Different materials will have different levels of resistance. A piece of plastic would have really high resistance because it is a very poor conductor; on the other hand copper would be a really good conductor because it has low resistance. One of the reasons copper is used in wiring is because it is a good conductor.
When the temperature is increased the resistance will also increase. This is because the particles in the object will start to move around faster. This would make it harder for the electrons to flow through the wire, as there would be more collisions obstructing the path of the electrons.
Ohm’s Law states that the current flowing through a metal wire is proportional to the potential difference (voltage) flowing across it, providing that the temperature is constant. The resistance equals voltage divided by current.
R= V
I
The steeper the gradient the lower the resistance, the flatter the gradient the higher the resistance. The resistance of the graph is equal to the inverse of the gradient of the line.
Resistance = 1 .
Gradient
To find the gradient you would use the formula: -
Gradient = difference in y co-ordinates (Y)
difference in x co-ordinates (X)
However the formula for resistance is
Resistance = Voltage (V) or difference in x co-ordinates
Current (I) difference in y co-ordinates
This is however the reciprocal of the gradient and to get the correct resistance the inverse of the gradient has to be used. Therefore giving the formula:
R= 1 .
Gradient
PREDICTION
I predict that the current will be proportional to voltage if all other factors are kept constant. I will be measuring the current, which is still able to pass through each resistor at different voltages. I think I will be able to draw a straight-line graph of current against voltage, which passes through the origin with my results. This would mean the results would be proportional and that it would abide by ohm’s law.
PLANNED METHOD
To make the experiment a fair test there will be only one independent variable, which will be the voltage. I will also only allow the voltage to go up to a maximum of 5 volts, a heating effect will occur at any current on the resistor but I limit its effect by keeping the voltage low. If the wire gets too hot Ohm’s Law will no longer apply. To make the results more accurate I will take several readings at constant intervals for each voltage. This will therefore allow me to work out the mean so that any anomalous will be cancelled out, therefore giving me a set or more accurate results.
I carried out a trial run before I began my experiment to ensure that all the equipment was working. After testing the Ammeter I realised that the dial was broken as it was moving backwards. Since I had carried out a trial run I was able to correct this and find an ammeter which worked correctly.
To carry out this investigation, I will set up my apparatus, which will consist of a power pack, ammeter, voltmeter, wires and various resistors. A diagram of the apparatus is shown below.
After setting up my equipment I will ensure that the ammeter and the voltmeter are set at zero before I begin to start the investigation. I will take reading as at 0.5 volt intervals and will work out the resistance after taking each of the readings. This will give me 10 readings for each resistor and with repeats I will have 20 readings per resistor. Overall I will have a total of 60 results.
Secondary Data of a Filament Bulb
The results in the table below are actual measurements performed on a 12V Car lamp bulb. Digital metres were used to record these results: current was measured to an accuracy of 0.01A and voltage was measured to an accuracy of 0.01V. My teacher supplied this data and I had not made the measurements myself. This data will also be analysed.
Reference
Potential Difference (Voltage) and Current Measurement for a Filament Bulb.
Claremont High School Physics Department.
RESULTS
ANALYSIS
The results in my results tables and graphs do show that the resisters do comply with Ohm’s law, the current is proportional to the voltage. This is also what I said in my prediction and therefore my prediction was correct. The lines of best fit for the blue resistor are not completely accurate. This is could be the result of inaccurate readings off the ammeter. I expected the voltage to be proportional to the current as if the voltage increases there are more coulombs of charge flowing per second and therefore there would be more energy. This can be proved by looking at the graph. All the points are close to a straight line, which passes through the origin
However the higher the resistance the lower the current because less of the electrons will flow through the wire. This is because the wire particles would be delaying them. For example, the black resistor at a voltage of 2.0V allows 0.4A through. However for the blue resistor, at 0.4A, the voltage is 3.0V. This means that the blue resistor has a higher resistance than the black one because more energy (voltage) is required for the electrons (current) to flow thought the wire. Similarly the blue resistor requires only 3.0V for 0.4A to flow where as the red resistor requires 4.0V for 0.4A to flow through the wire. From these results it can be confirmed that the black resistor has the highest resistance, followed by the blue and lastly the red.
Analysis of Secondary Data on a Filament Bulb
The graph for the second set of data does not show that current is proportional to voltage. The line curves as the voltage is increased. This is because the filament in the bulb will be heated up as the voltage is increased, therefore increasing the resistance. According to Ohms Law or the current and voltage to be kept proportional the temperature of the wire has to be kept constant, which is not the case in this experiment. The resistance is constantly changing making it harder to find its value.
EVALUATION
The results were quite accurate for the red and black resistors; they both form a perfect straight-line graph, which passes through the origin. The results taken for the blue resistor seemed out of place. I found out the mean to get even more accurate results. The graphs give a better view of all the results as they show which of the results were out of proportion to the rest and they can therefore be ignored. The, means that have been calculated from the result tables are not as accurate as the graphs because they do not discount the anomalous results. As a lot of the points do lie close to the line of best fit it implies that most of the results are fairly accurate.
The anomalous results that I have obtained could be the result of inaccurate or faulty equipment. Another reason could be that the meters were analogue meters and therefore had to be read by the human eye so mistakes in the accuracy of the readings could have easily occurred. There could have been a slight temperature increase as the power pack heated up. The blue resistor had a few results, which did not fit in a straight line. As it was the last resistor to be tested the wire could have heated up during the course of the experiment, making the readings less accurate.
To improve the experiment digital meters could have been used to avoid inaccurate readings by the human eye. Also the wire could be given time to cool down in between each test so inaccurate results could be avoided.
Sources of Information
* CD- ROM - Encarta 2000
* SPG GCSE Double Science Physics Revision Guide
2001 Edition
* Potential Difference (Voltage) and Current Measurement for a Filament Bulb.
Claremont High School Physics Department.