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Testing ohms law on a light bulb and a resistor connected to a source of potential difference.

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  Testing ohm’s law on a light bulb and a resistor connected to a source of potential difference.

Aim of investigation.

To test if ohm’s law applies to a light bulb or a resistor connected to a source of potential difference.


According to ohm’s law at a constant temperature the current through a conductor (light bulb, resistor, wire etc) is proportional to the potential difference across it.

Materials used

Voltmeter, amphometer, light bulb, resistor, source of potential difference, connecting wires.


There are three variables in this experiment.

  • The independent variable is the ones that we vary to see its effect on the dependent variable i.e. voltage applied.
  • The dependent variable is current. It varies according to the changes in the voltage applied.
  • The controlled variables are the resistor and bulb used. It should be the same one throughout the whole test. temperature should be constant as well.

Method of experiment.


First a light bulb or a resistor was taken. Then these were connected to the voltmeter and amphometer in a way which the figure above shows. The voltmeter was set at 20 V and the amphometer was set at 200 m. then the light bulb or the resistor was connected to a potential difference. The measurements of the voltmeter and amphometer were recorded.

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Graph 1.1 voltage versus current graph for a light bulb.


Table 1.1 the current and resistance of a light bulb when         

  the voltage is varied.

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the product of current flowing through a conductor and the resistor R is always equal to the potential difference V across the two ends of the conductor as long as the temperature and the other physical conditions of the conductor do not change.(V=R . I). By checking the literature value of the resistor used the resistance was found to be 270 Ω. using this experiment the resistance was found within the range of 260 up to 267 Ω ±3. The slight difference can be due to systematic error.

Conclusion: - The light bulb did not obey ohm’s law while the resistor obeys ohm’s law. In addition it can also be concluded that resistance can be dependent on temperature as seen in the resistance of the light bulb.

Limitations:-  the limitation of this experiment may be that the voltmeter and amphometer may have their own resistance which may affect the measuring of values. Since the experiment was only done once for each conductor systematic and random errors may affect the results obtained.

Improvement:   doing the experiment several times to get accurate readings.

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