History on Goerge Ohm
Goerge Ohm devised the relation ship between voltage and current. This was; Resistance = Voltage (V)/Current (A)
There is also a way to work out the voltage and current by using the resistance and either the voltage or current and these are;
Voltage (V) = Current (A) * Resistance
Current (A) = Voltage (V) / Resistance =
The circuit below can be used to find how the current through a conductor depends on the voltage across it. The conductor in this case is a coil of nichrome wire measured at a length of 20cm kept at a steady temp.
If you double the voltage by doubling the batteries the current doubles and so on. The voltage divided by current will always have the same value. This is Ohm’s law. All alloys obey ohms law as long as the temperature is constant.
Ohmic and non Ohmic conductors
Here are some Ohmic and none Ohmic conductors;
A resistor is a none Ohmic conductor because it has a resistance which increases slightly when the temperature rises.
A thermistor a none Ohmic conductor because has a high resistance when cold but low resistance when hot.
A light dependent resistor is a non Ohmic conductor because has a high resistance in the dark but a low resistance in the light.
A diode is a none Ohmic conductor because it has an extremely high resistance in one direction but low in the other.
A wire is an Ohmic conductor but a bulb is not.
Preliminary Investigation
The purpose of the investigation is to prove Ohms law. This states that the current is directly proportional to the voltage as long as the temperature is constant. For instance if I doubled my 2nd result in the following table I should get the same outcome as I did for the fourth result.
Method
- To set up a circuit like the one below. (The thicker part of the wire indicates the 20cm of nichrome wire.)
- Measure out an exact length of nichrome wire that is 20cm long – 20swg and connect it to the circuit on the part on the diagram where the red line is.
- Set up a power pack and set it to 2 volts to start with
- Take accurate readings of 1 volt and higher
- Switch off the power pack after each reading so it can cool down
- Repeat 3 times at voltages 3, 4, 5 and 6.
Results table
Graph
The graph is on the next page.
Analysis
From these results we can see that the resistance is constant. The voltage at test are i.e. 0.96V, when doubled gives a doubling in the current. This can be seen on the graph.
On the graph for this experiment I have circled two points that are not on the line these would be points in the graph where the wire got too hot and therefore gave me a miscalculation this might be because the meters are not accurate.
My preliminary work has proved that the wire obeys Ohms law, as long as the temperature is constant. You can see from the calculations of resistance that is relatively constant throughout the experiment only vary by 1/10 of an ohm at the higher voltages.
From my results I can see that this is an Ohmic conductor because it follows ohms law.
In the main investigation we will examine what happens in a light bulb and compare the two.
Prediction
For the main experiment where you do the same experiment as the one we did in the preliminary experiment apart from we will be using a bulb instead of a wire.
I predict that the bulb will not obey Ohms law because in order for a bulb to work electricity is passed through a thin piece of wire (the filament) and the gases inside of a light bulb force the filament to burn brightly in order for it to light up a room so the temperature is never constant it is always rising. But as it happened twice in the previous experiment the wire got hot because I did not leave it long enough to cool down and therefore it gave me an inaccurate result therefore the temp wasn’t constant. But with a bulb there is not a chance that it will be able to cool down unless you turn it off for a period of time but that would not be done in a house for it is not practical. So the filament will just keep getting hotter and hotter therefore giving you a result which will be correct to begin with where current is proportional to the voltage and then it will start flattening out because of the heat therefore I will have proved Ohm’s law, where the temperature must be constant so it does not increase resistance.
Method
When I carry out this experiment I shall follow this procedure;
- I will set up a circuit so it looks like the diagram below.
- Set up a power pack and set it on 1 volt.
- Take a reading and write it on my results table then turn off the power pack for 10 seconds then turn it on and take another reading.
- I will repeat this until I have 3 results then repeat this on voltages 2-10 volts
I am using a power pack instead of batteries and a voltmeter so my readings will be more accurate.
Results
Analysis
On the graph I have drawn some lines on top of the curve and labeled them A and B, if the bulb was an Ohmic conductor there would be a straight line in stead of a curve because you should be able to double A (voltage 4, 1.2 amps) and by doubling it you get voltage 8, 2.4 amps and that would be on the straight line if it was an Ohmic conductor and obeying Ohm’s law but this is not an Ohmic conductor because if I doubled the voltage from 4 to 8 it gives you a current of 1.725.
On my preliminary experiment I used a 20cm long piece of nichrome wire. The nichrome wire didn’t heat up as quickly as a filament and it cooled down quicker than the filament so my results did not get affected as much from the heat this is why I got a fairly straight line with only a couple of points at most 1/10 off the line.
My prediction was correct because the light bulb is not an Ohmic conductor because the heat is increasing the resistance more and more as the experiment goes on and when I pass 10 volts through it the temperature rose therefore the resistance rose. As I explained in my preliminary work where the heat increases the molecules vibration and therefore increasing the chances that the electrons will collide with them.
A light lights up because you are passing a current through a thin piece of wire (the filament) and the gases in side of the bulb forces it to glow brightly. There is no way of controlling the temperature in side of a light bulb so the heat increase resistance by forcing the molecules inside of the wire to vibrate therefore slowing the electrons down and increasing resistance and proving Ohms law only works when the temperature is constant. That heat increase resistance therefore current is not proportional to the voltage.
On my results table where I worked out there resistance the does not seem to be any pattern in my results only that they rise every time. The results show that the increase of resistance is very erratic where it can rise about 0.70 ohms at on result to another and about 0.30 at another result. This does not prove anything.
Therefore I conclude that when you pass a current through a wire of some sort and it heats up and it will increase resistance. But if the wire cools down the molecules do not have the energy to vibrate therefore there is a minimal chance that the electrons will collide into them and it lowers resistance therefore when Ohm’s law is present current is proportional to voltage just as it was in my preliminary experiment.
Evaluation
The experiment that I have carried out is good enough to produce reliable results but I think that it would be more accurate by repeating it a few more times and instead of repeating 1 volt three times then moving on to the next one that I should of done 1 volt once then 2 volts and when I got to 10 volts started it again and done it that way three times and also the results at the end began to go further and further away on the same voltage.
All of the results that I did fit the pattern none where really off but I could of got them better if I had left the bulb to cool down a little longer and for a set time. I think that my results on both experiments the preliminary one and the main one are good enough and reliable enough to support my conclusion.
The only improvements that I can think of for further work is to take three or fourth decimal places instead of just 2, to have a set time to leave my bulb to cool down for say 30second, to repeat the experiment up to 15volts to give you a bit more of a bigger picture, by repeating 1 volt three times then moving on to the next one that I should of done 1 volt once then 2 volts and when I got to 10 volts started it again and done it that way three times, also to use more scientific meters to record my results with and also by repeating the experiment with different light bulbs to see if you would get different results. The only reason that I did not make these changes is because I did not have enough time in the lesson to do so or the extra equipment if I did have these I think my results would be much more accurate.
