Resistance is opposition to the flow of current. Different materials have different levels of resistance. For example glass and rubber are excellent resistors whereas silver and copper offer little resistance to current flow. We utilise resistors to control the flow of current in a circuit.
> Resistance and insulation are not the same thing although they arise from and utilise the same natural phenomena. Resistance is a propery of an electrical circuit whereas insularity is a property of substances found in the world. Similarly, inulators are naturally occuring materials that resist the flow of current to different degrees whereas a resistor is a man-made electrical component that is used to _control_ the flow of current in a circuit.
The opposite to resistance is **conductance**. Again conductance is not the same thing as a [conductor](/Electronics/Physics_of_electricity/Electrons.md#conductivity-and-insularity): it is a measure of the capacity for current to flow through an electrical component and is function of the fact that certain materials are better conductors than others.
## Factors affecting resistance
- The larger the diameter of an electric wire, the lower the electrical resistance to the current flow.
- As a conductor heats up (e.g. copper, aluminium wire), its overall resistance increases.
## Notation and scientific expression
### Resistance
- We use $R$ to represent resistance
- Resistance is measured in **ohms** ($\Omega$)
> One ohm is the resistance of a circuit or circuit element that permits a steady current flow of one [amp](/Electronics/Physics_of_electricity/Current.md#formal-expression) (one coulomb/second) when one [volt](/Electronics/Physics_of_electricity/Voltage.md#voltage) is applied to the circuit.
