Summary
Highlights
Electric current is defined as the flow of net charge through a given area. It's measured as the amount of charge (coulombs) flowing per second, which is called an Ampere. Household appliances draw currents ranging from fractions of an Ampere to 10-15 Amperes, while lightning can be tens of thousands of Amperes.
For electric current to flow, two things are crucial: a voltage (electric potential difference) and a conducting medium. Materials like glass and plastic are insulators because their electrons are tightly bound, preventing charge flow. Metals are conductors because they have free electrons that can move when a voltage is applied. The voltage can be supplied by batteries in small circuits or large generators in power stations.
A circuit must be closed for current to flow; an open circuit, like one with air gaps, acts as an insulator. When a circuit is closed, a voltage creates an electric field that causes free electrons in a conductor to drift, constituting the current. Electrons are not at rest without voltage; they are always moving randomly at high speeds.
Positive charges move from high to low electric potential (downhill), converting potential energy into kinetic energy. Negative charges, like electrons, move in the opposite direction, from low to high potential (uphill), but they are still losing potential energy from their perspective, as the reference for potential is based on positive charges.
The conventional direction of current is defined as the direction positive charges would flow. For negative charges like electrons, the conventional current direction is opposite to their actual direction of drift. This convention, although sometimes counterintuitive when dealing with electron flow, is standard in physics.
Lightning is a powerful electric current through the air, which is typically an insulator. During a thunderstorm, charges build up in clouds (positive at top, negative at bottom), inducing opposite charges on the ground. When the potential difference becomes millions of volts, electrons are ripped from air molecules, creating a conducting channel. This allows charges to flow to the earth, creating a lightning bolt, which is essentially a massive spark, similar to static electricity discharges.