Electrons move more easily through some conductors than others when a pd. is applied. The resulting opposition of a conductor to the current is called the Resistance. A good conductor has a low resistance and a poor conductor has a high resistance. The resistance of a wire of a certain material: -
- Increases as it’s length increases
- Increases as it’s cross-sectional area decreases
- Depends on the material
A long thin wire has more resistance than a short thick one of the same material.
Resistors are Ohmic or linear conductors
R does not change (resistance), when V does (Voltage)
R = V R = Resistance
I
V = R x I V = Volts
I = V I = Current
R
Discussion of preliminary experiment.
Before we did the final experiment to find out if the length of a wire affected the resistance, we did a preliminary experiment. This was going to be exactly the same as the final experiment so we worked in pairs to get used to and so that we would know how to do the final experiment quickly and efficiently.
Here is my table of results for the preliminary experiment.
My Prediction
My prediction for this experiment involving the resistance for a length of wire is that when the current is moved further up the wire; i.e. from0-10, 10-20, 20-30, 30-40, 40-50, 50-60, 60-70, 70-80, 80-90, 90-100, the resistance will get higher due to the fact more current or electricity is passing through the wire. So a long strip of wire will have a high resistance and a short wire will have a low resistance.
From the book “Physics, by Tom Duncan”
“A long thin wire has more resistance than a short thick one of the same resistance.”
Apparatus
Variable power supply, wires, Digital Voltmeter, Digital Ammeter, ruler with thin wire (0.7mm) from 1 end to the other, crocodile clips.
Method
- Firstly take the power supply and set it to 4 volts
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Take one of your wires and connect it to the black terminal of the power supply (this is the ‘negative wire’)
- Now take one of your crocodile clips and connect it to the end of the black wire we just connected to the power supply and connect this crocodile clip to one of the ends of the wire on the ruler (preferably the end that starts with 0, makes recording the results easier.)
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Now take another wire and connect it to the Red Positive terminal of the power supply.
- Connect an Ammeter onto the positive wire and take another wire with a crocodile clip on the other end and use this to connect to the wire we are using to measure the resistance.
- Finally take a Voltmeter and connect it over the length of wire on the ruler with 2 wires (this is used to measure the Voltage going through the wire.)
Safety
CAUTION
The wire can become very hot and will burn you if used incorrectly, always wear a lab coat and also wear gloves to protect fingers and hands.
How many times shall I repeat the experiment to get a fair test.
To get a fair test I will be doing this particular experiment 5 times. I will use all the same components and same wires each time and the same amount of power (4volts).
Range of Results
I will be doing the same experiment 5 times. Each time I do the experiment I will connect and reconnect the wires together and tape down the wire along the ruler again if necessary. My range of evidence will come from the 5 times I do the experiment, then when I have all the results I will average them and get the range, e.g. from 0.34 ohms to 0.50 ohms.
My observations for this experiment.
My observations for this experiment were: -
That the wire got very hot when the length of wire was only 10cm long
- It would also burn the wood and plastic it was attached to.
- As the length of wire increased the resistance got greater.
- The voltage decreased as the length of wire increased.
- The Current increases as the length of wire is increased.
This is my Table of Results for the readings of Volts and Amps. V=Volts I=Current
This is the resulting Resistance of the experiment I did measured in ohms.Ω
This is the average of the 5 times I did the same reading on the same length of wire (resistance).
Observing my Results.
I have observed that from my final results (see above) that when the length of the wire increases the resistance also increase letting less current (I) through but more Voltage. I have also noted that the 5 times I did the experiment the results should stay the same but they didn’t, so I can conclude from this that the power supply was not 100% constant, this cannot be helped by the fact that we are using mains electricity. The reason why mains electricity does not stay constant is because it is A.C which means that the current is Alternating and cant stay constant, so the best source of electricity would be from a source with a D.C current.
Discussing my apparatus, was it reliable?
I will know discuss my equipment and see if it is reliable or not.
Ammeter – This was a Digital Ammeter and therefore would give me a reliable reading although it changed a small degree over time but was reliable overall. This was accurate to 1/100th of an amp.
Voltmeter – This was also Digital so it gave me a clear and precise reading so this was also reliable. This was accurate to about 1/100th of a Volt.
Variable Power Supply – I think that this is definitely more reliable and precise than the one where you have to plug the wires into the correct sockets and you could be out be about 1 volt every time, so this I think was also reliable. This was reliable to about ½ a volt.
Mains power – I think that this was the most unreliable part of my equipment, but couldn’t be helped because there is nowhere else to get power.
Wires – These were reliable because they were constant.
Length of Wire to be Tested – I think that this was unreliable because of when you wanted to clip on the crocodile clip you had to lift up the wire and this then in turn made the wire longer so increasing the resistance and altering your results.
This Particular graph is a ‘line of best fit’ Graph and shows me that when the length of wire increases the Resistance (Ohms Ω) also increases by about 0.66 ohms (averaged).
My Conclusion
My conclusion about this experiment is that when a wire that is subject to an electrical current this produces resistance and this is measured in Ohms. As the length of the wire increases the voltage (amount of power) decrease and now that there is more ‘space’ in the wire more current can pass through and in turn the resistance grows with the current passed through the wire. I can use my scientific knowledge to back up my conclusion. I can explain the change in voltage by “Voltage is the difference in Electrical Potential between 2 points”,(this is my scientific knowledge to explain my validity of my conclusion); this is saying that is the length in between the 2 points increases and the potential energy grows less and therefore passing less voltage through a circuit.
I think that my initial results support my final results because I predicted the right result and my prediction supported the final result by the way it explained what would happen in the experiment.
Was the method good?
I think that if I did this same experiment again I would change some things like some of the equipment, but the method I used worked well enough for what I wanted it to do, so if I could change some things I would choose to change some of the equipment like the power supply and use more accurate equipment. I would also change the way I connected the crocodile clips to the length of wire because when you try to connect them to the wire you have to lift up the wire and this in turn makes the 100 cm longer by about 3cm so thus making the results more unreliable.
Reliability of Observations?
I think that my observations were reliable and that my graph showed me that there were no anomalies that I was aware of, the normalness of the results of the graph supports my firm conclusion.
