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.
V = I R I = V / R R = V / I
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.
A 1 kΩ resistor connected across a 12 V DC supply draws:
1 kΩ = 1000 Ω I = 12 / 1000 = 0.012 A = 12 mA P = V I = 12 × 0.012 = 0.144 W
The last calculation uses 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.
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 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.