistance

These three will all be varied. The Voltage will be set at a specific setting on the power pack and not be altered until all voltage and current results are recorded for all resistance values. Then the voltage will be changed and once again the voltage and current values taken for each resistance level.

This means the results for each voltage setting will be compared making voltage a fixed variable for a certain set of results. However the voltage setting is changed to repeat the same experiment.

The main variable in this experiment is the external resistance. As the external resistance increases the voltage will decrease. This is because fewer electrons can pass, also this increase in resistance will cause an increase in current.


        In this investigation the voltage and current levels as resistance changes will be recorded.  The results for each resistance will be recorded 3 times for both voltage and current to improve accuracy. These results will also be recorded to a two decimal place degree of accuracy.

Apparatus…in detail:

        Power pack: The power pack used was the same throughout. It has five voltage settings 2V, 4V, 6V, 9V and 12V. However for my particular power pack there was a fault with the final two voltage settings so it was only possible to take results up until 6V. The power pack also has the option to change current form AC to DC and vice versa.

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Wires: The wires used are made of copper with plastic insulation. The plastic insulation comes in a variety of different colours so different component links can be easily identified.

Variable Resistor: A larger component made of many coils of wire. Theses coils create resistance throughout the circuit. The amount of coils the electric flow encounters can be altered using a manual slide on the resistor.

Voltmeter: This apparatus is  used to measure the voltage of a circuit. It measure the voltage to two decimal places. Therefore it is quite precise.

Ammeter: This apparatus is ...

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