In order to carry out the experiment, the following equipment will be used:
- Resistance wire board (specifically the wire of 30swg)
- Power supply
- Voltmeter
- Ammeter
- Electrical leads
- Micrometer screw gauge
Resistance is the property of any object (this case a metal) of resisting the flow of an electrical current. The quantity of resistance in an electric circuit determines the amount of current flowing in the circuit for any given voltage applied. The unit of resistance is ohms / Ω. The standard abbreviation for electric resistance is R.
The resistance of an object is determined by a property of the substance of which it is composed, known as the resistivity, and by the length and cross-sectional area of the object, and by the temperature. At a given temperature, the resistance is proportional to the object's resistivity and length, and inversely proportional to its cross-sectional area. Usually, a material's resistance increases with increases in temperature.
In the diagram above, the variable resistor represents the resistance wire, which is made of the metal allow, constantan. Using the resistance wire, I am employing both an ammeter & a voltmeter in order to calculate the resistance along the wire using Ohm’s law.
Ohm’s law states that the potential difference is the same as the current multiplied by resistance (as long as temperature is kept as a constant) or:
V=I x R
In order to calculate resistance you are able to change the equation to form ‘resistance = voltage divided by current’, or:
R=V / I
As previously stated, Ohm’s law can only be engaged when temperature is constant. As it is difficult to keep temperature constant when conducting the experiment, allowances must be made – though it is possible to calculate such heat changes.
When the temperature of a metal increases the resistance of that metal increases. This is because when the temperature increases the atoms of the metal vibrate more vigorously because of the increase in energy. This means that the electrons have more difficulty getting through the wire as they collide with the atoms which are in their pathway. This increases the amount of collisions therefore there is more resistance. However it is hard to keep the temperature exactly the same as the room temperature might change from day to day. It is essential to use a low voltage because it means a low current that will not heat up the wires. If a high voltage is used the energy would be in form of heat which would make the experiment unfair. The investigation will be done at room temperature. The temperature cannot be investigated because it is hard to control the range of temperature needed without the correct apparatus.
I have found the constantan (which the wire is to be made of) is an alloy. It consists of 55% Copper, 44% Nickel & 1% Manganese alloy. This manganese alloy is an impurity, and such impurities can defect the atomic structure of the wire – increasing the resistivity as the current is not able to flow evenly through the wire. Electrons passing through the wire will collide with the impurities and will dissipate – triggering a (tiny) temperature change. Constantan has a very small resistivity. At 20 oC, it is 4.9 x 10-7 ρ/Ωm. The resistivity change per oC is only +0.001%!
Resistivity is directly related to resistance it can be calculated using the formula below. Using it, it is possible to relate the results to what they should be. With that, the amount that the resistance has changed according to its heat can be determined.
ρ = (R x a) / l
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ρ is resistivity
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R is resistance
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l is length of wire and
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a is cross sectional area of wire (thickness)
This experiment is generally pretty safe because a person carrying it out will not be subject to dangerous chemicals or hazardous situation; a few measures still need to be undertaken. This includes not using too high a current as this will overheat the wire and cause burning and possible fires. This can be achieved by either lowering the voltage (which also directly affect the current supplied) or by increasing the overall resistance of the circuit. Also, I will make sure that the current is not left on for a prolonged amount of time. As well as, I must ensure that no water is present around my work-space, as this could cause an electric shock.
I predict that when the length of a wire is increased, its resistance will also increase. I also think that the rate at which the resistance increases will be constant and directly proportional to the length. I think that I can explain this. Electric current is the movement of electrons through a conductor – a metal wire in this case. When resistance is high, conductivity is low. Metals such as constantan conduct electricity well because the atoms in them do not hold on to their electrons very well. Free electrons are created, which carry a negative charge, to jump along the lines of atoms in a wire, which are in a lattice structure. Resistance is when these electrons, which flow towards the positive, collide with other atoms; they transfer some of their kinetic energy. This transfer on collision is what causes resistance. So, if we double the length of a wire, the number of atoms in the wire doubles. This increases the number of collisions and energy transferred twice, so twice the amount of energy is required. This means the resistance is doubled.
However; it may not obey Ohm’s law, due to the temperature increases in the resistance wire.