I therefore predict that when I increase the current through the filament, the resistance will increase. Since we have done work on this in school, I know this will happen because I know that when a current passes through a metallic conductor, it will increase in temperature. The filament in a bulb is made from a metal material, tungsten because of its high boiling and melting points. Also, I know that bulbs heat up when we pass current through them (they glow). Also, Ohm’s law states “Provided the current stays the same, as the temperature varies, the current cannot go up in proportion, so this means the resistance will vary.” In an equation, Ohm’s law is V=IxR. From schoolwork I have done over the years, and by the fact that the current will change, I can easily work out that the resistance will increase.
To measure the resistance for different power levels I will be using the ‘Standard Test Circuit’. This includes a voltmeter to measure the voltage around the bulb, an ammeter to measure the current, and the actual light bulb. However I will be unable to use a variable resistor due to unforeseen circumstances. However, the power source allows me to change the voltage to whatever I please, so that makes up for it.
I predict my graphs to look like the ones below:
These graphs show the relationship between voltage and resistance and voltage and current respectively.
To make this a fair test, I will follow some strict guidelines. They are:
- I will use all of the same equipment for the whole experiment. This is because each piece of equipment has their own faults and niggles within them. If I use a consistent set of equipment, than I can work around their faults, but if I use different equipment, their different faults and different niggles will reek havoc on my results and make them unreliable. If the investigation takes more than one Science lesson, than I will have to group my equipment and store them in a safe place so that nobody changed their status after we have left.
- I will use the same person to record the results throughout the investigation. This is because each person will have a different opinion on each result, and it is better to have one person’s opinion on the results rather than different people’s opinions on the results because the former means that the results will be consistent, whereas the latter will be very inconsistent.
- I will measure each voltage a total of three times. Then using the mean method of working out averages, I will work out an average, which will be used on my graph. This will ensure that any anomalous results won’t totally destroy the experiment. If any result is blatantly wrong on my part, I will retake it to make it slightly more reliable. I will write down the results that I have retaken.
- Because we are working in a classroom environment, there are many factors such as minor room temperature fluctuations, contaminations in the wiring materials, non-accurate measuring instruments and a non-accurate power supply. All the above factors could be improved upon to give more accurate and true results, but this would be quite unpractical on the level that we are doing the experiment for GCSE’s
For the experiment, the ideal amount of results is thousands, but because of the restraints of the accuracy in the classroom and the time period, I will be working with about 36 results, across different voltages. This should let me do enough results to draw a firm conclusion, but will be within any time constraints.
The Results:
Observations and Conclusion:
From my two graphs, it is quite clear that my hypothesis is correct. That when the voltage is low, the current and the resistance will be lower because when there is less voltage, there is no need for high resistance as there is less current. However when there is a high voltage, the resistance will be high as there is more current flowing through. The increase in the current passing through the bulb means that it will get hotter.
This relates to my results because when the voltage was at one volt, the amps and the Ohms were also very low, as there are not as many electrons passing the bulb at one time, so it means that there is less resistance. When the voltage was at six volts, the amps and the Ohms followed accordingly, as there were a few more electrons pass the bulb at once, needing higher resistance, but still very low. When there were twelve volts on the Voltmeter, there were a lot of electrons passing through the bulb at one time, the resistance would need to be high as if not, the bulb could become to hold and melt the filament.
From the conclusion I have drawn, if I keep adding voltages on, the resistance will carry on getting higher and higher as the electrons are passing through the bulb quicker and in greater number. When the voltage gets too high and there is not enough resistance, the filament will get too hot and reach its melting point. Rendering it obsolete.
This is why light bulbs always blow when they are first switched on. When the filament is left dormant, it cools down and the resistance is low. When there is a power surge, the filament quickly heats up and since the resistance is still momentarily low, it causes the bulbs to melt. This is also why filaments have to be made from compounds with extremely high melting points.
Evaluation:
The procedure I used to carry out this investigation I believe was a very good procedure because it has given me relevant results and has obtained me enough evidence to produce the above conclusion. The fact that I didn’t have any anomalous results proved the above statement. The graphs I have drawn show a very clear upward curve, which has proved my hypothesis correct. Also, the results that have been grouped together for each voltage are very similar to each other, thus creating an average very close to the original results. I believe that the results were accurate and quite similar because I had carried out the experiment accurately and thoroughly throughout the test. Either that, or I failed miserably at attempting the experiment each and every time. The latter is quite unlikely.
To improve the experiment, I would probably have to be really professional and a lot more scientifically precise in how I carry out the experiment. I would need to make sure there is a constant classroom temperature, as varying temperatures may affect the filament’s own temperature. If I was going to create a really detailed graph and draw the most perfect upward curve ever, I would probably need to take down hundreds, maybe even thousands of results at least five times to get a very concrete conclusion, but this is quite unrealistic with the limited time and patience GCSE students have. Also, the school equipment is pretty poor standard compared to laboratory equipment, and it comparatively unreliable and inaccurate, and it could produce a glitch in my results. If I had more time, I could try the investigation with different equipment to what I had already used. This could prove that the equipment I used doesn’t matter in the experiment, and it’s the fact that how I carry it out which is crucial.
To extend the project further, we could carry on the voltages to twenty, or twenty-five to see what happens to the results and graphs then. We even could carry the experiment on until the filament melts, and see how much voltage it can tolerate. Also, we could add a lamp or two into the circuit and see if this affects the resistance, voltages or the resistance in any way. Maybe instead of a variable power supply, replacing it with a couple of cells and also adding a variable resistor into the circuit. I would like to know if these affect the filament in anyway, and see if affects the resistance or current in anyway. It would be interesting to find out anyway.
