Ohm's Law Explained — Voltage, Current, and Resistance

Blog › Engineering · 6 min read · Published 2026-02-17

Master the fundamental electrical law that governs all circuits. Learn V=IR with practical examples and applications.

What Is Ohm's Law?

Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit. Formulated by German physicist Georg Ohm in 1827, it is one of the most fundamental principles in electrical engineering and electronics. Simply stated: the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance.

The Formula

V = I × R

V = Voltage (Volts, V) — the electrical "pressure" driving current
I = Current (Amperes, A) — the flow rate of electric charge
R = Resistance (Ohms, Ω) — opposition to current flow

Rearranging for any unknown:

V = I × R (find voltage)
I = V / R (find current)
R = V / I (find resistance)

Practical Examples

Example 1 — LED circuit: You have a 5V power supply and want 20mA (0.02A) through an LED with 2V forward voltage. The resistor needed: R = (5V − 2V) / 0.02A = 150Ω. Use the nearest standard value: 150Ω.

Example 2 — Home wiring: A 120V circuit with a 1,200W heater draws I = P/V = 1200/120 = 10A. With 14 AWG wire (resistance ~0.004Ω/ft), voltage drop over 50 feet: V = 10A × (0.004 × 100 ft) = 4V drop (acceptable, ≤3% of 120V).

Example 3 — Battery circuit: A 9V battery powers a 470Ω resistor. Current: I = 9V / 470Ω = 0.019A = 19mA.

Power and Ohm's Law

Combining Ohm's Law with the power formula (P = V × I) gives useful derived formulas:

P = V × I (power = voltage × current)
P = I² × R (power from current and resistance)
P = V² / R (power from voltage and resistance)

Where Ohm's Law Applies (and Doesn't)

Ohm's Law applies to ohmic conductors — materials where resistance remains constant regardless of voltage or current (most metals at constant temperature). It does NOT apply to:

Diodes and LEDs: Resistance changes dramatically with voltage
Transistors: Non-linear devices controlled by other signals
Lightbulb filaments: Resistance increases with temperature
Electrolytes: Ionic solutions have complex resistance behavior

For these components, more complex models (Shockley diode equation, transistor models) are needed. But for resistors, wires, and most passive components in everyday circuits, Ohm's Law is perfectly accurate and indispensable.