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Investigating The Resistance Of A Light Bulb As The Voltage Is Increased.

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Investigating The Resistance Of A Light Bulb As The Voltage Is Increased.

Resistance is the resist of electrons flowing through an electrical component. The resistance slows down flow meaning that the components will heat up due to friction. The current in a circuit gets smaller the bigger the resistance is. To work out resistance you can use this formula Resistance = Volts/Current, R=V/I

To measure the resistance in a circuit you would need to find out what the voltage and the current is in the circuit. To find the voltage of an light bulb in a series circuit and hence the voltage of the circuit you would need to use a voltmeter which would be connect parallel with the light bulb to find the voltage across the bulb (see diagram below). Then you would need to find the current in the series circuit by placing an ammeter in the circuit which would give you the current of the circuit as well as the bulb current (see diagram below) Now you would then divide the volts by the amperes to find the resistance.  

Voltmeter and Ammeter in a circuit diagram


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Below are the results from the preliminary work and a line graph on the next page shows the results in a line graph.

Voltage (V)

Current (I)

Resistance (Ω)







































Current results 1 in Amperes

Current results 2 in Amperes

Current results 3 in Amperes

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These results show that if the resistance on the resistance graph is too high it will then be lower on the current graph.

I think that some results are wrong due to the rheostat being unreliable.  One reason for this could have been that the rheostat could have had a loose connection.  Also the coiling of the wire in the rheostat could have become pulled apart with use and this could make the rheostat inaccurate.   I tried to avoid this by checking if any rheostats worked better but they did not work as well. To have solved this the school could have had some new rheostats.

My results were good enough to support the prediction and for this reason I think my investigation was worth carrying out.

Further work to this experiment could have been done with a new reliable rheostat which would give me more accurate results. Also I could have changed the make of the light bulb to see if a different make of light bulb would have given similar results and verified the prediction.  Using a different light bulb may give different results as the metal filament may be different but the general pattern should be the same.

Patrick Pitts

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