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 (?), 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.

ii.) The number of components in a circuit: Electrons require more energy to travel through components in a circuit than if they merely through just a plain wire. Therefore, the more components in a circuit, the higher the resistance will be.

iii.)The cross sectional area of a wire: As the cross-sectional area of a wire increases, the resistance in the circuit will decrease. This is because there is more room in the wire for electrons to pass through, and therefore less energy is wasted as the electrons move through the circuit.

iv.) Material of the wire: Some materials are better conductors of electricity than others. The better the conductor, the less resistance there will be in the wire and the easier it is for electrons to flow around the circuit.

v.) Temperature of wire: As the temperature of the wire increases, the resistance in the wire also increases.

Invented by Thomas Edison (1847-1931), the filament light bulb is a common feature in practically every modern home around the world. In theory the structure of a filament light bulb is quite simple. At the base of the light bulb there are two small metal contacts, which connect into a circuit. These contacts are attached to two stiff wires, which are in turn attached to a thin metal filament, hence the name ‘filament lamp.’ The filament is a long thin piece of metal wire, which is coiled, and then coiled again. The filament is constructed in such a manner to make the resistance in the wire as high as possible. This is because, as the resistance in the wire increases, the filament in the lamp heats up more quickly, and therefore causing the filament to also glow more quickly. To increase the resistance in the wire, and also the temperature, the filament is made out of a metal, a good conductor of electricity, the cross-sectional area of the wire is as small as possible, in order to increase the resistance. Finally, the filament is coiled tightly twice so the length of the wire can be as long as possible. This is because as length of the wire increases, the resistance will also increase. The most commonly used material to manufacture the metal element out of is tungsten, chosen because of its high melting temperature. The tungsten filament sits in the middle of a bulb, held up by a glass mount, and the wires and filament are housed in a glass bulb which is filled with an inert glass, such as argon.

The whole objective of a filament lamp is to pass electrical current through the tungsten filament in order to give off light. However, as well as giving off light, heat is also given off as wasted energy, which has no usage. As the voltage increases, the current flowing through the filament will also increase, meaning both more light and heat are also given off. Filament lamps are not particularly efficient as most of the energy produced is given off as heat energy, which has no use, instead of light energy.

Inert gases are used in the glass bulb of a filament lamp to reduce the evaporation of the tungsten filament, which therefore greatly prolongs the life of the bulb, making it more economical.

In order to investigate the characteristics of a filament lamp, a simple circuit could be set up in order to discover how much the current in the circuit changes as the voltage is altered. From these two results, Ohm’s law could be employed to work out the resistance. The results could then be plotted onto a number of line graphs in order to prove the prediction either correct or incorrect. If the results from an experiment involving current and voltage are plotted onto a line graph and the line of best fit forms a straight line, the current (I) is therefore proportional to the voltage (V), an indication of Ohm’s law. As well as being a useful method for working out the resistance in a circuit (R=V/I), Ohm’s law also states that the steeper a graph will be, the lower the resistance in the circuit. The flatter the graph is, the higher the resistance. A substance which produces a straight graph such like this is referred to as an ‘Ohmic conductor.’ Copper wire and all other metals give this shape of graph, unless a change in temperature occurs.

It is a scientifically proven fact that filament lamps are non-ohmic conductors because any results taken from experiments involving filament lamps do not produce straight lines with the values recorded. Instead, successful experiments result in the line of best fit becoming a gentle curve towards the top end of the graph. However, as it says above, the material most commonly used to manufacture filament lamps is tungsten, a metal. As the lines of best fit for all experiments involving current, voltage and metals are an indication of Ohm’s law, unless a change of temperature occurs. It is therefore possible to state that a change of temperature does indeed occur when a filament lamp is used in a circuit. As the voltage in a circuit is increased, more current will flow. As more current flows the metal filament will get hotter, causing the resistance to increase. This will cause the line of best fit to become flatter as the voltage increases.

In order to determine which instruments and what values would be suitable to ensure that the best results possible could be obtained from the experiment, a preliminary investigation should be carried out, using potential apparatus. From these results the range of measurements needed could also be discovered.

After carrying out the preliminary investigation, the best way to display the results from the main investigation itself would be to record each repeat into a simple results table, and then to work out the average current. Once the mean current has been calculated, the resistance could be also calculated. It is important to repeat the experiment a number of times, as if the experiment was only calculated once, the result would not be as accurate, and would be more likely to be anomalous.

After both the mean current and the average resistance had been mathematically calculated, line graphs could be drawn and compared against the typical filament lamp graph above. In all graphs, the voltage should feature along the x axis as it is what is being controlled and changed in order to see what effect change will have on the current.

