Preliminary Work: As part of the first module to be studied at GCSE (module 5), a number of experiments were carried out to determine different variables affected resistance. Experiment 1 – Proving Ohm’s Law The circuit opposite can be used to investigate the relationship between the current flowing through the fixed resistor and the potential difference (voltage) across the fixed resistor. The variable resistor is used to change the current flowing around the circuit. If the position of the variable resistor is changed in regular increments, the ammeter and voltmeter readings can be recorded and the results plotted on a graph.
Ohm’s Law states… The current through a metallic conductor is directly proportional to the potential difference across its ends, providing the temperature and other conditions are constant. V=IR Experiment 2 – Investigation the equation R=? L/A L = Length, ? = resistivity, A = Cross – sectional area For the second set of experiments, a number of lengths of wire were crammed between two pieces of A4 white card and fixed in place using staples. These lengths of wire were used as fixed resistors.
The same apparatus was used for Experiment 2 as in Experiment 1 above, but the fixed resistor was replaced by each of the wires, in turn, stapled onto the card. Crocodile clips were attached to the staples at each end of the wire and the ammeter and the voltmeter readings were recorded. To make the experiment a fair test each wire had the same cross sectional are and was the same length between the staples. The following wires will be used: 1. Single length of nichrome wire 2. Single length of copper wire 3. Single length of steel wire 4. Single length of constantan wire 5.
Double length of constantan wire 6. Triple length of constantan wire 7. Single length of constantan wire (10cm, 20cm, 30cm) The conclusion drawn Types of wire – Comparing wires made from different materials. It was found that each different type of wire had a different resistance. Therefore, it is fair to conclude that the resistance of a wire depends upon the resistivity of the wire, i. e. its ability to resist the flow of charge. Thickness of wire – Comparing wires which had different thicknesses. It was found out that as the thickness of the wire increased the resistance decreased.
Therefore, it is fair to conclude that the resistance o f a wire depends upon the thickness of the wire, i. e. its cross section area. Length of wire – The crocodile clips were attached at three different points along the wire, in turn in order that the resistance could be calculated. It was found out that as the length of wire increased the resistance increased. Therefore, it is fair to conclude that the resistance of a wire depends upon the length of the wire. The investigation to be undertaken Of these three factors tested in experiment 2, the length of wire is the easiest variable to be investigated.
Length is continuous variable and it is very simple to gather a large range of measurements. The cross sectional area and resistivity of the constantan wire used in experiment 2 will be used in the investigation because it gave some good results in the range of apparatus. Consequently, the investigation to be undertaken will be into how the length of a conductor affects the resistance of the conductor. Aim: The aim of this investigation is to determine how the length of a wire affects resistance. Theory Introduction The Theory is split up into four different parts.
They are: The Metallic Structure of a Conductor How a Metal Conducts Electricity The Obstacle Model of Resistance The Factors affecting Resistance a) Length of a Conductor b) Cross-sectional Area of a Conductor c) Resistivity (type of Conductor) d) Temperature of a Conductor How current and voltage are related – Ohm’s Law What is Current? What is Voltage? What is Resistance? The Metallic Structure of a Conductor The atoms which make up a metal are arranged into four different layers. Metals which are in a regular structure are packed closely.
There are spaces between the atoms, when they are arranged in layers because the atoms are spherical in shape. The second layer can fit into the gaps created by the previous layer because it is offset. Each following layer can fit into the gaps created by the previous layer for the reason that it is offset as well. You can compare the structure to the way oranges are stacked up on a fruit stall. In the total volume, atoms of the first two types take up78% of the space, whereas the other types take up 68% of the entire volume. Metal atoms will always try to occupy the greatest amount of volume.
For instance, they have the smallest amount of gaps. The type of metallic structure a metal gets relies of the radius of the atom and the charge it generates when it loses its valence electrons. The valence electrons of a metal atom are fairly easily removed, with the construction of metal cations. Then the valence electrons from every atom consequently come under the influence of a huge amount of cations. The valence electrons are free to travel through the structure and are no longer situated in the outer shell of any one atom and are consequently called delocalised electrons, frequently identified as the ‘sea of electrons’.
The elimination of the electrons leaves layers of cations behind. The cations electrostatically repel away from each other, however they are held in position by the attraction of the cations to the delocalised electrons clouds between them. How the Charge is conducted A chemical reaction (called electrolysis) takes place in the inside of a cell or battery of cells, which causes electrons to be pushed or forced away from the cell. Therefore the connecting wire attached to the negative terminal becomes negatively charged. The electrons in the connecting wire are repelled away from the build up of charge.
The next segment of the wire in turn will become negatively charged and this procedure will carry on around the circuit. The electrons are able to perform like this for the reason that they are locomotive due to their delocalised nature. There is no time limit as one set of electrons repels another; this movement of electrons take place similarly. Correspondingly as electrons are pushed out of the cell, the electrons are pulled back in to replace them. The circuit remains detached overall even though there is a movement of electrons. The Obstacle Course Model of Resistance.