Introduction to Ohm's Law — Voltage, Current & Resistance Explained
Blog › Engineering · 8 min read · Published 2026-02-28
Learn Ohm's Law fundamentals, the V=IR formula, and practical applications in electrical engineering and everyday circuits.
What Is Ohm's Law?
Ohm's Law is the foundational relationship in electrical engineering, stating that voltage (V) equals current (I) multiplied by resistance (R): V = I × R. Discovered by German physicist Georg Simon Ohm in 1827, this deceptively simple equation governs everything from smartphone circuits to power grid design. Understanding it is essential for anyone working with electricity.
The three variables are interdependent. Knowing any two, you can calculate the third: V = I × R (voltage), I = V / R (current), R = V / I (resistance). Power can also be derived: P = V × I = I²R = V²/R. These relationships form the complete foundation of DC circuit analysis.
Understanding the Variables
Voltage (V): Measured in volts, voltage is electrical pressure — the force that pushes electrons through a conductor. Think of it as water pressure in a pipe. A standard US outlet provides 120V, European outlets provide 230V, and a car battery provides 12V.
Current (I): Measured in amperes (amps), current is the flow rate of electrons. It's analogous to the volume of water flowing through a pipe. A typical LED draws 20mA, a phone charger draws 1-2A, and an electric stove draws 30-50A.
Resistance (R): Measured in ohms (Ω), resistance opposes current flow. It's like the narrowness of a pipe. Copper wire has very low resistance; rubber has extremely high resistance. Resistance depends on material, length, cross-sectional area, and temperature.
Practical Examples
LED Circuit: An LED needs 2V and 20mA. From a 5V source, the resistor needed is R = (5 - 2) / 0.020 = 150Ω. Power dissipated by the resistor: P = 0.020² × 150 = 0.06W, so a 1/4W resistor works fine.
Home Wiring: A 1,500W space heater on a 120V circuit draws I = 1500 / 120 = 12.5A. A standard 15A circuit can handle this, but adding other loads risks tripping the breaker. Two heaters on the same circuit would draw 25A — dangerous on a 15A circuit.
Voltage Drop: 100 feet of 14-gauge copper wire has about 0.25Ω resistance. At 15A, voltage drop = 15 × 0.25 = 3.75V. On a 120V circuit, that's 3.1% — within the recommended 3% maximum for branch circuits but borderline.
Series and Parallel Circuits
In series circuits, resistances add: R_total = R1 + R2 + R3. Current is the same through all components, but voltage divides proportionally. In parallel circuits, resistances combine as reciprocals: 1/R_total = 1/R1 + 1/R2 + 1/R3. Voltage is the same across all branches, but current divides.
Limitations of Ohm's Law
Ohm's Law assumes linear resistance — not all components are "ohmic." Diodes, transistors, and gas discharge tubes have non-linear resistance that changes with voltage or current. Temperature also affects resistance: incandescent bulbs draw 10x more current when cold (low resistance) than when hot. Superconductors have zero resistance below critical temperature, where Ohm's Law breaks down entirely.
Key Takeaways
Ohm's Law is the starting point for all electrical analysis. Master V = IR and its power variants, understand series vs parallel behavior, and always consider real-world factors like temperature and non-linear components. Use an Ohm's Law calculator for quick computations when designing circuits, troubleshooting electrical problems, or sizing components.