Practice Assignment 10
Measuring changes in electrical properties - same materials
It’s always important to use the right material for a job. It’s also important to use the right size.
Electrical wiring must have the right resistance for the job – for example, low resistance in cables, higher in electric heaters. Assignment PA09 compares the wires of the same size made of different metals. The assignment below compares wires of the same metal, but different sizes.
The size of a wire has two variables: length and diameter (or 'thickness'). Your task is to find out about the effects of length and of diameter on the resistance of wires. To do that in detail you will have to plot two graphs: one for length and one for diameter.
What you have to do
- Get a copy of Standard Procedure SP 0010:2005
- Before starting:
- read the Standard Procedure and the hint below carefully;
- check that a risk assessment has been carried out;
- list the equipment and materials you need.
- Use the procedure to measure the resistances of different lengths of wire. Plot a graph of resistance in ohms (y-axis) against length of wire in cm (x-axis). Describe the effect of the length of wire on its resistance.
- Use the procedure to measure the resistances of 1 metre lengths of different diameter wires, all made of the same metal. Plot a graph of resistance in ohms (y-axis) against diameter of wire in mm (x-axis). Describe the effect of the diameter of wire on its resistance.
Some hints
- In these investigations, the three variables that can affect resistance are: length of wire, diameter (or thickness) of wire and type of metal. (Note: Temperature of the wire is another variable that can affect resistance but, in these investigations, the temperature of the wire changes very little.)
Some questions
Don’t forget to write up your Test Report in your Laboratory Notebook.
- In the investigation of the effect of length of wire on resistance you kept some variables the same. Which variables?
- Have a look at the filaments of some lamps. What shape are the filaments? Can you explain this?
- Make a list of the units of current, voltage and resistance. Will you remember these units? Make up a memory aid to help you. (Your own memory aids usually work better than ones other people have made up.)
- The wires have small diameters, which can be measured by special instruments. Find out how to measure diameters of wires. Make some sample measurements of diameters of wires.
- Look back at what you have done. Suggest what you could have done better.