Definition 4.1.9.
One ohm (unit symbol: Greek Capital Omega, \(\Omega\)) is defined as that amount of resistance that will produce one ampere of current when a one volt potential difference applied across the conductor.
Subsection 4.1.5 Resistance
Resistance is a property of a material or component in an electrical circuit that opposes the flow of electric current. When a voltage is applied across a circuit component, its resistance restricts the flow of charged particles and reduces the magnitude of the current. Resistance is measured in ohms and is denoted by the symbol \(R\text{.}\) Large resistance, particularly electrical insulation resistance is measured in megohms, where 1 megohm = 1 million ohms.
The resistance of a conductor depends on three factors
- The material properties of the conductor. If current flows easily the material is said to be a good conductor, with low resistivity. Gold, silver, copper and aluminum are all good conductors. Copper and aluminum are commonly used for wires and cables, but gold and silver only used for small electrical contacts due to the cost.
- The conductor length. The resistance of a conductor is directly proportional to the length of the current path.
- The conductor’s cross-sectional area. As with fluid in a pipe, the narrower the path, the greater the resistance to flow.
The resistance of a conductor is given by
\begin{equation}
R = \frac{\rho L}{A}\tag{4.1.1}
\end{equation}
where:
- \(R\ \) is the resistance of the conductor, in ohms.
- \(L\ \) is the length of the conductor, in meters.
- \(A\ \) is the cross-sectional area of the conductor, in square meters.
- \(\rho\ \) is the resistivity of the conductor, in ohm-meters.
