With electricity, the property that transforms electrical energy into heat energy, in opposing electrical current, is resistance. A property of the atoms of all conductors is that they have free electrons in the outer shell of their structure. (All metals are conductors and have an arrangement in similar form to diagram A)
As a result of the structure of all conductive atoms, the outer electrons are able to move about freely even in a solid. When there is a potential difference across a conductive material, all of the free electrons arrange themselves in lines moving in the same direction. This forms an electrical current. Resistance is encountered when the charged particles that make up the current collide with other fixed particles in the material and the atoms become positively charged ions. As the resistance of a material increases so to must the force required to drive the same amount of current. In fact, resistance, in ohms(R) is equal to the electromotive force or potential difference, in volts (V) divided by the current, in amperes (I) – Ohm’s law.
As the length of the wire is increased, the number of collisions the current carrying charged particles make with fixed particles (ions) also increases as the ions provide resistance to the electrons. Therefore, the value for the resistance of the wire becomes higher and in hitting the ions creates heat via friction. The material and cross sectional area of the wire is constant throughout the experiment. Therefore, it is clear from the formula that the resistance should be directly proportional to the length.
I will be using a rheostat (or variable resistor) to moderate the flow of electrons keeping the temperature of the wire constant to prevent anomalies in my data caused by heat. (see above for reason why)
Key factors: in this experiment, we will only change one factor, the length of the wire. This should affect the resistance of the wire in the ways stated above.
Fair test: in this experiment we are only changing one factor – the length of the wire, the factors that we are going to keep the same are as follows:
- We must keep the surrounding room temperature the same or the particles in the wire will move faster (if the temperature is increased) and this will therefore have an effect on the resistance.
- The cross sectional area of the wire must be kept constant throughout as well. Because if it isn't kept the same the resistance will decrease as the width increases.
- The material of the wire must also be kept the same as different materials have different conductivity. The last two factors will be kept the same by using the same wire all of the way through the experiment.
- The current that we pass through the wire is to be kept the same, also. If this is changed the temperature of the wire might change in a way that is not constant making the results more confusing and my test unfair.
I will be using a piece of nicrome wire 28swg at 6 cm intervals from 0-30cm. The power pack will be set at 4 volts and using the variable resistor, I will keep the current flowing constantly at 0.25 amps.
Equipment:
- 1 x Power Pack
- One x Voltmeter- I will use a digital voltmeter of range 0-5V to avoid parallax error when recording the voltage readings.
- One x Ammeter- I will use a full-scale deflection ammeter of current range from 0-1A because I only want to measure small currents passing through the circuit and not large currents. I am keeping the current passing through the circuit constant, by passing a small current of 0.25A throughout the whole experiment. This will maintain the temperature of the wires in the circuit, because large currents would heat up the wires thus damaging the circuit and affecting the resistance of nichrome.
- 5 x wires (with crocodile clips)
- Wire of varied length (Controlled variable) – We are given strips of nichrome wire attached to cardboard with 2 cm measurements drawn onto them up to 30cm, as we didn't know how accurate they were decided to still measure the wire length and then attach the crocodile clips. Therefore, we decided to measure lengths at 6, 12, 18, 24, and 30cm.
- Temperature (room temperature)
- Wire material
(Dependent variable) nicrome wire 28swg
(Independent variable)
Plan: Set up circuit B using apparatus. We will move the crocodile clips attached to the nichrome wire, up or down to make the distance between them the specified amount of cm (6, 12, 18, 24, 30cm). We will record the current (I) and the voltage (V). Making sure we keep the current constantly at 0.25A. To do this we will adjust the rheostat (variable resistor) to control the current. Which will hope can keep the temperature down?
We need to keep the temperature down because as the current is passed through the wire, the electrons collide with the ions, and the ions resist the electrons, this creates heat via friction. We will use Ohm's law to find the resistance of the wire with the equation
Safety: This is not a very dangerous experiment but despite this, you must always handle electricity with care, keep the current low, handle with dry hands, and try not to poke eyes out with the wire or burn fingers.
