|Electricity · Magnetism|
The electrical resistance of an electrical conductor is a measure of the difficulty of passing an electric current through a substance. It explains the relationship between voltage (amount of electrical pressure) and the current (flow of electricity). With more resistance in a circuit, less electricity will flow through the circuit. The inverse of resistance is conductance, a measure not much used.
Resistance, discovered by Georg Simon Ohm in 1827, is the ratio between voltage and current. Ohm's law said that the voltage between any two points in a conductor changes directly as the current between the two points, given the temperature remains the same. He described it with the equation:
which models the ratio, where:
- is the resistance of the object, measured in ohms (Ω)
- is the voltage across the object, measured in volts (V)
- is the current going through the object, measured in amperes (A)
Calculating resistance[change | change source]
The resistance of a wire increases as it becomes longer and decreases as it becomes wider. (A simple analogy is a road - the more lanes there are, the less traffic there is.) The resistance R of a wire with a constant width, therefore, can be calculated as:
where is the length of the conductor, measured in meters [m], is the cross-sectional area of the conductor measured in square meters [m²], and ρ (Greek: rho) is the electrical resistivity (also called specific electrical resistance) of the material, measured in ohm-meters (Ω m).
Example: Calculate the resistance of copper wire with a radius of 2mm and a length of 5 meters.
- The resistivity () of copper is Ω m.
- The cross sectional area () is square meters
- The length () is meters