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tags: [physics, electricity]
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# Resistance

- Resistance is opposition to the flow of [current](./Current.md).

- 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.

## Relationship between _resistance_ and _insulation_

Resistance and insulation are not the same thing although they relate to the
same natural phenomena.

Resistance is a property of an electrical circuit whereas insularity is a
property of substances found in the world.

Similarly, insulators 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.

## Factors affecting resistance

- The larger the diameter of an electric wire, the lower the electrical
  resistance to the current flow. (If we use the analogy of electricity as
  water-flow: the wider the pipe, the more water that can 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
- The unit of resistance is **ohms** ($\Omega$)

> One ohm is the resistance of a circuit or circuit element that permits a
> steady current flow of one [amp](Current.md#formal-expression) (one
> coulomb/second) when one [volt](Voltage.md#voltage) is applied to the circuit.

### Conductance

- We use $G$ to represent conductance
- The unit of conductance is **Mho** (ohm spelled backwards) ($\mho$)

> Conductance is the ability of a material to pass electrons

> Given that resistance is the opposite of conductance, the relation between the
> two quantities can be expressed via a recipricol: $R = 1/G$ and $G = 1/R$

## Ohm's Law

The relationship between current, resistance and voltage is expressed in
[Ohm's Law](Ohms_Law.md).

## Resistors

A resistor is an electrical component that can be used in circuits to introduce
specific amounts to resistance where needed. This is typically done in order to
temper the total voltage and so keep the current flowing within certain set
parameters.
