Calculate voltage, current, resistance, or power from any two known values.
Ohm's Law is the fundamental relationship between voltage, current, and resistance in electrical circuits. It states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance.
V = I × R
Where V is voltage in volts, I is current in amperes, and R is resistance in ohms.
P = V × I
Power in watts equals voltage times current. Combined with Ohm's Law, this gives us 12 useful formulas.
5V supply, LED with 2V forward voltage, need 20mA current:
R = (5V - 2V) / 0.02A = 150Ω
Device draws 200mA from 3.7V battery, what's the power consumption?
P = 3.7V × 0.2A = 0.74W
12V heater with 6Ω resistance, what current and power?
I = 12V / 6Ω = 2A
P = 2A × 12V = 24W
Motor draws 5A, max wire resistance to limit drop to 0.5V:
R = 0.5V / 5A = 0.1Ω max
1kΩ resistor with 10V across it, what wattage needed?
P = 10² / 1000 = 0.1W
Use 1/4W or larger
Thermistor shows 10kΩ, powered with 3.3V, what current?
I = 3.3V / 10000Ω = 330µA
Ohm's Law applies to resistive (linear) devices. Diodes, transistors, and other non-linear components don't follow V=IR directly.
Resistance changes with temperature. Metals increase resistance when heated; semiconductors typically decrease.
For AC circuits, impedance (Z) replaces resistance and includes inductive and capacitive reactance.
Always select components rated for at least 2× the calculated power dissipation for reliable operation.