Table of Contents
Calculating Current with Ohm's Law
Current is the rate of flow of electric charge through a conductor, measured in Amperes (A). One Ampere equals one Coulomb of charge passing a point per second. Using Ohm's law, current is calculated by dividing the voltage across a component by its resistance. This is the most common calculation in circuit analysis.
Understanding current flow is essential for sizing wires, selecting fuses, designing power supplies, and ensuring electrical safety. Excessive current causes overheating and can damage components or start fires. Circuit protection devices like fuses and circuit breakers limit current to safe levels.
Formula
Where I is current in Amperes, V is voltage in Volts, and R is resistance in Ohms.
Current Examples
| Application | Typical Current |
|---|---|
| LED indicator | 10-20 mA |
| USB device | 0.5-3 A |
| Household outlet | up to 15 A |
| Electric vehicle motor | 100-400 A |
| Lightning bolt | ~30,000 A |
Frequently Asked Questions
What is the difference between AC and DC current?
DC (direct current) flows in one direction continuously, as from a battery. AC (alternating current) reverses direction periodically, typically at 50 or 60 Hz in power grids. Ohm's law applies to both, but AC calculations use RMS values and impedance instead of simple resistance.
How much current is dangerous?
As little as 10 mA AC through the heart can cause fibrillation. Current above 100 mA is often fatal. The danger depends on path through the body, duration, and frequency. This is why electrical safety standards focus on limiting current through ground fault protection.