Making your own Resistor

Shade an area on grid paper 1cm x 20cm

Make sure that you shade this area really, really well! It must be really shiny otherwise the resistor will not work!!

Now get an orange multimeter. Connect two wires to the bottom two sockets:

Set the dial of the multimeter to Resistance (this symbol: Ω) and to the setting that says 200k

Using the multimeter, measure the resistance of the shaded area 1 cm at a time. Record the resistance each time.

Using the multimeter, measure the resistance of the shaded area 1 cm at a time. Record the resistance each time.

Using the multimeter, measure the resistance of the shaded area 1 cm at a time. Record the resistance each time.

Using the multimeter, measure the resistance of the shaded area 1 cm at a time. Record the resistance each time.

Using the multimeter, measure the resistance of the shaded area 1 cm at a time. Record the resistance each time.

Using the multimeter, measure the resistance of the shaded area 1 cm at a time. Record the resistance each time.

Make sure that the wires are pressed firmly down on the shaded area. If you don’t get a reading, shade the area again some more!

Measure the resistance from lengths of 1cm to 20cm and record your data on the following table:

Length / m

Resistance / Kilo Ohms

Plot a scatter graph of your results. Put length on the x axis (horizontal) and resistance on the y axis (vertical)

Do not join all the points up, instead draw a line of best fit!

Your graph should look something like this…

Resistance / Kilo ohms

Length / cm

x

x

x

x

Conclusion

• How did the resistance of the shaded area change as you changed the length?

• Why do you think this happened?• What do you think would happen to the

resistance if you made the shaded area even more shiny? Why?

Resistance

• R is proportional to the length of wireR α L

• R is inversely proportional to the cross sectional area of wire

R α 1/A

• R depends on the type of material – WHY?

Resistivity

R = ρL A

where R = resistance in OhmsL = Length of conductor in metresA = cross sectional area of conductor in m2

ρ = resistivity of the material in Ohms.meters

Example

The resistivity of copper is 1.7 x 10-8 Ωm. What is the resistance of a piece of copper wire 1 m in length with a diameter of 0.1mm?

Example

The resistivity of copper is 1.7 x 10-8 Ωm. What is the resistance of a piece of copper wire 1 m in length with a diameter of 0.1mm?

radius = 0.05mm = 5 x 10-5mcross sectional area = πr2 = 3.14x(5 x 10-5)2 = 7 x 10-9 m2

R = ρL/A = (1.7 x 10-8 x 1)/ 7 x 10-9 = 2.42 Ω