Ohm's Law & the Power Triangle

What is Ohm's Law?

Ohm's Law is the fundamental principle of electricity. It describes the relationship between Voltage (Volts, V), Current (Amps, I), and Resistance (Ohms, R or Ω). The law states that current is directly proportional to voltage and inversely proportional to resistance.

Joule's Law (The Power Formula)

While Ohm's law deals with resistance, Joule's law (often just called the Power formula) calculates the actual work being done or energy consumed per second, measured in Watts (P).

The Ohm's Law Wheel (12 Formulas)

By combining Ohm's Law and the Power Formula algebraically, we can calculate any missing variable as long as we know at least two others. This creates the classic 12-formula wheel.

To Find	Formula 1	Formula 2	Formula 3
Voltage (V)	V = I × R	V = P ÷ I	V = √(P × R)
Current (I)	I = V ÷ R	I = P ÷ V	I = √(P ÷ R)
Resistance (R)	R = V ÷ I	R = V² ÷ P	R = P ÷ I²
Power (P)	P = V × I	P = I² × R	P = V² ÷ R

The AC Power Triangle

In DC (direct current) circuits, power is simple (Watts = Volts × Amps). But in AC (alternating current) circuits with inductive or capacitive loads (like motors), voltage and current can get "out of phase." This requires the Power Triangle to explain.

The Power Triangle consists of three vectors:

• apparent power (S): Measured in Volt-Amps (VA). This is the raw multiplication of Volts and Amps in the circuit. (The hypotenuse).
• Real Power (P): Measured in Watts (W). This is the actual power performing useful work. (The horizontal leg).
• reactive power (Q): Measured in Volt-Amps Reactive (VAR). This is the "wasted" power bouncing back and forth to sustain magnetic fields in motors. (The vertical leg).

Example Calculation

An AC motor draws 10 Amps at 120 Volts. The Apparent Power (VA) is 1,200 VA (10 * 120). However, the motor's Power Factor is 0.8.

Real Power (Watts) = 1,200 VA × 0.8 PF = 960 Watts.