====== Ohm's law ======

**Ohm's law** relates the voltage across an ohmic conductor to the current through it. It is a useful starting point for analysing resistors, power supplies and simple electronic circuits.

===== Relationship and units =====

<code>
V = I R
I = V / R
R = V / I
</code>

V is voltage in volts (V), I is current in amperes (A), and R is resistance in ohms (Ω). Voltage is measured between two points; current is the flow of charge through a branch.

For an ohmic component, resistance remains approximately constant under the stated conditions. Temperature changes can change resistance. Diodes, transistors and a filament warming up do not behave like one fixed resistor over their entire operating range.

===== Worked example =====

A 1 kΩ resistor connected across a 12 V DC supply draws:

<code>
1 kΩ = 1000 Ω
I = 12 / 1000 = 0.012 A = 12 mA
P = V I = 12 × 0.012 = 0.144 W
</code>

The last calculation uses [[electric_power|electric power]]. A resistor's power rating must accommodate the dissipation and its operating temperature; the resistance value alone does not specify how much heat it can safely release.

===== Measuring a circuit =====

A voltmeter goes across the component. An ammeter goes in series with the branch. A resistance measurement normally requires an unpowered circuit and may require isolating the component from parallel paths. A current input connected directly across a supply can create a short circuit.

For alternating current, [[impedance|impedance]] extends the relationship to include frequency and phase. Dividing an RF voltage by 50 Ω is justified only when that 50 Ω describes the relevant load under the measurement conditions.

===== See also =====

  * [[resistor_circuits|Resistor circuits]]
  * [[electric_power|Electric power]]
  * [[impedance|Impedance]]

===== References =====

  * [[https://openstax.org/books/physics/pages/19-key-equations|OpenStax Physics: electrical circuit equations]].
  * [[https://openstax.org/books/university-physics-volume-2/pages/9-4-ohms-law|OpenStax University Physics: Ohm's law]].