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Find how changing the amount of voltage (potential difference) coming out of the power pack will affect the current while it is flowing through a filament bulb of 24W.

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Physics Coursework

Filament Bulbs


My aim is to find how changing the amount of voltage (potential difference) coming out of the power pack will affect the current while it is flowing through a filament bulb of 24W.










The power pack supplies a direct current (DC) to the circuit. The voltmeter is in parallel to measure the potential difference across the bulb and the ammeter is in series to measure the current flowing through the circuit.



  1. Get all the equipment and set it up as shown in the diagram.
  2. Set power pack so that the voltage shows 10V on the voltmeter.
  3. Record the number of amps that are flowing through the circuit by reading the ammeter.
  4. Repeat steps 2 + 3 for 20, 30, 40 and 50 Volts.
  5. Repeat the steps 2 through 4 two more times to get three sets of results. Make an average and this leaves you with a set of good, accurate results.

Fair Test:

...read more.



The graph shows that the voltage goes up in proportion to how much current there is. But I predict that the resistance of the bulb will not change throughout the experiment.

However as the filament is used for a longer period of time, the filament inside the bulb will get hotter.

How resistance is caused:image13.png

It is stated in the particle theory that as molecules and atoms are heated, they will begin to vibrate. Its vibration will cause the resistance to increase by scattering the electrons so that less electrons can get through.





So the hotter the filament gets, the higher the resistance. In theory, this should cause the light bulb to slowly stop increasing in current due to the resistance, as shown in the graph below.


...read more.


th or 5th) we get a trend line which represents the data more accurately.

Results Graph 02:image23.png

This graph shows that the resistance is going up in a complex way. This could be because the temperature is not rising in a steady manner. This will affect the amount of resistance as we know that resistance is linked to temperature. This could be one explanation for having a complex shape curve. Another explanation for having points outside the trend line is that there could be experimental error (eg. Not reading the ammeter correctly).


        I think this experiment went very well as I got the results I had expected. I executed the experiment very well as I did a very fair test, I did everything I mentioned in my fair test section.

        However, there was an anomaly in my data, I think this is down to human and equipment error as the equipment is not 100% accurate.

        All in all this was a successful experiment as I got the results I needed, analyzed them and found out what I wanted to know.

...read more.

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