Resistance is a force. It opposes the flow of an electric current around a circuit. A circuit itself can resist the flow of particles if the wires are either very thin or very long. E.g. the filament across an electric bulb is very thin, as it needs to resist the flow of particles so that the bulb can glow.
Resistance is measured in ohms. This discovery was made by George Ohm. He found out that the voltage of a circuit is directly proportional to the current flowing through the circuit.
It means that if the voltage is doubled then the current is also doubled. . He also discovered that a circuit sometimes resists the flow of electricity. He called this resistance. He also made a general rule for working out the resistance of a circuit:
V/I = R
V – Volts
I – Current
R – Resistance.
Extra Information:
Before starting my coursework I have decided to write down the factors that will affect the resistance of a wire.
Below is a list of factors and reasons why they affect the resistance of a wire. To explain the how the factors would affect the resistance of a wire I have drawn a diagram to show how resistance occurs.
WIRE=
ATOMS=
ELECTRONS=
Resistance occurs when the electrons travelling along the wire collide with the atoms of the wire. These collisions slow down the flow of electrons causing resistance. Resistance is a measure of how hard it is to move the electrons through the wire.
Factors affecting resistance.
1.Temperature: If the wire is heated up the atoms in the wire will start to vibrate because of their increase in energy. This causes more collisions between the electrons and the atoms as the atoms are moving into the path of the electrons. This increase in collisions means that there will be an increase in resistance.
2.Material: The type of material 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 energy shell of the atoms. If the metal has a high atomic number there will be a high number of electrons causing a lower resistance because of the increase in the number of electrons. Also if the atoms in the material are closely packed then the electrons will have more frequent collisions and the resistance will increase.
3.Wire length: If the length of the wire is increased then the resistance will also increase as the electrons will have a longer distance to travel and so more collisions will occur.
4.Wire width: If the wires width is increased the resistance will decrease. This is because of the increase in the space for the electrons to travel through. Due to this increased space between the atoms there should be less collisions.
Prediction:
I think that if the length of the wire is increased the resistance will also increase. I also think that the rate at which the resistance of the wire increases will be directly proportional to the length. The graph to show this should therefore look something like this:
Reason: Due to the collisions, resistance is changed from electrical energy into heat energy. 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 this:
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 metal 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. Resistance is measured in ohms(R), potential difference in volts (V) and the current in amperes (I)
Key factors: in this experiment I 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:
1. 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.
2. The width of the wire must be kept constant throughout as well.
3. The material of the wire must also be kept the same as different materials have different conductivity. Factors 2 and 3 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.
Apparatus:
1. 1m long
2. Power supply
3. Six connecting wires
4. Two crocodile clips
5. Voltmeter
6. Ammeter
Plan:
1. Connect circuit as shown in the diagram.
2. Adjust Power Pack until the voltmeter reads 4V.
3. Secure the connections.
4. Perform the experiment with the following lengths of wire, connected between the two crocodile clips and record the voltage and current for each length of wire.
5cm, 10cm, 15cm, 20cm,… and so on, up to 100cm.
5. Use Ohm’s law to find the resistance of the wire
Circuit Diagram:
Safety: this is not a very dangerous experiment but despite this we must always handle electricity with care, keep the current low, handle with dry hands etc.
Accuracy: to keep this experiment as accurate as possible we need to make sure of the following:
1. 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.
2. 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 in the wire may affect the resistance, also.
3. The reading that we take of 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 least affecting the test.
Upon testing to see if the experiment would work I found no problems with the plan I described earlier. I was able to get the following results:
Length (cm)
Current (I)
Voltage
Resistance (ohms)
10
1.87
2.35
1.26
15
1.47
2.70
1.84
20
1.18
2.95
2.5
25
1.00
3.18
3.18
30
0.87
3.34
3.83
35
0.77
3.44
4.48
40
0.70
3.48
4.97
45
0.63
3.63
5.77
50
0.59
3.68
6.24
55
0.51
3.69
7.24
60
0.49
3.76
7.67
65
0.45
3.79
8.42
70
0.45
3.80
8.44
75
0.43
3.82
8.88
80
0.41
3.84
9.37
85
0.38
3.85
10.13
90
0.36
3.89
10.81
95
0.32
3.92
12.25
100
0.31
3.94
12.71
Figures have been rounded off to two decimal places for the resistance.
Observations:
1. I observed that the reading on the voltmeter and ammeter change as we change the length of the wire. The voltage increases as the length of wire we use increases.
2. The range of current was quite high in the beginning. From 10cm to 50cms the range was 1.28amps. From 50cms to 100cms, the range was only 0.28
Evidence: to make sure our overall values are as accurate as possible we will repeat our readings once more and then take the mean resistance of the 2 readings. I will also be able to spot any anomalies from our results.
Results:
Length (cm)
Current (I)
Voltage
Resistance (ohms)
10
1.85
2.35
1.27
15
1.48
2.72
1.84
20
1.15
2.95
2.57
25
1.00
3.18
3.18
30
0.87
3.34
3.95
35
0.75
3.45
4.60
40
0.70
3.48
4.97
45
0.62
3.63
5.86
50
0.57
3.68
6.46
55
0.51
3.69
7.24
60
0.49
3.76
7.67
65
0.45
3.80
8.40
70
0.45
3.80
8.40
75
0.42
3.82
9.10
80
0.40
3.84
9.60
85
0.36
3.85
10.69
90
0.34
3.89
11.44
95
0.30
3.92
13.10
100
0.29
3.94
13.59
To check to see if my prediction was correct I performed a test with another type of wire. This wire was thicker than the nichrome wire so it should have a lower resistance than the nichrome wire. The apparatus and the circuit diagram are the same as shown for the previous wire.
Table of results. The experiment was performed twice to make the readings more accurate.
Length (cm)
Current (I)
Voltage
Resistance (ohms)
10
20
30
40
50
60
70
80
90
100
This was the first set of results. The next sets of results are as follows:
Length (cm)
Current (I)
Voltage
Resistance (ohms)
10
20
30
40
50
60
70
80
90
100
Trends: 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.
Conclusion: I think that from my results I can safely say that my prediction was right. The resistance did change in proportion to the length. This is because as the length of the wire increased the electrons that made up the current, had to travel through more of the fixed particles in the wire causing more collisions and therefore a higher resistance. We can work out what the resistivity of the wire should be from our results using the formula
Evaluation
I feel that overall our results were quite accurate. This is can be seen when we look at the graph, which shows a straight line with all of the points apart from one being very close to or on that line. The one point that was not that close to the line was a slight anomaly, but it was only slight and did not effect the final gradient of the graph. I have found out that for the wire I was using, the resistivity at 20(c)C is 4.9 X 10-7 ohm-meter. From this we can then work out the percentage error of our results:
The accuracy for this experiment is, theoretically, � 15.7%, but as one can see this does not seem to be the case from looking at the graph. The reason for this could have been due to a number of different factors. Firstly the temperature of the wire was not necessarily 20C when we conducted the experiment and the material of wire may not be as pure as it should have been. The main reason for this was probably due to the equipment that we used being inaccurate. This did not stop us from seeing the trend, though, because the equipment would have been out by a constant amount each time therefore there was a constant error. So the trends that were predicted in the plan still were shown. Also 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. Read also does length of wire affect current essay
The crocodile clips and the connecting leads could have affected the fairness of the experiment. They are a different type of metal from the Nichrome wire and may have different properties and therefore different resistance. Therefore the resistance of the Nichrome wire showed up on the multimeter was slightly more than it actually was.
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