Resistance and Resistivity

Not all materials allow electric current to flow through them with the same ease. Some conduct very well (metals), while others conduct very poorly (insulators). Resistance quantifies this opposition to the flow of current.

Resistance

The electrical resistance of a component reflects its ability to oppose the flow of current at a given voltage.

You can think of it as a pipe of varying width in a water circuit:

• if the pipe is wide, water flows easily → low resistance;
• if the pipe is very narrow, it is harder for water to pass through → high resistance.

Similarly:

• a component with low resistance allows current to flow easily;
• a component with high resistance severely limits the current.

Symbol and Unit

Resistance is denoted by \(R\).

Its unit is the ohm, denoted by \(\mathrm{\Omega}\) (the Greek letter "omega").

Examples:

• \(10\ \mathrm{\Omega}\), \(100\ \mathrm{\Omega}\), \(1\ \mathrm{k\Omega}\) (\(1000\ \mathrm{\Omega}\)) for resistors used in electronics;
• very low values (a few milliohms, \(\mathrm{m\Omega}\)) for high-quality electrical cables;
• extremely high values (\(\mathrm{M\Omega}\), megaohms) for insulators.

The following prefixes are often used:

• \(1\ \mathrm{k\Omega} = 1000\ \mathrm{\Omega}\)
• \(1\ \mathrm{M\Omega} = 1\ 000\ 000\ \mathrm{\Omega}\)
• \(1\ \mathrm{m\Omega} = 0{,}001\ \mathrm{\Omega}\)