What is Ohm's Law?
Electricity powers almost everything around us — from the phone in your pocket to the lights in your classroom. But how do we predict and control how electricity behaves in a circuit? That's where Ohm's Law comes in.
The relationship between voltage, current, and resistance in an electrical conductor: the current through a conductor is directly proportional to the voltage across it, provided the temperature remains constant.
Ohm's Law is named after German physicist Georg Simon Ohm, who published his findings in 1827. It is one of the most fundamental and useful relationships in all of physics and engineering.
Think of electricity flowing through a wire like water flowing through a pipe. Voltage is like the water pressure pushing the water along. Current is like the flow rate — how much water passes a point per second. Resistance is like the narrowness of the pipe — a narrower pipe resists flow more. A higher pressure with the same pipe gives more flow; a narrower pipe with the same pressure gives less flow. Ohm's Law describes exactly this kind of relationship for electrical circuits.
The Three Key Quantities
Before we can use Ohm's Law, we need to understand the three quantities it connects.
The potential difference between two points in a circuit. It is a measure of the energy transferred per unit of charge and is measured in volts (V). Voltage is what drives current around a circuit — it is sometimes called the "electrical pressure."
The rate at which electric charge flows through a conductor. It is measured in amperes (A), often called "amps". One ampere equals one coulomb of charge passing a point every second.
A measure of how much a component opposes the flow of electric current. It is measured in ohms (Ω). Materials with high resistance make it harder for current to flow; materials with low resistance allow current to flow easily.
| Quantity | Symbol | Unit | Measuring Device |
|---|---|---|---|
| Voltage | V | Volt (V) | Voltmeter |
| Current | I | Ampere (A) | Ammeter |
| Resistance | R | Ohm (Ω) | Ohmmeter |
The symbol for current is I (from the French word intensité), not C. This catches many students off guard — make sure you use the correct symbol in equations and circuit diagrams.
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