Resistance Of The Wire
As we know the resistance of the wire we are using is 7.2 (Ohms) per metre, we can calculate its resistance for the 40cm of wire we will be using.
7.2 / 100 = 0.072 resistance of wire per cm
0.072 x 40 = 2.88 resistance of wire per 40cm
Now by knowing this we can use the formula P = V² / R to predict the power from certain set voltages.
For wire of 2.88 per 40cm resistance and a voltage of 1 Volt we can calculate the power like this:
1² x 2.88 = 2.88
we can now use this formula to predict this for more voltages:
2² x 2.8 = 11.52
3² x 2.8 = 25.92
4² x 2.8 = 46.08
5² x 2.8 = 72
6² x 2.8 = 103.68
Fair Testing
To ensure my method is done fairly, I must consider a few points. To make my results regular and correct, I will use the same wire in each experiment. If any different wire is used, there could be variation in length and thickness. These small alterations could change the results and the resistance of the wire. If a different material is used in the wire, the results will also change, because different metals conduct different amounts electricity, making the voltages and currents different, and therefore the power and the resistance.
Safety
To prevent any accidents or putting anyone in danger, I should consider some aspects of my experiment that could lead to harming people. I will make sure that the voltage on the power supply does not exceed a limit of 15 volts, as this can lead to dangerous electrical accidents. I will also ensure that I do not touch the coiled wire, or the crocodile clips, as they will be hot as the electricity passes through them, and could give me a painful burn.
I also shouldn't touch any wire in the circuit as I could receive a painful electrical shock, that could be fatal if the voltage was high enough.
Apparatus
Power Supply with voltage available up to 14V
Voltmeter
Ammeter
40cm coiled wire
Crocodile clips
Wire connections
Method
After setting up your apparatus as shown, put the ammeter in the series circuit and the voltmeter in parallel.
Also make sure the electrical wires are well connected and that the voltmeter and ammeter are set to the suitable readings.
Starting at 1V by the reading on the voltmeter, take the current from the ammeter three times on each volt.
Stop reading at 14 volts, as this is when the experiment will become dangerous.
This method will ensure accuracy because I will take three repeats of each reading on the ammeter, so I can take averages and obtain more accurate results. I will use a voltmeter and an ammeter to take my readings, which will give me quite accurate results. I am using this method because it can give accurate results, which can be obtained in normal lab conditions, without using very expensive equipment or putting anyone in danger.
Prediction
From my research and observation of the wire, I can make a prediction that should be proved by my results.
As I found in my research, the formula P = V² / R can be used to predict the power obtained in my experiment.
This should show that as the voltage increases, so will the resistance of the wire, lowering the current. This is because the wire isn't an Ohmic Conductor, as it can only be an Ohmic Conductor if the temperature of the wire remains constant. But as the wire's temperature increase as electricity passes through it, it doesn’t obey Ohms law and there is resistance.
To Prove my prediction correct, I will predict the approximate power obtained by the formula P = V² / R :
2² x 2.8 = 11.52
3² x 2.8 = 25.92
4² x 2.8 = 46.08
5² x 2.8 = 72
6² x 2.8 = 103.68
Preliminary Tests
To check my method works, and check my prediction, I will take a few preliminary tests. This will also show me if 1 volt will give me good evidence, or if 13 volts will be too strong for the wire and melt it.
Voltage (volts) Current (amps)
1 0.15
7 1.03
13 1.95
To find the power from the voltage and the current, I can use the formula Power = Voltage / Current.
Voltage = 1 volt Current = 0.15 amps
Power = 1 x 1.05
Power = 1.05 watts
My preliminary tests seemed to prove my prediction, and went quite well, with no explosions or melting wires. The voltage increased, as did the power of the circuit. This preliminary test has showed that my method is a good way of carrying out this experiment, but as I only read off 3 voltages from the experiment and didn't take any averages, I can still be made a lot more accurate.
Results
Analysis
To show more accurate results on my graph, I took my results and made averages, to even out the results and make a better line of best fit. I found the mean average of each reading by using the three repeats I took, and then used the averages to calculate the power by using the formula
P = V x I (Power = Voltage x Current)
By looking at my results and at my graph, I can see that this experiment proved my prediction correct. This showed that as the voltage increases, so did the resistance of the wire, lowering the current. This was because the wire wasn't an Ohmic Conductor, as it could only be an Ohmic Conductor if the temperature of the wire remained constant. But as the wire's temperature increased as electricity passes through it, it didn’t obey Ohms law and there was resistance.
My predictions for the power using the formula P = V² / R were not correct, as my power for voltage of 2V was 0.6 watts instead of 11.52 as in my prediction. This may have been due to anomalous results or error in the experiment or the prediction.
Looking at my graph, I can see that there is a positive correlation in the results, and the steep positive curve proves that as the power increased, as did the voltage, whilst the resistance in the wire decreased. My precautions taken on this experiment were good, as no one was injured, and the results were even and fairly accurate, which means my fair testing measures were taken into consideration.
Evaluation
Looking at my results table and at my graph, I cannot see any anomalous results. This could be because my experiment was performed so carefully that there was no chance of an error, or because there was not any large gaps in the readings for anomalies to take place anyway. To make sure there were no anomalous results, I checked the results table, as sometimes anomalies can be overlooked when the results are evened out into averages.
From my results and my graph, I can see that the results I obtained are of good enough quality to draw a sensible conclusion. I therefore can say that my prediction is correct, that as the power increased, as did the voltage, whilst the resistance in the wire decreased.
If I were to make any improvements on my experiment, to obtain better, more accurate results, or to ensure safer conditions, I could carry the experiment out in other ways.
One of these would be to use a data logging software, to take readings at set intervals from sensors, without human error, and then the results could immediately be plot into a graph and analysed on a computer with the suitable software.
Another improvement would be to prepare the coiled wire in a lot more care. With the coiled wire I used in my experiment, the wire wasn’t exactly 40cm long, as a small part of the wire had to be used to attach the wire to the two crocodile clips at each end, to ensure a successful electrical connection. If this was to improved somehow, by finding an alternative way of attaching the wire to the circuit or by using a longer peice of wire, the results may be more accurate.
To make another improvement to the experiment, I could have used a computer input device to correlate my results automatically, and also to reduce the risk of human contact with the dangerous electrical circuit and hot wires.
To further this investigation, I could look into another aspect of the factors that affect the resistance, voltage and power. I could change a different variable, such as length of wire, thickness or material of wire, instead of voltage as I chose to investigate this time.
