What is Ohm's Law?

Understanding Ohm's Law

Ohm's Law describes the connection between voltage, current, and resistance in an electrical circuit. Voltage provides the electrical pressure that moves charge. Current describes the rate of charge flow. Resistance opposes that flow.

Related glossary terms: Voltage, Amperage, Ohm.

The basic equation is V = I × R. V represents voltage in volts. I represents current in amperes. R represents resistance in ohms. The same relationship can be rearranged to find current, I = V/R, or resistance, R = V/I. These forms describe the same electrical relationship from different starting points.

How Ohm's Law Works, Is Measured, or Is Used?

To apply the law, identify two known values and calculate the third. For example, a 120-volt source connects to a 24-ohm load. It produces 5 amperes because 120 divided by 24 equals 5. If resistance rises to 48 ohms, voltage remains 120 volts. Current falls to 2.5 amperes.

A digital multimeter measures voltage, resistance, or current. Each setting requires a different connection method. Voltage is measured across two points. Resistance is measured with power turned off. Current is measured through the circuit. A technician also considers temperature, conductor length, and the load's rated values. These factors can change resistance or affect safe operation.

• Use V = I × R when voltage, current, and resistance must be compared.
• Use I = V/R to estimate current before selecting circuit conductors or protection.
• Use R = V/I to identify unexpected resistance during troubleshooting.

Why Ohm's Law Matters?

Ohm's Law helps explain why a circuit may draw too much current. It can also explain low voltage or low output. A damaged conductor, loose connection, or failing load can change resistance. These problems can create abnormal readings. Comparing calculated values with measured values gives a technician a clear way to locate the problem.

The law also supports decisions about wire size, overcurrent protection, and equipment ratings. For example, a heater with lower resistance can draw more current. The supply voltage stays the same. That current must stay within the circuit's capacity. This includes the conductors and protective devices.

When Ohm's Law Matters Most?

Ohm's Law becomes especially useful during electrical troubleshooting, new circuit design, and equipment testing. It can help identify an open circuit, a short circuit, or an unexpected load. This can happen before a component is replaced. The calculation is simple. The measurement must be taken safely with the correct meter setting.

The law doesn't describe every device perfectly. Incandescent lamps, motors, electronic power supplies, and other non-ohmic loads may change resistance. Temperature, speed, or operating conditions can cause these changes. In those cases, a single resistance value may not predict the device's behavior. This applies across its full operating range.