Ohm's Law Explained: V = IR Formula, Power Calculations & Circuit Examples
Ohm's Law: V = I × R, where V = voltage (volts), I = current (amps), R = resistance (ohms). This guide covers V=IR, power formulas, series vs. parallel circuits, and worked examples.
The V = IR Formula
Ohm's Law describes the linear relationship between voltage, current, and resistance in a conductor. The law applies to ohmic materials — conductors where resistance remains constant regardless of the applied voltage, including most metals and resistors. The Ohm's Law triangle is a useful mnemonic: cover the variable you want to find, and the remai
Power Formulas
Electrical power (P) is the rate of energy dissipation in a circuit element, measured in watts (W). By substituting Ohm's Law into the basic power equation P = V × I, we derive three equivalent power formulas, each useful when different variables are known. Example: a 100W light bulb on a 120V US household circuit. I = P/V = 100/120 = 0.833 amps. R
Series Circuits
In a series circuit, components are connected end-to-end in a single path. The same current flows through every element, but voltage divides across each component proportional to its resistance. Total resistance: R_total = R₁ + R₂ + R₃ + … Current is the same throughout: I = V_total / R_total Voltage divides: V₁ = I × R₁, V₂ = I × R₂ Example: 12V b
Parallel Circuits
In a parallel circuit, components connect between the same two nodes. Voltage is the same across every branch, but current divides among the branches inversely proportional to resistance. Total resistance formula for parallel: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ For two resistors: R_total = (R₁ × R₂) / (R₁ + R₂) Current in each branch: I₁ = V/R₁, I₂ = V
Frequently Asked Questions
What are the units in Ohm's Law?
Voltage is measured in volts (V), current in amperes (amps, A), resistance in ohms (Ω), and power in watts (W). SI relationships: 1 volt = 1 watt per amp = 1 ohm × 1 amp. 1 watt = 1 volt × 1 amp = 1 amp² × 1 ohm.
Does Ohm's Law apply to AC circuits?
Ohm's Law applies to resistive AC circuits exactly as in DC. However, AC circuits with capacitors and inductors have reactive impedance (measured in ohms but frequency-dependent) rather than pure resistance. The generalized form uses impedance Z instead of R: V = I × Z, where Z i
What is the difference between resistance and impedance?
Resistance (R) is the opposition to current flow in purely resistive elements — constant regardless of frequency. Impedance (Z) is the total opposition in AC circuits including resistance, capacitive reactance (Xc = 1/2πfC), and inductive reactance (XL = 2πfL). For DC and purely
How do I calculate the resistance of a wire?
Wire resistance: R = ρ × L / A, where ρ is resistivity (Ω·m), L is length (m), and A is cross-sectional area (m²). For copper (ρ = 1.68×10⁻⁸ Ω·m), a 100-foot (30.5m) run of 14 AWG wire (2.08mm²) has R = 1.68×10⁻⁸ × 30.5 / 2.08×10⁻⁶ ≈ 0.25Ω. Higher resistance means more voltage dr
What is a safe current for common wire gauges?
Standard US ratings (NEC, for residential wiring): 14 AWG = 15 amps max, 12 AWG = 20 amps, 10 AWG = 30 amps, 8 AWG = 40 amps. Always derate by 80% for continuous loads (running more than 3 hours). Never exceed the rated ampacity — overloaded wires overheat and cause fires.