Resistance is measured in ohms. A resistor has the resistance of one ohm if a voltage of one volt is required to push a current of one amp through it. George Ohm discovered that the emf of a circuit is directly proportional to the current flowing through the circuit. This means that if you triple one, you triple the other He also discovered that a circuit sometimes resisted the flow of electricity. He called this resistance. He then came up with a rule for working out the resistance of a circuit:
V/I = R or
The type of material will make a difference because the electrons have to pass through the material. These electrons find it easier to pass through some materials than others. In this experiment I am going to use copper and nichrome wire. I predict that the nichrome wire will have a higher resistance than the copper wire. I can say this because I know that the electrons have to squeeze together more in order to be able to pass through nichrome wire than they do in order to pass through copper wire. (The more the electrons bump together, the higher the resistance.)
The length of the wire will make a difference. This is because when you have a long wire, the electrons have to squeeze together for longer to be able to pass through the wire than they do in order to be able to pass through a short wire. I predict that the longer the wire, the greater the resistance. If I had a 30 cm wire and a 60 cm wire, the 60 cm wire would have a resistance twice that of the 30 cm wire.
Apparatus List
100 cm rule with
Nichrome wire along it,
A power pack
Crocodile clips,
Wires and
Ammeter.
Step by Step Method
- Get out all the equipment and make sure that the content of the apparatus list (see bottom of page 4) is there.
- Connect the apparatus to look like the circuit diagram above.
- Switch on the power pack at the required voltage (4 V) to make sure everything is in working order and switch off again.
- Move the crocodile clip to the desired length on the wire
- Switch on the power pack and the ammeter. Take the reading off the ammeter and switch off power pack leaving enough time for the ammeter reading to settle. The switch off power pack.
- Make a note of the results (on your table) and repeat two more times.
- Then change the length of the crocodile clip on the wire to whatever length is going to be used, such as 80, 70, 60, 50 etc. cm, and take the reading off the ammeter three times in total for each length. As above.
- Once all the readings for each length that is going to be used have been taken work out the average length and the resistance (to work out resistance use V=IxR).
- Once done change the voltage up to 6V and do the same process as above for each of the lengths. Then work out the average and resistance.
Fair Test
To make this experiment a fair test I will take the results of each length of wire three times in order to get an average reading and to find the resistance. I will also need to do this because if just one result was taken then it could not be determined if that result was a true reflection of the amount of amps in the current or not or whether the reading on the ammeter had fluctuated.
In order to obtain a fair test in this experiment the same 100cm rule and nichrome wire must be use, in order to get a correct result. This is because if the wire is changed the thickness and resistance in the wire will be changed. Therefore to say that this experiment has been a fair test the same wire (which is attached to the rule) must be used.
The same power pack must be used throughout this experiment because the voltage on each power pack may vary even to a decimal number, but the same power pack must be used so that the readings aren’t affected by this slight vary in the voltage.
The same ammeter must also be used to make this experiment a fair test, because the reading on the ammeter will change if a different one is used, which in turn will affect the whole of the experiment!
If the wire happened to break because of too much heat from the amount of current that was going through. Then I would get a new wire of the same material, length and width and start the whole experiment over, so that my results would not be changed drastically right in the middle of the experiment. Then I could write about it analysis and evaluation and say how it could be improved next time.
What results need to be taken?
The results that need to be taken are the:
- Range i.e. 20, 30, 40, 50, 60 etc. cm
- The amps – which would be as a result of the resistance of the length of the wire.
