An Investigation into the Electrical Resistance of Wires
Introduction
My Physics coursework is about the electrical resistance of wires and to see what affects the resistance. Electrical resistance is the measure of the degree to which an object opposes the passage of electrical current. Ohm’s Law states that at a constant temperature, the resistance is the amount if current flowing for any given voltage. This can be written by the formula:
R is the resistance in Ohms
V is the voltage in Volts
I is the current in Amperes
Scientific Background
A metal consists of a grid of atoms, each with a shell of electrons. The outer electrons are free to separate from their own atoms and travel through the grid, making the metal a conductor. When an electrical potential (a voltage) is applied across the metal, the electrons drift from one end of the conductor to the other under the influence of the electric field causing electrical conduction. These electrons can collide into impurities, the lattice and other free electrons and so cause resistance. A rise in temperature causes the atoms to vibrate more strongly, creating even more collisions, making it more difficult for the electrons to get through and so increasing resistance.
Variables
The possible independent variables for the investigation are:
- Thickness (Diameter, Circumference, Radius or Area) of wire: The cross sectional area of a piece of wire greatly affects the resistance. If the wire is very thick it will allow a high current through because there is more room for more electrons to transfer the current. However as shown in the diagram, in a narrow wire (a) there is less room and therefore fewer electrons to carry the current, hence a higher resistance. This means that the thickness is inversely proportional to resistance so if you double the thickness, you cut the resistance in half.
- Temperature of wire: When current passes through a metal the electrons in the metal are trying to get past the protons. In a cold wire the protons are not vibrating much so the electrons can pass between them rather quickly. As the conductor heats up, the protons start vibrating and moving slightly out of location. As they begin to vibrate more and more they are more likely to get in the way and disrupt the flow of the electrons. This means that the higher the temperature, the higher the resistance. An example of this is when you turn on a light bulb. At first, the wire (filament) is cold and has a low resistance but as the wire heats up and gives off light it increases in resistance. As a result we can say that Ohm's law holds true unless temperature changes.
- Type of wire: Different types of wires are made from different metals and therefore have different resistances. I am not going to use this as my independent variable as it is an internal and not an external factor affecting resistance.
- Length of Wire: If the wire is very long then it will have many atoms in it. This means that there is more material for the electrons to bump into as they move from one end to the other and so there is a greater resistance. If you double the length, you double the resistance of the wire. Therefore at a constant temperature the length will be directly proportional to the resistance.
