However, for our experiment I would predict a graph like this:
This is because I think the resistance of the light bulb will not remain constant, but will increase due to an increase in temperature. As the temperature rises the filament in the bulb will begin to heat and the tungsten atom’s vibration will get larger. This makes it harder for the electrons in the circuit to pass through the filament. The gradient for the graph represents the resistance and therefore an increasing steepness means an increasing resistance.
The equation for resistance is:
Resistance (Ω)= p.d across resistor, in volts (V)
current flowing through it, in amperes(I)
As the current passing through the circuit is increasing less than an ohmic conductor would due to the obstructing atoms of tungsten, and the voltage is increasing at the same amount as it would in an ohmic conductor the resistance will therefore increase making a curve in the graph.
My preliminary work involved carrying out the experiment to help decide upon my range of values, to find out any things that I was unsure about in the set up of apparatus, and to discover the position of the curve for taking more readings.
From this preliminary work I discovered the position of the curve. It appears that the heat rises, and therefore resistance (shown by the gradient on the graph) forms a curve. Then when the heat reaches a constant, maximum temperature, the graph becomes a straight line.
Here is the graph plotted from the readings I attained for my preliminary experiment:
I have now adjusted my choice, and I am going to take my voltage readings from 1.50 volt to 6 volts increasing by half a volt each reading and then from 6 volts to 11 volts increasing by one volt for each reading. In this way I will be able to gain more information about the curve in the graph, and plot it more accurately.
I will also take some results on a lower setting on the power supply in order to investigate the bottom of the curve.
From my experiment I attained the below readings, having carried out the experiment two times. My readings were taken to 2.d.p and my averages have been rounded to 2.d.p.
Results taken on a 4V setting
I also chose to take results on a lower voltage from the battery pack to see the effect it had on the current and voltage, and therefore the resistance. It enabled me to take readings on a much lower voltage, and see if my results were constant. I also wanted to see if the curve happened in the same place, and whether a smaller voltage would form a continuation of the graph. I took these results twice also rounding my averages to 2.d.p again:
Below is a graph to represent the average data:
- From my graph I can see that as the voltage increases so does the current, but they are not directly proportional to each other.
- A filament light bulb is not an ohmic conductor due to the curve in the graph. Ohm’s law states: ‘the current flowing is proportional to the voltage as long as the temperature remains constant’ Therefore the only reason for this curve is an increase in temperature from the heat energy generated by the light bulb.
The graph had this curve shape because the resistance of the light bulb did not remain constant, but increased due to an increase in temperature. As the temperature rose the filament in the bulb began to heat and the tungsten atoms' vibration got larger. This made it harder for the electrons in the circuit to pass through the filament. The gradient for the graph represents the resistance and therefore an increasing steepness means an increasing resistance.
My prediction that the light bulb was not an ohmic conductor and there would be a curve in the graph was correct, however I predicted the curve in the graph higher up because I thought it would remain a similar temperature and then heat up after a period of time. I found out that as the temperature begins to increase, causing the resistance to increase, a curve is formed. Then the temperature reaches a constant maximum temperature and so the resistance remains the same forming a straight line.
From placing both sets of results on the same graph I can see that they almost form a complete graph, which curves mainly at the bottom due to the immediate temperature increase. From both graphs, but particularly the graph taken on a 4V setting because that is where the curve began I can see that the biggest temperature increase happened at the beginning and then became smaller and smaller. As seen from the table below resistance began at 1.11Ω, then increased a large amount to 1.42Ω, and then increased a smaller amount to 1.69Ω. This tells us that the temperature increasing rapidly forms a rapidly increasing resistance, and then as the temperature increase slows so does the resistance.
These numbers show the increasing resistance (Ω) every 0.25 volts passed through the bulb, worked out using the formula R=V/I to two decimal places. I have used the correct averages, not the averages after they were rounded. This clearly shows that as the current increased the resistance also increased due to a change in temperature. If I was to continue this graph using the 12 V supply I would expect at the end to see that the resistance remained constant, as shown by the straight line on the graph. This would be because the temperature had reached a maximum and was no longer rising.
- My conclusion is definitely reliable because I have repeated my results twice, receiving very similar findings. I then repeated my results a third time and fourth time, on a lower voltage setting and found similar results.
- However, I have found some anomalies. The 4V supply readings are slightly out with the 12V supply readings and do not form one complete graph due to the change in power supply. When I did the table of resistance above I also found some unexpected results. The resistance increase appears to gradually decrease apart from between the 11th and 14th points where it appears to remain constant, except for a dip at the 12th point to a 0.11Ω increase. I do not know why this happened, perhaps because these results were taken towards the end of my 4V readings the temperature had almost reached its maximum.
- I didn’t find many problems with the procedure, except for the variable resistor. It was difficult to control to receive voltage readings with the same gaps and if I were to repeat the experiment I would want a device in which I could type in the voltage making it more accurate. However, I could have improved this by measuring along the resistor so the slide could be set at exactly the same point each time.
There is one extension exercise I would like to carry out to provide additional relevant evidence. This involves discovering the length of time it takes for the bulb to reach a constant temperature and therefore become an ohmic conductor. To do this I would need the following apparatus:
- A 12 Volt power supply
- The ammeter setting on one digital multi-meter
- The voltmeter setting on another digital multi-meter
- A variable resistor
- A tungsten light bulb
- A mercury thermometer
- A clamp to attach the thermometer to the bulb
- A stop clock
Firstly I would set up the same circuit as for the experiment above:
- To begin with I would repeat the above experiment, but using a stopwatch and thermometer to mark down the temperature increase at certain intervals.
- I would use a preliminary experiment to discover the best intervals for this.
- I would then repeat this to ensure I had similar results.
- After finding out the length of time to reach a maximum temperature I would then carry out some different experiments:
- To allow the bulb to reach this maximum temperature and take a set of results to investigate ohm’s law.
- To validate my previous experiment by leaving the same amount of time between each reading. Using my temperature chart I could investigate the reason for the curve being more profound at the beginning and whether it was because the temperature increased in much larger intervals.
I would want to receive a table like the one below:
From this I could plot various graphs and perform various investigations such as resistance plotted against temperature and time plotted against temperature.