What is resistance?
A potential difference (V) applied across a wire of length (l), there is in the conductor, an electric field (E). In this electric field the free electrons are not however under continuous acceleration (Ee/m). This is because they repeatedly collide with the moderately massive vibrating atoms losing their kinetic energy. The vibrating atoms having gained this kinetic energy now vibrate more. The resulting increase in the average vibration kinetic energy is rise in temperature.
Movement of charge carriers in any medium must necessarily be subject to such collisions causing loss of kinetic energy and generating heat in the medium. This heating of the medium due to the passage of charge carriers is a general property of all materials and is due to the resistance offered by the material to the flow of charge. The resistance of any material is measured as the potential difference required per unit current in that material. Hence the resistance (R) is determined as:
Where (V) is the applied potential difference and (I) the current in the material It should be noted that all materials require to have a potential difference applied in order to maintain an electric current in the material. Consequently all materials have resistance. Some materials become more heated than others despite the same rate of flow of charge.
Energy is required to push the charged particles around the circuit. The circuit itself can resist the flow of particles if the wires in the circuit are very thin and very long. For example, a filament in an electric light bulb is extremely thin and incredibly long. Due to the resistance, energy is given out as heat and light. Numerous household appliances, such as electric heaters, hair dryers, toasters, oven and electric fires, use a high resistance wire in their elements so that heat is given out.
There are four different factors, which affect the resistance in a wire:
1. Thickness – thin wires have more resistance than thicker wires, because there is not as much space inside the wire, which means the electrons have a lot more resistance inside the thinner wire than the thicker wire.
2. Length – The larger the length of the wire, the larger the resistance. This is because the longer the wire there is more chance that the electrons can collide with more wire particles therefore it creates more resistance. In a longer circuit, it is more of a struggle for electrons to get around the circuit without any collisions. There are many more wire particles (acting like obstacles) to avoid. Electrons cannot increase or decrease speed, but they can collide. They collide with the particles in the wire; therefore fewer electrons are able to flow than in a shorter length wire. This is because there are more atoms from the metal so there is more chance that the electrons would collide with one of the atoms therefore there are more resistance.
3. The material used – Different materials have different resistances because the materials’ atomic structures are different so some metals have low resistances and some have high resistances. Therefore it is important to keep the material the same throughout the experiment unless a different material is used to check if the conclusion or theory works for all materials. The type of material will affect the amount of free electrons that are able to flow through the wire.
The number of free electrons depends on the amount of electrons in the outer shell of the atoms, so if there are more or larger atoms then there must be more electrons available. If the material has a high number of atoms there will be high number of electrons causing a lower resistance because of the increase of the number of electrons. If the particles in the material are tightly packed together, the electrons will have more collisions and therefore more resistance.
4. Temperature – 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 that 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 temperature cannot be investigated because it is hard to control the range of temperature needed without the correct apparatus.
In this experiment the intended factor to be investigated was length. The experiment was completed with so many problems. The results were practically the same and as a consequence the results were not very accurate. The problems, which occurred in the experiment, were that the power supply kept on tripping and the ammeter was fluctuating a lot. Another problem was that it was not a fair test because the wire got really hot which meant that there was more then one factor affecting the resistance in the wire.
I have decided to investigate how the thickness of a wire affects its resistance because other factors such as temperature are hard to control or vary. There is not a large enough range of materials to investigate how materials affect the resistance of a wire. The way in which the thickness of a wire affects the resistance is an efficient experiment to do. A graph can be plotted easily, there is a large range of results and the results can be recorded easily.
My theory is that the thinner the wire, the higher the resistance. The thicker the wire, the lower the resistance.
This is because the thinner the wire is, the fewer routes there are for electrons in the wire; therefore the harder it is for current to flow. This results in the energy not being able to spread out as much, so the resistance will be higher. If the diameter of a wire is thicker more electrons can go through the wire, therefore less resistance. The atoms from the metals cannot stop or collide with as many electrons because the diameter of the wire is larger.
If the area of the wire doubles, so does the number of possible routes for the current to flow down, therefore the flow of electrons are twice as spread out, so resistance might halve,
i.e. Resistance 1/Area.
This can be explained using the formula
R = V/I
Where R=Resistance, I= Current, V=Voltage
When there is twice the current, and the voltage is the same, resistance will halve. The resistance of a wire is inversely proportional to its area, or R 1/A. The relationship between resistance and thickness is R = 1/T(R=Resistance and T=Thickness). So if T=1 then R=1 and T=10 then R=0.1
What is Current?
Current electricity is a flow of charged particles, usually through a circuit. In all dry conductors, the flow is of electrons and therefore of negative charge. The electrons flow from negative to positive. They are attracted to the positive terminal and repelled from the negative terminal. Current is measured in amps (A) by using an ammeter in the circuit. All circuits require an energy supply to push the particles around the circuit, or mains electricity.
Cross sectional area
Voltage is the measurement of the energy of each charge. A voltmeter is placed in a circuit in parallel with the place where energy is being converted. Voltage is the electrical force, or “pressure”, that causes current to flow in a circuit. It is measured in VOLTS (V or E). Take a look at the diagram.
“The amount of current flowing in a circuit made up of pure resistances is directly proportional to the electromotive forces impressed on the circuit and inversely proportional to the total resistance of the circuit.”
Or in simpler terms ohm’s law is a steady increase in voltage, in a circuit with constant resistance, produces a constant linear rise in current.
1. We measured the thickness of a wire in three different places to find the average because in some places it could thicker than others.
2. We then attached it to the crocodile clips and turned the power supply on.
3. We took the readings of the ammeter and the voltmeter. Because the readings were fluctuating a bit we took the first reading.
4. We turned the power supply off and repeated the readings another two times.
5. we then did the same approach to the rest of the wires.
Whilst doing the investigation, it is important to keep safety into consideration. The scissors should only be used for cutting the wire to the appropriate length and for no other reason. Before using the power pack, the pointer should point at 0 volts. It is important to be careful while using the power supply. While handling live wires, it is essential to be careful. The voltage should be kept low because of the safety factor and the wires heating up.
To make the experiment reliable, all apparatus must be checked to see if it is functioning properly and is giving a true reading. This will partly avoid systematic error. Another way to make the experiment reliable is to use two methods: to do the investigation in two different ways to measure the resistance when the diameters are changed. If one method contains systematic error or is very inaccurate, the other method will be used to recognise that.
I will take as many different results as possible so that there a wide range of results and that I am able to arrive at a good conclusion. To increase the accuracy of the experiment I will do repeats for all the experiments so when the mean is taken, an accurate table is drawn up and if one result is anonymous the other two results would contrast the anonymous result.