**Ohm’s Law, Resistance and Resistivity**

__Resistance__

- Current, I - Flow of electrons around the circuit (Amps): how fast the electrons travel around
- Voltage, V - Driving force that pushes electrons (Volts): electrical pressure
- Resistance, R - Slows down the electrons (Ohms): resists the flow of the electrons

**Resistance = Voltage / Current**

R = V / I

Unit of resistance: Ohm, Ω

The graphs below represent typical results obtained for a metal wire at constant temperature, a filament lamp, and a diode:

Wire Bulb Diode

A potential divider is used to investigate how the current passing in a component is dependent on the voltage across it.

__Resistivity__

unit: Ohm metres, Ωm

The physical dimensions and the cross sectional area have a direct effect on the resistance of a resistor. The resistance of a sample of material is directly proportional to the length and inversely proportional to its cross sectional area.

Hence: *R**∝** I / A*

The resistive properties of a resistor are measured by its resistivity, *ρ*

When this is taken into account, the formula becomes:

* R = ρL / A*

where *L* = length of material, *A* = cross sectional area