Key
Variable resistor
Ammeter
Lamp
Voltmeter
Battery pack
Lead
- I will do a preliminary test to find the range of current in amps. I will do this by moving the variable resistor to its lowest and highest points and noting down the amps. In this case the lowest point is 0.5 and the highest is 1.6 amps.
- I will start with 0.5 amps, working systematically, I will note down the voltage every 0.1 amps. I will do this until I reach the highest point which is 1. 6 amps. I will then repeat this to see if my results have changed or are the same.
Fair test: To make this investigation a fair test I will use the same equipment and the same method every time I repeat the experiment so that nothing will change. I will also wait until the lamp cools down so that it is at the same temperature that it was when I first used it.
Obtaining evidence
A table of my results:
I will now find the average voltage. I will do this u by adding the voltage for experiment 1 and 2 and then dividing it by 2. Using the average voltage and current I will find the resistance using the following formula:
R= V/I
(The resistance has been rounded to 1 decimal place.)
Using the results on my table above, I will now draw a graph ( page 5) to show current against resistance. I will draw a line of best fit.
Graph showing the current against resistance
Analysing and considering evidence
Looking at the graph I can see that the resistance is directly proportional to the current. This means that as the current increases so does the resistance. For example, when the current is 1.3 amps the resistance is 6.2 ohms. Further on, the resistance is 6.9 ohms when the current has increased to 1.6 amps. This shows an obvious increase in resistance when the current increases.
During the experiment I changed the amount of current so that it increased. The increase in current causes the lamp to get hotter but also means that it will be harder for the current to flow. All solids (including the leads used in the experiment) are made of atoms that are constantly vibrating. The increase in heat gives more energy to the atoms making them vibrate even more. The leads are metal conductors; increased vibrations means that it will be harder for current to flow as the electrons will collide with the vibrating atoms more. The collisions will slow the electrons down; this is resistance.
The resistance of a filament lamp increases as the temperature of the filament lamps increases. Temperature increases with more current. Therefore resistance will increase as the current increases. This agrees with my prediction.
Evaluation
I think that the results were accurate as the second experiment showed similar results to the first. This shows that the experiment was carried out correctly and as accurately as possible. To be more sure I could have repeated the experiment a further 2 or 3 times.
To make this experiment more accurate I could have used electronic equipment which would accurately measure the volts and amps so that there would be no human errors. To extend this investigation I could have used different types of bulbs with a different size of maximum voltage. I could have taken more readings, perhaps repeated the experiment a few more times and used a wider range of current.
Using my graph I can see that there are two anomalies; the other results are all close to the line of best fit. These two anomalies are near the higher end of the graph and are not near the line of best fit. The reason for these odd results may be because there was human error involved or factors such as the temperature of the lamp. With the time that I was given I think that enough results were taken to support the conclusion.
To help me with my investigation I used the following secondary resources:
- AQA syllabus
- Modular science for AQA
