Foundations
Ohm’s law: current
- where
- current (A)
- voltage (V)
- resistance (Ω)
Explain this equation
Works out the current through a resistance from the voltage across it. The everyday check on how much current a resistive load, heater element or test resistor will draw.
I is the current through the component. V is the potential difference across its two ends, which is the push. R is how strongly the component opposes the flow.
Ohm’s law says current rises in step with voltage for a fixed resistance. More push gives more flow; more opposition gives less.
Sense check: 12 V across 6 Ω gives 2 A. Double R and the current halves; double V and it doubles.
Watch out: use the resistance at operating conditions. On AC with inductance or capacitance you need impedance Z, not R.
Voltage provides the potential difference across the resistor. Resistance determines how much current flows for that voltage. Divide voltage by resistance.
Sketch the resistor and place the voltage across its two ends. Increasing resistance at fixed voltage reduces current; use that as a reasonableness check.
Enter your values to see the result.
Common mistake
Using an appliance power rating as its resistance.
Limits
A resistive element at the stated operating condition; general AC impedance needs Z.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.