Ohm's Law Calculator

Ohm's Law states that the voltage (V) across a conductor is directly proportional to the current (I) flowing through it, with resistance (R) as the constant of proportionality: V = I × R. Named after Georg Simon Ohm (1827). The triangle trick: cover the variable you want (V, I, or R) and the remaining two show the formula. Power formulas: P = V × I = I² × R = V² / R. Ohm's Law applies to ohmic (linear) conductors where resistance is constant — not to non-linear components like diodes or transistors.

Voltage (V): Volts (V). Named after Alessandro Volta. 1V = energy per unit charge = 1 J/C. Current (I): Amperes (A). Named after André-Marie Ampère. 1A = 1 coulomb per second. Resistance (R): Ohms (Ω). Named after Georg Ohm. 1Ω = 1 V/A. Power (P): Watts (W). Named after James Watt. 1W = 1 J/s = 1 V×A. Common prefixes: milli (m, 10⁻³), micro (μ, 10⁻⁶), kilo (k, 10³), mega (M, 10⁶). A typical household outlet: 120V/240V at 15A = 1800W/3600W max.

The power triangle relates P, V, I, and R: P = V × I (Joule's Law). Combined with Ohm's Law (V = IR): P = I² × R (power dissipated in a resistor — useful when you know current and resistance). P = V² / R (useful when you know voltage and resistance). The power triangle has P at the top, V×I at the bottom. Cover P to see V×I. Cover V to see P/I. Cover I to see P/V. Power dissipated as heat in a resistor: P = I²R — this is why resistance wires in toasters and heaters get hot.

For most metals (positive temperature coefficient, PTC): resistance increases with temperature. Formula: R(T) = R₀ × (1 + α × ΔT), where α is the temperature coefficient. Copper: α ≈ 0.004 /°C. Nichrome (heating elements): α ≈ 0.0004 /°C — stable resistance at high temps. Semiconductors (NTC — negative temperature coefficient): resistance decreases as temperature increases — used in thermistors. Superconductors: resistance drops to exactly 0 below critical temperature. This non-linear behavior means Ohm's Law doesn't strictly apply to temperature-varying components.

Series circuit: components connected end-to-end. Same current through all. Voltage divides: V_total = V₁ + V₂ + ... Resistance adds: R_total = R₁ + R₂ + ... If one component fails open, circuit breaks. Used in: old Christmas lights, fuses. Parallel circuit: components share same voltage terminals. Same voltage across all. Current divides: I_total = I₁ + I₂ + ... Resistance: 1/R_total = 1/R₁ + 1/R₂ + ... (lower than any single R). If one component fails, others still work. Used in: household wiring, batteries in parallel.