resistor as a two-terminal passive electronic component

A resistor is a two-terminal passive electronic component which implements electrical resistance as a circuit element. When a voltage (V) is applied across the terminals of a resistor, a current (I) will flow through the resistor in direct proportion to that voltage. This constant of proportionality is called conductance, (G). The reciprocal of the conductance is known as the resistance (R), since, with a given voltage (V), a larger value of (R) further “resists” the flow of current (I) as given by Ohm’s law:

The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm. An ohm is equivalent to a volt per ampere. Since resistors are specified and manufactured over a very large range of values, the derived units of milliohm (1 mΩ = 10⁻³ Ω), kilohm (1 kΩ = 10³ Ω), and megohm (1 MΩ = 10⁶ Ω) are also in common usage.

The reciprocal of resistance R is called conductance G = 1/R and is measured in Siemens (SI unit), sometimes referred to as a mho. Thus a Siemens is the reciprocal of an ohm: S = Ω ^{− 1}. Although the concept of conductance is often used in circuit analysis, practical resistors are always specified in terms of their resistance (ohms) rather than conductance.

Ohm’s law

The behavior of an ideal resistor is dictated by the relationship specified in Ohm’s law:

Ohm’s law states that the voltage (V) across a resistor is proportional to the current (I) passing through it, where the constant of proportionality is the resistance (R).

Equivalently, Ohm’s law can be stated:

This formulation of Ohm’s law states that, when a voltage (V) is present across a resistance (R), a current (I) will flow through the resistance. This is directly used in practical computations. For example, if a 300 ohm resistor is attached across the terminals of a 12 volt battery, then a current of 12 / 300 = 0.04 amperes (or 40 milliamperes) will flow through that resistor.

Series resistors

Parallel resistors

The parallel equivalent resistance can be represented in equations by two vertical lines “||” (as in geometry) as a simplified notation. For the case of two resistors in parallel, this can be calculated using:

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