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Calculate any electrical quantity using Ohm's Law. Enter any two of voltage (V), current (I), or resistance (R) to instantly find the third, plus power. Essential for electronics engineers, students, and hobbyists.
Engineers rely on Ohm's Law for everything from choosing the correct resistor value for an LED circuit to designing complex power distribution systems. Students use it to understand why higher voltage can push more current through the same resistance, and why thicker wires (lower resistance) are needed for high-current applications.
Ohm's Law is the fundamental law of electrical circuits, discovered by German physicist Georg Simon Ohm in 1827. It states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. This simple relationship governs the behavior of virtually every electronic circuit ever built.
The law is expressed as V = I × R, where V is voltage in volts (V), I is current in amperes (A), and R is resistance in ohms (Ω). From this single equation, we can derive all other relationships between these three fundamental electrical quantities.
The Ohm's Law triangle is a memory aid: place V at the top, I and R at the bottom. Cover the quantity you want to find — the remaining two show the formula. Cover V → I × R. Cover I → V / R. Cover R → V / I.
Electrical power (P) measured in watts combines with Ohm's Law to give four power formulas: P = V × I, P = I² × R, P = V² / R. A 60W light bulb on 120V draws 0.5A of current and has a resistance of 240Ω when hot.
| Voltage (V) | Current (A) | Resistance (Ω) | Power (W) |
|---|---|---|---|
| 12V | 2A | 6Ω | 24W |
| 120V | 1A | 120Ω | 120W |
| 5V | 0.02A | 250Ω | 0.1W |
| 230V | 10A | 23Ω | 2300W |
| 9V | 0.5A | 18Ω | 4.5W |
Ohm's Law applies to resistive (linear) components at constant temperature. Non-linear components like diodes, transistors, and LEDs do not strictly follow Ohm's Law — they have variable resistance depending on operating conditions.
Ohm's Law governs every electrical circuit from the simplest LED flashlight to complex server farms. Understanding how voltage, current, and resistance interact allows engineers to design safe, efficient circuits. Every resistor value chosen, every fuse selected, and every wire gauge specified relies on Ohm's Law at its foundation.
When designing any circuit with LEDs, transistors, or sensors, Ohm's Law determines the correct series resistor. For a 5V microcontroller output connected to a 3.3V device: the voltage difference is 1.7V, and if we want 10mA current, R = 1.7V / 0.01A = 170Ω — use a standard 180Ω resistor.
Technicians use Ohm's Law daily to diagnose faults. Measuring voltage across a component and current through it allows calculation of resistance, revealing short circuits (near 0Ω), open circuits (infinite Ω), or degraded components. A motor drawing twice its rated current likely has a winding short — R has halved, so I doubled at the same V.
Every resistor and wire dissipates power as heat: P = I²R. A 100Ω resistor carrying 100mA dissipates P = 0.01² × 100 = 0.1W. Choose a resistor rated for at least 2× this (0.25W minimum). Insufficient power rating causes resistors to overheat, change value, and potentially catch fire.