Scientific knowledge
Here are the scientific principles behind this investigation with help from physic books.
All materials, solid, liquid or gases are made up of atoms. The atoms themselves consist of a central bit, called the nucleus, made up of particles called protons (which have a +ve electrical charge) and neutrons (which have no charge) Orbiting around the nucleus are electrons which are very tiny and have a -ve electrical charge. E.g.; the electrons orbiting in layers like the rings of an onion, and it's the ones in the very outside layer, the outer shell, that are the most important when thinking about conduction.
In metals, the outermost electrons are held only very weakly to the atom and often wander away from it and go to the nearby atom or one a bit further away. These wandering electrons are called conduction electrons and the more of these there are, for a given volume of metal, the better the metal will be as a conductor of electricity. When you connect a battery across a wire, one end becomes +ve and attracts the conduction electrons, which drift towards that end of the wire. But the electrons have obstacles to face because the metal atoms are jiggling about because of their thermal energy and so the electrons collide with them and are knocked all over. It's this difficulty that the electrons have in moving along the wire that we call resistance. Resistance involves collisions of the current-carrying charged particles with fixed particles that make up the structure of the conductors. A resistor is a material that makes it hard for electrons to go through a circuit. Without resistance, the amount from even one-volt would be infinite. Resistance occurs when electrons traveling along the wire collide with the atoms of the wire. The unit of resistance is Ohms. The higher the resistance, the lower the current. If there is high resistance, to get the same current a higher voltage will be needed to provide an extra push for the electricity. With electricity, the property that transforms electrical energy into heat energy, in opposing electrical current, is resistance. A property of the atoms of all conductors is that they have free electrons in the outer shell of their structure. All metals are conductors and have an arrangement in similar form to this:
As a result of the structure of all conductive atoms, the outer electrons are able to move about freely even in a solid. When there is a potential difference across a conductive material all of the free electrons arrange themselves in lines moving in the same direction. This forms an electrical current. Resistance is encountered when the charged particles that make up the current collide with other fixed particles in the material. As the resistance of a material increases so to must the force required driving the same amount of current. In fact resistance, in ohms(R) is equal to the electromotive force or potential difference, in volt (V) divided by the current, in ampere (I) - Ohm's law.
Some metals have less resistance than others. Wires are always made out of copper because copper has a low resistance and therefore it is a good conductor.
Resistance opposes the flow of an electric current around a circuit so that energy is required to push the charged particles around the circuit. The circuit itself can resist the flow of particles if the wires are either very thin or very long.
The material and cross sectional area of the wire is constant throughout the experiment. Therefore it is clear from the formula that the resistance should be directly proportional to the length.
Apparatus
Nichchrome wire
Ammeter
Voltmeter
Power supply
Crocodile clips
Switch
Connecting wires
Meter ruler
Wire cutters or scissors
Safety
What I am planning to do is a safe procedure. I have pointed out some key safety precautions: when we are using the electricity we have to be careful at all times not to shock or burn our selves, due to the current getting to hot. We must make sure that all the lab rules are followed and that we don't use faulty equipment. I will carry out the experiment in the laboratory in a safe environment. Also shut off appliances when changing wires and when not in use.
Circuit diagram
Preliminary test
We did a pre test to see how to do the test. We measured to wires, a 10cm wire and a 100cm wire. Results as shown:
In my pre-test I used a similar circuit to the one above but used a multi meter in series to act as an ammeter and then across the wires to act as a volt meter.
From my pre-test results I can see that my plan of using wires from 10cm to 100cm in 10cm steps will provide adequate results.
Method
First of all the wires were cut from 10cm to 100cm in intervals of ten. The wire was taken off the same reel so the thickness and material will be the same and will not affect my results. The wire was then taped on a meter ruler and cut to length needed as accurately as possible. Two people were doing this to make it easier. After all the wires were cut they were then tested with the smallest first going up to a 100cm. A wire was carefully put in to the circuit with crocodile clips on each end of the wire and the circuit was put on to measure the volts and current. This was done three times on each wire to get more accurate results and an average. After each wire was tested the circuit was shut off for safety reasons and the next one was tested. The crocodile clips were taken off and put back on to make sure it was put on properly for each test of the same wire.
Results
Wire length
These are the measurements of the wires I used. They have been measured three times and average was made.
Voltage and current
These are the results of the average wires from above. These also have been tested three times and average was made for voltage and current.
Resistance
These are the results for the resistance from the average length of wire.
Conclusion
My results show clearly that as the wire becomes longer its resistance increases. My graph shows that there is a strong correlation between resistance and length. This was predicted by my scientific knowledge that as the wire becomes longer there are more collisions between the current electrons and the atoms in the wire so resistance increases in length.
I think that my results are very reliable as the line on my graph goes through almost all the points. Therefore my results back up my prediction completely.
Evaluation
My results are of very good quality as there are no anomalies and the line goes through all almost all the points and if they are not on the line they are very close.
This experiment was easily carried out so I could not think of many ways of improving it, except I could have used better equipment for reading the voltage because we used an old volt meter, we could have used a digital one for more accurate and reliable readings.
There are no anomalies in my graph so these results were very good and reliable. Although not all of the points are exactly on the line. This could be because we measured the length of the wire we were trying to use wrong so we got a different reading to what we wanted. Also the temperature could have gone up a bit if we didn't disconnect the circuit after each reading, and as we know the resistance will change if the temperature changes. Also we could have used a different cross sectional area but still changing the length. Another reason why the points were a little off the line sometimes is that we might not have measured the length of wire right as the crocodile clip does not always test the full length of the wire. To improve the results I could use screw clips which would test the full length of the wire and measure the thickness of the wire at various points to ensure it’s the same.
To gather more relevant evidence we could have done the experiment with a different metal just to see if the resistance still went up when the length was increased. The resistance would change in this new wire depending on how many atoms were in the wire. Also we could have changed the width of the wire and I think that as the width of the wire got larger the resistance would decrease, as there would do more room for the electrons to move.