In 1826 Georg Ohm discovered that ‘the current flowing through a metal wire is proportional to the potential difference across it’. In other words if you double the potential difference the current is also doubled. However this law is only obeyed when all the conditions stay constant. However the shorter the wire the more energy is converted into heat due to more atoms for the electrons to collide with. So the wire temperature increases which was one factor that Ohm did not take into consideration.
A battery supplies voltage to the circuit between its terminals. Ideally the voltage should stay constant, however it does not, this is because the battery itself has resistance and acts like a resistor. This is called internal resistance.
Prediction:
I predict that as I increase the length of the wire the resistance will also increase, because as the current goes through the wire the electrons lose energy because they collide with the atoms of the metal and they lose the energy via heat loss. This causes there to be a decreases in the push force that move the electrons around the circuit. If the wire is longer there will be more collisions and so the electrons will lose even more energy thus losing the push force.
If I double the voltage the current will also increase because voltage and current are both proportional. However as stated above in my research I shall not expect this to be exactly correct because not everything in my circuit is not remaining constant the temperature of the wire should increase.
Equipment:
- Energy source (batteries)
- Ammeter
- Volt Meter
- Metal wire
- 5 wires
Plan:
- I will set up my equipment ensuring that the volt meter is parallel to the metal wire
- I will then insert the 30 cm long wire and measure the amps and volts
- Then I will take away 5 cm off the wire and read off the ammeter and volt meter
- I will repeat this till I have a 5 cm piece of wire
Diagram:
Ammeter Volt Meter
Wires
Metal wire
Batteries
Preliminary Results and Changes:
After doing my preliminary experiment I have decided to do the test every 3 cm so it will go 30cm, 27cm, 24cm, 21cm etc.
Results
Results table 1
Results Table 2
Results table 3
Average Result for resistance
Conclusion
I can now conclude that the resistance increases as the voltage (and current) increases because energy is lost as the electrons go through the wire and collide with the atoms. Having looked at my results and graph I can also conclude that the resistance is directly proportional to the length of wire because my graph is fairly straight. Despite the fact that hardly any off my points are exactly on the line, they follow the line of best fit and are very close to it. This shows that the resistance increases as the length of the wire increases due to more atoms for the electrons to collide with (see prediction and research for further details).
Gradient
y1 – y2 Two points: (25 (x1), 2 (y 1) (15 (x 2), 1.2 (y2)
x1 – x2
2 – 1.2 1.667 0.1667
25 – 15 10
This shows that my line of best fit goes up very gradually. The graph that I have drawn is very misleading because it looks as though the line goes up steeply; this is because my scale is very big. If I half the scale my line would appear to go up at half the steepness.
Resistance per centimeter
2.3 / 30 = 0.076
2.0 / 27 = 0.074
1.8 / 24 = 0.075
1.6 / 21 = 0.076
1.3 / 18 = 0.072
Evaluation
My results in my opinion are fairly reliable we have the odd result as with most experiments, however on the whole our results and graph show that the current is directly proportional to the voltage and that was our aim in the beginning. Our graph is reliable despite the fact that only two results actually touch the line (this could be due to back readings off or a fault in our method) because all of our results follow the line and are very close to it.
Regardless of getting reliable results except a few anomalous results we could have improved our method by ensuring that we had the set amount of wire in between the clip, for example 30cm, because when you add the clips it takes of about 2 cm so we could all of the experiment with the wire 2cm longer. Also as stated above in my research temperature has to remain a constant, which it was not because the smaller the wire got heat was lost so the resistance would be higher so maybe our lower results are not as reliable as the higher ones.
