Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Electric Current

An electric current is a movement of charge. When two objects with different charges touch and redistribute their charges, an electric current flows from one object to the other until the charge is distributed according to the capacitances of the objects. If two objects are connected by a material that lets charge flow easily, such as a copper wire, then an electric current flows from one object to the other through the wire. Electric current can be demonstrated by connecting a small light bulb to an electric battery by two copper wires. When the connections are properly made, current flows through the wires and the bulb, causing the bulb to glow.

Current that flows in one direction only, such as the current in a battery-powered flashlight, is called direct current. Current that flows back and forth, reversing direction again and again, is called alternating current. Direct current, which is used in most battery-powered devices, is easier to understand than alternating current. Most of the following discussion focuses on direct current. Alternating current, which is used in most devices that are “plugged in” to electrical outlets in buildings, will be discussed in the Alternating Current section of this article.

Other properties that are used to quantify and compare electric currents are the voltage (also called electromotive force) driving the current and the resistance of the conductor to the passage of the current. The amount of current, voltage, and resistance in any circuit are all related through an equation called Ohm’s law.