EngineerBenchUsing Ohm's law with two known values
Ohm's law connects voltage, current, and resistance: voltage equals current multiplied by resistance. Electrical power is voltage multiplied by current. Choose the pair you know to calculate the other quantities, such as the current drawn by a resistor or the resistance needed for a specified current.
Keep track of prefixes
A milliamp is one thousandth of an amp; a kilohm is one thousand ohms. Select the units that match your measurement rather than moving decimal points mentally. The calculated power is the power dissipated at that operating point, not a recommended component wattage.
Where the relationship applies
These modes describe a resistive circuit. They also apply to a resistive AC load using RMS voltage and current. Motors, capacitors, inductors, and nonlinear devices can need impedance or a device-specific model instead of a single resistance value.
Following the electrical relationships
For a 12 V source across a 6 Ω resistance, current is 2 A and dissipated power is 24 W. The same operating point can be recovered from resistance and power, or from voltage and current. These inverse modes are useful when a specification gives a different pair of known quantities. They describe one consistent operating point rather than several independent ratings that can be mixed arbitrarily.
For a fixed resistance, doubling voltage doubles current and quadruples power. For a fixed voltage, increasing resistance reduces both current and power. Those relationships explain why a small voltage change can have a larger heating effect than expected. Use a component's tolerance and operating temperature when deciding how representative the entered resistance is. The result describes electrical dissipation, not the component's temperature rise. Cooling, enclosure conditions, duty cycle, and the manufacturer's derating guidance are needed to turn calculated watts into a suitable component selection.
Formula
V = IR. P = VI = I²R = V²/R.