This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong. ====== 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]].
