I will begin the investigation by carrying out a preliminary experiment to check whether it is fair, and if I am collecting enough results.
Factors
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Temperature: I know from my studies that if the wire is heated up, the atoms in the wire will begin to vibrate because of an increase in energy. This causes more collisions between the atoms and the electrons, which causes an increase in resistance. From this information, I know that it is important to try and keep the temperature constant so that my results are fair. The big heavy metal atoms that the wire is made of just “sit” there while the current of electrons flows along, and some of the electrons collide with them. When an electron collides with an atom, some of the electrical energy it is carrying is transferred to the atom as kinetic energy. This makes the atom vibrate more – and we see this as the wire becoming hotter!
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Material: The type of material of the wire will affect the amount of free electrons, which are able to flow through the wire. The number of electrons depends on the amount of electrons in the outer shell of the atoms. If the material has a high number of atoms, then there will be more electrons. This causes a lower resistance because of the increase in the number of electrons. Also if the atoms in the material are tightly packed, then there will be more collisions, increasing the resistance so it is important that the Nichrome wire is kept the same throughout the experiment, and not changed for a different material.
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Wire Width: Since current is the movement of free electrons in a circuit then the number of atoms in a wire make a big difference as to how many electrons can flow at any given time. The bigger the diameter a wire is, the more atoms there are in the wire, so the more free electrons. The smaller the diameter of wire the fewer the number of atoms so the fewer the number of free electrons. In other words, large size wires will have more atoms therefore more current and small size wires will have fewer atoms and therefore less current. If we want to control the amount of current flowing in a circuit we can use smaller wire to allow less current and larger wire to allow more current. This is why it is important to keep the diameter of the wire constant.
Preliminary Experiment
I carried out a preliminary experiment to help me with my predictions, and also to help illustrate any flaws in the method, and to make sure I would get an accurate set of results for the main investigation. I first took the meter of wire, and taped it to the meter rule using selotape. Then I proceeded to take readings from the voltmeter and ammeter for every 10 cm that the wire was extended. I extended the wire by attaching 1 crocodile clip to one end of the wire, and moving the other one along the stick, thus increasing the length of the nichrome wire. I found out during the preliminary test that my initial prediction was correct, that as wire length increases, resistance increases. A problem highlighted during the preliminary test was that the lowest measurement of wire (10cms) got red hot while the readings were taken from the voltmeter and ammeter. This could have affected the results taken.
I have shown the results of my preliminary results in the table below. I set the input P.D on the power pack at 2v for this preliminary experiment.
See graph “preliminary results graph”
They show that as I increased the length of the wire, the resistance also increased.
Hypothesis
1. I predict that as the length of the wire increases, the resistance will also increase in proportion to it. I have come to this conclusion because of my previous studies of resistance, and because of my preliminary results. My logic is that I know that atoms in all conductors have free electrons in the outer shell of their structure. It is because of this structure that the outer electrons can move freely about. I have shown below in the diagram how the electrons move when there is a potential difference passed through a conductive material.
The atoms are arranged in a honeycomb shape.
The free electrons drift around inside the wire, in the general direction of the positive terminal. This is called the electron drift current. As the electrons drift toward the positive terminal, they collide with the metal atoms that make up the wire. These collisions cause the electrons to move more slowly, which in effect causes resistance. If the length of the wire is increased, then the resistance will also increase as the electrons have will have a longer distance to travel, thus more collisions will occur. Due to this, the length increase should be directly proportional to the resistance of the wire.
2. I also predict that as we increase the potential difference in the circuit, the resistance of the wire will increase. I know that current is directly proportional to P.D (potential difference) so if I increase one, the other should increase. I also know that an increased P.D causes the atoms in the wire to vibrate, thus causing heat, and more collisions between the atoms and free electrons. These collisions should slightly increase the resistance.