A conclusion can be drawn by analysing graphs of the results recorded and comparing the graph with the typical ‘filament lamp’ graph. The more similar the graphs collected from the results are to the typical filament lamp graph, the more accurate the experiment has been.

However, if the graphs begin to show the shape of the filament lamp graph, yet do not fully show the shape, the investigation could be extended by taking more recordings, and drawing a new graph. The investigation could be extended further by using the same circuit to investigate a different component, such as an electric heater and seeing how the current in the heater changes as the voltage is altered. Alternatively, a different variable could be controlled, such as the number of components in a circuit, instead of the voltage.

Already during the course of this academic year, I have carried out a number of experiments in class involving different components, such as diodes, although not in as much detail. The circuit diagram used in this experiment could merely be adapted from the other experiments, substituting the previous components for the filament lamp.

Bibliography of Scientific Knowledge:

i.) http://www.howstuffworks.com – “How Light Bulbs Work”

ii.)http://web.mit.edu/invent/www/inventorsA-H/edison.html –

“The Scientific Career of Thomas Edison”

iii.)Physics For You by Keith Johnson

iv.) GCSE Physics Revision Guide – Published by CGP publications

Variables:

i.) Cross-sectional area of the filament

ii.) Material type of the filament

iii.) Length of the filament

iv.) Voltage flowing through the circuit

*NB: As the voltage is increased during the course of the investigation, more current will flow through the wire, causing the temperature to increase. It is already known that temperature affects the resistance in a circuit, yet, it cannot be controlled as a variable because the filament glows, giving off light and heat as energy, as it reaches a high temperature. Therefore, if the temperature was controlled as part of fair testing, the filament would not glow, and there would be no experiment!

Chosen Variable: Voltage, as it can be easily controlled and accurately changed and measured, thus giving a conclusive range of results, and values which can be plotted easily onto a graph.

Apparatus:

Filament Lamp

DC Power Supply

Ammeter

Voltmeter

Wires

Circuit Diagram:

Fair Testing and Safety: In order to make the results acquired as accurate as possible, each voltage should be repeated three times, and the mean average of these three should be taken as the final reading. This would increase accuracy.

The same pieces of apparatus should be used for each repeat, as even a slight variation in a piece of the apparatus such as the length of one wire may change the resistance in the circuit.

Although the internal temperature in the filament lamp will increase as a indirect result of the voltage in the circuit being increased, the external temperature should be kept as consistent as possible. This is because the external temperature may affect the temperature of the wire, causing the electrons to gain more energy.

All typical safety rules for lab work should be followed. These include no running whilst experiments are in progress, hair tied back, jewellery tucked away and carrying out all experiments on benches clear of school books and standing up with stools tucked under benches. Also, as the voltage is increased more current flows through the circuit. As this happens, it is extremely likely that the apparatus shall become hot, so particular care should be taken whilst handling it. In order to protect the benches, any apparatus which is likely to become hot should be placed on a heatproof mat.

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. The results gained were recorded in a table, and once the voltage had been increased up to 6V, the experiment was repeated twice more. Once all the required values had been gained, the three separate results for each voltage were added together and divided by three, in order to find the average current. Once this had been discovered, Ohm’s law (R=V/I) was employed to work out the resistance in ?. This mathematical process was repeated for each of the voltages.

These values were then plotted onto a line graph with intention of discovering if the experiment followed the general trend of graphs involving filament lamps not being straight lines, therefore making them non-ohmic conductors.

After graphs had been plotted showing the relationship between voltage and current, the graphs were analysed and any anomalous results were accounted for. The graphs then were used in order to draw a conclusion and to help explain how the current in the lamp changed as the voltage was altered.

Results:

Voltage (V)

Current (I) 1st Repeat

Current (I) 2nd Repeat

Current (I) 3rd Repeat

Average Current (I)

Average Resistance ?

0.5

0.51

0.60

0.57

0.56

0.89

1.0

0.63

0.69

0.68

0.67

1.49

1.5

0.70

0.77

0.75

0.74

2.03

2.0

0.80

0.82

0.82

0.81

2.47

2.5

0.87

0.90

0.88

0.88

2.84

3.0

0.94

0.95

0.95

0.95

3.16

3.5

0.99

1.01

1.01

1.00

3.50

4.0

1.05

1.07

1.07

1.06

3.77

4.5

1.10

1.12

1.12

1.11

4.05

5.0

1.15

1.18

1.17

1.17

4.27

5.5

1.21

1.22

1.21

1.21

4.55

6.0

1.25

1.26

1.26

1.26

4.76

For each different voltage, the three current repeats were added together and then divided by three to calculate mathematically an average current. From these results, a line graph was plotted using the voltage (V) and the average current (I), in order to discover how the average current in the circuit changed as the voltage was increased. There were no anomalous results present on the graph, which is an indication that the test was carried out fairly and accurately.

