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Investigating The Characteristics Of A Filament Lamp

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Laura Higgins 10A

November 2002

GCSE Physics Coursework:

Investigating The Characteristics Of A Filament Lamp

Aim: To investigate how the current in a component changes as the voltage is altered.

Prediction: As the voltage in the filament lamp increases, more current will flow through the circuit, causing the tungsten filament to get hotter. According to Ohm’s law (R =V/I), the resistance in the circuit will therefore increase.

Scientific Knowledge: In order to give reason for the prediction above, already proven scientific theory can be used.

        A number of factors can affect how the current in a component, such as a filament lamp, can change as the voltage is altered. A predominant factor is the resistance in the circuit. The resistance in a circuit can be worked out by employing ‘Ohm’s law’, which declares that the resistance in a wire is equal to the voltage (V) over current (I). The final answer is given in Ohms, or (Ώ), and the results can be displayed on a line graph and a line of best fit can be drawn.

        The overall resistance in a wire can be affected, in itself, by a number of different factors. These are:

i.) The length of the wire: As the length of wire increases, the total resistance in the circuit will also increase. This is because the electrons in the circuit have to get past positive ions in order to travel around the circuit. Therefore, as the length of the wire increases, the wire will contain more positive ions, and the electrons will have to pass more positive ions in order to travel around the circuit. Travelling past the positive ions causes the electrons to use up more energy just to travel around, and so the resistance shall also increase.


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Preliminary Measurements: To get an idea of what range of measurements would get the best results, a circuit was set up using all of the apparatus which would be included in the final experiment. The preliminary investigation was also useful to discover whether I would prefer to use analogue meters or digital meters in the final experiment.

        After increasing the DC voltage by different ranges each time, using both types of meters, I concluded that it would be best to start from 0.5V and to finish at 6V, increasing in steps of 0.5V each time. This would give me quite an extensive range of numerical results. After this range had been repeated two more times, I could work out the mean current, and from the mean current I could work out the average resistance. Once I had discovered the average resistance, I could plot this value on the graph. When all the values had been plotted on the graph, a line of best fit could be drawn.

        By using both the meters in the preliminary measurements, I decided to work with the digital meters in order to measure the voltage. This was because I felt that it was easier to read accurately the voltage on the meter than with the analogue meters. On the analogue meters the needle kept moving around quickly, and did not seem to stabilize on one single value.

Number of Repeats: 2 (three in total)

Number of Measurements: 36 in total

Range of Measurements: 0.5V  - 6.0V (12 individual measurements per repeat)

Method: The circuit diagram mentioned above was set up using the specified apparatus. The fair test and safety requirements were strictly followed. The voltage was increased from 0.5V to 6.0V, in steps of 0.5V each time.

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        Out of the three line graphs I drew from the final results, there was only one identifiable anomalous result, in the graph showing the relationship between the voltage and the average resistance in the lamp. However, as it only just did not fit in with the line of best fit it may have been down to human error whilst reading from the ammeter, or whilst it was plotted onto the graph. It is relatively easy to make such a mistake when recording results because there are so many to read from the ammeter, and it fluctuates rapidly before settling on a final result. By choosing to use digital ammeters rather than analogue ammeters, I feel I decreased the chance of human error as it is easier to read a LCD screen which fluctuates rather than a swinging needle. Human error could also be reduced by being allowed to take more time over the experiment; more repetitions of the results would make anomalies more obvious and increase the reliability of the results further.

        This investigation could be extended by choosing to look at other variables which would affect the rate of resistance, such as the material which the wire is made from, the cross-sectional area of the wire, the number of components in the circuit, or the length of the wire. A different component, such as an electrical heater, could also be investigated with the variable of voltage.

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