Electrical principles - Properties
Resistance
Resistance is the opposition to current flow. In an electrical circuit everything has some resistance, including conductors.
Some factors that determine the amount of resistance in a conductor are explained below.
Conductor material
The amount of resistance of a conductor will depend on the atomic structure of the conductor material. The amount of resistance offered by a material is termed its 'resistivity'.
The symbol for the resistivity of a material is ρ (rho).
Quantity |
Symbol | Unit |
Abbreviation |
Meaning |
|---|---|---|---|---|
resistivity |
ρ |
ohm/metre |
Ω/m |
resistance of material |
The following table shows the resistivity of a number of conductors.
Metal |
Resistivity (Ω - metre) |
|---|---|
Silver |
1.65 x 10-8 |
| Copper | 1.72 x 10-8 |
| Aluminium | 3.2 x 10-8 |
| Nickel | 8.7 x 10-8 |
| Iron | 11 x 10-8 |
| Nichrome | 112 x 10-8 |
Temperature
As temperature increases or decreases, its resistance value changes.
A material that has a Positive Temperature Coefficient (PTC) will increase in resistance with an increase in temperature.
A material with a Negative Temperature Coefficient (NTC) will decrease in resistance with an increase in temperature.
Conductor length
This is how the resistance of a conductor changes, depending on the conductor's length.
Conductor length - resistance
Increased conductor length equals increased resistance in direct proportion.
Conductor cross sectional area
The thinner or smaller the diameter (cross sectional area), the greater the resistance.
Cross sectional view of two conductors
A wire with a 3mm diameter would have four times the resistance of a 6mm diameter wire.
Calculating resistance of a conductor
The following formula can be used to calculate the resistance of a length of conductor (at 20 degrees Celsius).
R = ρ l / A
where
R = resistance in ohms
ρ = resistivity in ohmmetres
l = length in metres
A = cross-sectional area in square metres (m2)