Results
Analysis
The first part of my predictions was correct, that as I increase the length of the wire, the resistance would increase in roughly a proportional manner. You can see this in my graphs. I have used lines of best fit to show the relationship between length of wire, and resistance. All the graphs are linear. By lengthening the wire, this increased the amount of wire that the electrons had to flow through. The electrons had further to travel, therefore made more collisions with the atoms of the nichrome wire.
My other prediction however was not correct. My theory was that, by increasing the P.D on a length of wire, the resistance should increase, due to the increased kinetic energy produced as the electrons flow around the circuit, thus causing more collisions with the atoms, and generating friction and heat. This makes the atoms vibrate faster, causing more collisions with the electrons. This however was not the case. If you refer to my combined results graph, I would expect the 8v circuit to have the steepest line of best fit, but instead the 2v circuit was the steepest. I think this is because when taking the readings from the ammeter and voltmeter, the values were constantly changing, so I had to try and record the average of the changing results. For example if the voltmeter read 1.04v then 1.06v then 0.07v, I would estimate roughly the middle value of the results to be 1.06v, and this is not a very accurate way of obtaining results. This could have caused my results to be unexpected, and against the theory. This method of reading could also have caused my anomalies (circled in pencil on the graphs). I could have recorded the wrong result by accident. This could not have been the case with the anomalies on the 8v circuit graph and the 6v circuit graph however, because the ohm reading has actually decreased from the previous result, whereas the rest of the results have increased. There are some possible reasons for this.
- I could have read the readings wrongly, and recorded the wrong results.
- I could have accidentally moved the crocodile clip, thus decreasing the amount of wire that the current must flow through.
- I could have inadvertently altered the potential difference flowing into the circuit on the power pack by knocking the tuning knob.
- There could have been a kink in the wire, therefore increasing its length.
Evaluation
I think on the whole, my experiment went smoothly apart from the anomalies that I collected, that did not correlate with the rest of my results. The test was fair, although there are some variables that I should have tried to keep constant. The temperature of the wire was a major factor. If I could have kept the temperature of the wire constant throughout the experiments, I would have obtained more consistent results. This is because as I have already explained in my predictions, as the temperature increases, the atoms vibrate in the wire more, and this created more collisions with the electrons causing more resistance. When the wire was shorter, and the P.D was 6v and 8v, the wire got read hot, and this would surely have affected the results that I took.
One way in which I could have combated this, would have been to submerge the wire in water, so the water molecules would absorb the heat from the wire. This would have kept the temperature of my wire constant, and therefore made it a fairer investigation.
I could also have carried out the experiment with a longer section of wire, so that I could note if the pattern of results differed if the wire was longer. I would have tested using a 200cm length of wire submerged in water.
I would also have carried out the whole investigation twice, and taken an average of the results. This way I would get more accurate results, and I could discard any anomalies.
My method was suitable, but with these changes, I would have got more accurate results, and made the experiment fairer.
Due to the knowledge that I have obtained from this investigation, I could carry out a follow-up investigation to learn more about current, and resistance. I could change the width of wire that I take the voltage reading from, and see if altering the width of the wire affects the resistance. I would predict that as the width of the wire is decreased, the resistance would increase. I have illustrated this in the diagram below.
*The diagram above shows a model of charge moving from a connecting wire to a thinner piece of test wire. The thin wire has less conducting electrons than the same length of connecting wire, and so the charge has to move faster to maintain the same current or flow rate through the thin wire. This should lead to heating of the thin wire, as the electrons transfer their energy in collisions with the metal atoms. The greater the number of collisions, the more energy is transferred as an electron passes through, and the greater the resistance to charge flow.
This is similar to trying to suck milkshake up through a thin straw, or through a thick straw. The milkshake will move more easily up the thick straw, which has a larger diameter. The relationship is one that is "inversely proportional". In other words, there is a direct link between the thickness of the wire, and the loss or resistance, and the larger the area of a cross section of the wire, the lower the resistance.
To keep this experiment fair, I would: use the same connecting wires, submerged the test wire in water, kept the P.D constant, kept the wire material the same and kept the length of the test wire constant.