However, the line of best fit only began to curve slightly towards the end of the results, and therefore was starting to show that the filament lamp in an non-ohmic conductor. I then decided to extend the range of results until 10.0V, again taking three repeats of the current. In order to continue ensuring that the test was indeed fair, I used the same equipment as I did for the first part of the results. I followed the safety and fair testing requirements as strictly as before.

Results (Part ii):

Voltage (V)

Current (I)

1st Repeat

Current (I)

2nd Repeat

Current (I)

3rd Repeat

Average Current (I)

Average Resistance

?

6.5

1.31

1.32

1.31

1.31

5.00

7.0

1.34

1.35

1.35

1.35

5.19

7.5

1.39

1.38

1.39

1.39

5.40

8.0

1.41

1.44

1.42

1.42

5.63

8.5

1.48

1.45

1.43

1.45

5.86

9.0

1.47

1.47

1.46

1.47

6.12

9.5

1.51

1.49

1.50

1.50

6.33

10.0

1.52

1.52

1.53

1.52

6.58

Although the aim of the investigation was to discover the relationship between voltage and current in a filament lamp, I also took the average resistance and plotted it against the voltage to show that as the voltage is increased in a circuit, the resistance will also increase, due to the change in temperature which occurs as a result of more energy flowing through the filament.

Voltage (V)

Resistance ?

0.5

0.89

1.0

1.49

1.5

2.03

2.0

2.47

2.5

2.84

3.0

3.16

3.5

3.50

4.0

3.77

4.5

4.05

5.0

4.27

5.5

4.55

6.0

4.76

6.5

5.00

7.0

5.19

7.5

5.40

8.0

5.63

8.5

5.86

9.0

6.12

9.5

6.33

10.0

6.58

Analysis of Results: The second line graph showing the relationship between the voltage and the average current in a filament lamp produces a curve. The curve begins very slightly at first and then gradually becomes steeper; similar to the one of x2 . It shows that as the voltage was increased, the current that was flowing through the wire also increased. There were no identifiable anomalous results in either of the two graphs showing the voltage/current relationship.

The line graph showing the relationship between the voltage and the average resistance in the filament lamp also produces a curve extremely similar to that of the line graph showing the relationship between the voltage and the average current. There is only one identifiable anomalous result on the voltage/average resistance graph at 7.0V. This is most likely to be down to human error when recording the results either during the experiment or when plotting the graph. It may have also been a mistake made when calculating the average resistance, but as I checked all the resistances three times, this scenario is less likely.

Conclusion: From the results recorded and the line graphs produced of the results, I feel that I have proved my prediction from the beginning of the investigation correct. The results turned out as I expected, because they show that the filament lamp was indeed a non-ohmic conductor. This is because as the chosen variable of voltage was increased, more current flowed through the circuit, yet it did not obey Ohm’s law (R=V/I) because the voltage was not directly proportional to the current, causing the line of best fit to curve. All graphs drawn from the results values were similar to the typical non-ohmic conductor sketch graph I included in the scientific knowledge.

As the both the voltage and the current in the circuit were increased, the temperature also increased. This was also reflected on the line graphs as it is already accepted that the higher the temperature is in a circuit, the higher the resistance will be. It is already widely scientific knowledge that Ohm’s law also states that the steeper the line of best fit is in a graph such as this one, the lower the resistance will be in a circuit and the flatter the line of best fit, the higher the resistance. Therefore, as the voltage increased, the line began to curve and become flatter, because of the higher temperature, and higher resistance, in the circuit.

Evaluation: As the aim of the investigation was fulfilled, and my prediction was proved correct, I feel that it was successful. I also think that the results were accurate enough to prove the hypothesis correct, as the measures taken to ensure that the results were taken under entirely fair conditions were strict. I also think that the results are reliable because the experiment was repeated three times and the average current was taken as the final result, which would be more accurate than taking merely the one set of results and plotting those values on the graph. The results were taken within one double period in the laboratory, allowing my group to use the same equipment for each repeat. When it was discovered that the first set of results only began to show the curve, the circuit was fortunately still set up, allowing us to continue from where we left off. However, this is the only part of the investigation where the fair testing was slightly lax, as the filament lamp had had a chance to cool down when we started to continue the experiment, meaning that the resistance in the circuit would probably have decreased slightly. With hindsight, I should have drawn graphs using the preliminary results, which would have shown that the values only started to curve. We would have then known to continue to 10V without stopping for a while at 6V.

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.