## What is meant by Electrical Resistance?

*Resistance* is a measure of the opposition to current flow in an electrical circuit. *Resistance* is measured in ohms, symbolized by the Greek letter omega (Ω). Ohms are named after Georg Simon Ohm (1784-1854), a German physicist who studied the relationship between voltage, current and *resistance*.

If we make an analogy to water flow in pipes, the resistance is bigger when the pipe is thinner, so the water flow is decreased.

## Resistance calculation

The resistance of a conductor is resistivity of the conductor’s material times the conductor’s length divided by the conductor’s cross sectional area.

R is the resistance in ohms (Ω).

ρ* *is the resistivity in ohms-meter (Ω×m)

l is the length of the conductor in meter (m)

A is the cross sectional area of the conductor in square meters (m^{2})

It is easy to understand this formula with water pipes analogy:

- when the pipe is longer, the length is bigger and the resistance will increase.
- when the pipe is wider, the cross sectional area is bigger and the resistance will decrease.

### Resistance calculation with ohm’s law

*R* is the resistance of the resistor in ohms (Ω).

*V* is the voltage drop on the resistor in volts (V).

*I* is the current of the resistor in amperes (A).

### Temperature effects of resistance

The resistance of a resistor increases when temperature of the resistor increases.

*R*_{2} = *R*_{1 }× ( 1 + α(*T*_{2 }–* T*_{1}) )

*R*_{2} is the resistance at temperature T_{2} in ohms (Ω).

*R*_{1} is the resistance at temperature T_{1} in ohms (Ω).

*α* is the temperature coefficient.

### Resistance of resistors in series

The total equivalent resistance of resistors in series is the sum of the resistance values:

*R _{Total}* =

*R*

_{1}+

*R*

_{2}+

*R*

_{3}+…

### Resistance of resistors in parallel

The total equivalent resistance of resistors in parallel is given by:

See Also

Kirchoff Law

Ohm’s Law

Ampere

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