In general terms ohms law can be stated as the law stating that the electric current i flowing through a given resistance r is equal to the applied voltage v divided by the resistance. The International Standard unit of resistance is the ohm, designated by the Greek letter Ω. One ohm of resistance is equal to the resistance of a circuit in which a potential difference of one volt produces a current of one ampere. Mathematically Ohm's law is written as: resistance is the sum of voltage over current; r = i / v.
Resistance is unwanted if you are either transporting electricity or using a modern device that include a circuit board or complex wiring. The two main effects is wasting electrical energy by the heating effect of high resistance and controlling that heat produce. Also high resistance resistors only let high energy electrons through which controls the quality of power. However, it is resistance that allows us to use electricity for heat and light. The heat that is generated from electric heaters or the light that we get from light bulbs is due to resistance. In a light bulb, the electricity flowing through the filament, or the tiny wires inside the bulb, cause them to glow white hot. If all the oxygen were not removed from inside the bulb, the wires would burn up.
Resistance in a circuit is a physical constant and can only be modified by changing components, exchanging resistors for those rated at more or fewer ohms, or by adjusting variable resistors. A variable resistor is a device placed in a circuit can alternate the amount of resistance. All resistors produce heat when a current flows through them. Whenever a current flows through anything with electrical resistance then the electrical energy is converted into heat energy. The more current that flows, the more resistance, the more heat. However the higher you make the resistance the less heat produced. This is because a higher resistance means less current will flow, and that reduces the heating.
There is a less resistance I a parallel circuit than I a series circuit because in a parallel circuit the electrons have more freedom of choice.
Aim
The aim of this investigation is to show that the resistance of a wire depends on either:
- Length of the wire
- Thickness or diameter of wire
- Temperature of the wire
How these factors affect resistance:
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The longer the wire, the longer it will take electrons to get to the end of the wire. This will cause more collisions between electrons and atoms. So, in theory, the length of the wire should be directly proportional to the resistance.
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The thickness or diameter of the wire will cause resistance to decrease because of the increase in space in the wire. The increase in space means that there is more space for the electrons to flow freely because there would be fewer collisions with atoms.
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If the wire is heated, the atoms will move around more because there will be an increase in energy. This would cause more collisions between the atoms and the electrons. The increase in collisions would cause the resistance to rise.
Another factor which affects resistance is the material utilised. If the material being used contains atoms with a large number of electrons on the outer shells, then this means there are more electrons available. So, in theory, if the material has a large number of atoms, there should be less resistance, because of the higher number of electrons .If the atoms in the wire are closely packed, then this will cause an increase in resistance, due to frequent collisions.
Apparatus & Method
- A circuit was set up, including a voltmeter and ammeter in order to measure the current and voltage. Two different lengths of wire between 40 and 100 cm were used and so were two different diameters.
The apparatus which will be used are:
· Crocodile clips
· Ammeter
· Voltmeter
· Power supply
· Meter ruler
· Connecting wires
· sticky tape
· Thin Constantine wire
Safety Issues
The experiment was carried out with great caution. No protective equipment was necessary. The power pack reduced the mains voltage to a safe level for working. The working area was kept tidy and dry, to avoid short circuiting.
Prediction
The prediction is that as the length or diameter of the Nichrome wire is increased, the resistance will go up. This is because
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Thinner the wire, the more resistance. This is because the thinner it is the more narrow it is for electrons to go through making it harder, while when there is more diameter, there is more space for electrons to flow through.
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The longer the wire of course, the more resistance as well because the electrons will have to travel more.
Results
Analysis
It was a fair test because the voltage was the same for all the experiments. The voltage was four volts. Also, all the same lengths of wire were used, the only variable being the thicknesses and lengths of wire.
For the first factor, length, all three graphs confirm the prediction: all three graphs show that the as the length decreases, so does the resistance. But for the diameter the results were not expected. The wire with the least diameter, (0.19) had the most resistance.
The experiments do not support the original prediction. It is a fact that the diameter affects resistance, so there is obviously an error.
Evaluation
Possible reasons for this error is maybe because of the procedure used. The wire was measured using metre-rulers, and the recording could have been innacurate.
The changes I would make to improve would be to take care in measuring, and to try and use more accurate equipment.
I don't there was enough data to support the prediction. To improve this I would do more experiments. The more results there are, the more likely there'll prove the theory.