Accuracy: to keep this experiment as accurate as possible we need to make sure, firstly, that the length of the wire is measured precisely from the inside edge of the crocodile clips, making sure that the wire is straight when we do this. We must also make sure that the wire is straight when we conduct the experiment. If it is not, short circuits may occur and bends and kinks in the wire may affect the resistance. The reading that is taken from the voltage should be done promptly after the circuit is connected. This is because as soon as a current is put through the wire it will get hotter and we want to test it when heat is affecting it the least, i.e. at the beginning.
Observations: we will observe the reading on the voltmeter change as we change the current to 0.25 A. We also observe a general increase in the voltage, as the length of wire we use gets longer. The rheostat will also be set at different positions for the different lengths of wire that we use.
Evidence: to make sure our overall values are as accurate as possible we will repeat our readings and then take the mean resistance. We will also be able to spot and discard any anomalies from our results.
Results:
Analysis
Trends: The graph is a straight line through the origin, which means R is directly proportional to L. This means that if the length is 40cm, and resistance is 4ohms, then if length is doubled to 80cm, resistance also doubles to 8ohms.
This is because of the scientific idea that if you double the length, you also double the number of atoms and electrons in that length. In doing so, the electromotive force also has to double. The results support my predictions well, the results turned out the way I had expected, and they match the predicted line well. I had predicted a straight line through the origin, which means R is directly proportional to L.
Therefore, this effectively means that if the wire was trebled or quadrupled then the resistance would also treble or quadruple.
from the graph, we can see one very clear trend, which is, as the length of the wire increases so does the resistance of it. Another, more significant thing is that the increase is constant. This is indicating by the fact that the line drawn is a straight one. One may also note that the gradient of the line drawn is __________
Conclusion:
In my prediction, I said that: If the length increases than the resistance will also increase in proportion to the length.
From my graph, I have shown that my prediction was correct, as the Line of Best Fit is a straight line proving that the resistance of the wire is proportional to the length of the wire. The length of the wire affects the resistance of the wire because the number of atoms in the wire increases or decreases as the length of the wire increases or decreases in proportion. The resistance of a wire depends on the number of collisions the electrons have with the atoms of the material, so if there is a larger number of atoms there will be a larger number of collisions, which will increase the resistance of the wire. If a length of a wire contains a certain number of atoms when that length is increased, the number of atoms will also increase.
Evaluation
I feel overall, our results were quite accurate. I know this because my results table does not show any individual anomalous results this means that I did not have to leave any results out of my averages because they were anomalous. In addition, on the graph I can see that none of the averages plotted are anomalous because all the averages lie along the same straight line.
If the results where inaccurate it could be due to the temperature of the wire not necessarily being the same when we conducted the experiment and the material of the wire not being as pure as it should have been. The main reason for the results being inaccurate would probably be due to the equipment that we used being inaccurate. This would not stop us from seeing the trend though, because the equipment would have been out by a constant amount each time therefore there would be a constant error. So the trend that was predicted in the plan would still be shown (the above is speaking hypothetically).
Most errors in our experiment were encountered in the measuring of the wire. This is because it simply was not very practical to hold a piece of wire straight, whilst holding it next to a ruler and then trying to accurately fix crocodile clips to the right part on the wire. In addition, I do not feel that the crocodile clips were always fixed securely to the wire with a good connection. This also meant that they were easy to move about on the wire, changing the length of it. Errors rarely occurred in the setting of the current and the reading of the voltage. The wire was never totally straight when we started the experiment, which may also, as was said earlier on, affect the resistance.
I do not think that doing any more results in our experiment would have made it any more accurate.
During my experiment, I have noticed several modifications I could make to improve on the Investigation if I was to repeat it. I could use pointers instead of crocodile clips; I would do this because pointers would be more accurate. The pointers would be more accurate because the tips have a much smaller area than the crocodile clips giving a more accurate measurement of the length of wire. Perhaps where we to have a bar that didn't bend in place of the wire, it could have been more accurate.
As well as making these modifications, I would also improve my Investigation by testing the same wire but with different widths of that wire. I would do this to expand on my Investigation. I could also try to work out the resistivity of the wire, we could do this by using the formula ( ) and then work out the percentage of error in our results.
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