How Circuits Work

Circuit Basics

This circuit board contains many individual circuits.
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You've probably heard these terms before. You knew they had something to do with electricity, but maybe you weren't sure quite sure how.

Just as your heart produces the pressure to make blood circulate, a battery or generator produces the pressure or force to push electrons around a circuit. Voltage is the force and is measured in volts (V). A typical flashlight battery produces 1.5V, and the standard household electrical voltage is 110V or 220V.


­Electrical current, or flow of electrons, is measured in amperes (A). The product of electric force (in volts) and current (in amperes) is electrical power, measured in watts (W). A battery generating 1.5V and producing a current flow of 1A through a flashlight bulb delivers 1.5V x 1A = 1.5W of electrical power.

The blood flowing through your body doesn't get a free ride. The walls of the blood vessels impede the flow, and the smaller the blood vessel, the more the resistance to flow. Some of the pressure produced by your heart is just for pushing blood through blood vessels. As electrons move through wires, they bump into atoms. This impedes the flow of the electrons. The wire offers resistance to the flow of the current. The amount of resistance depends on the material, diameter and length of the wire. The resistance increases as the diameter of the wire decreases. Resistance is in units of ohms (Ω).

Ohm's Law relates voltage, current and resistance:

Resistance (Ω) = Voltage (V)/ Current (I)

Ohm's Law can be written as R = V/I.

Electric circuits are composed of wires and other components -- like light bulbs, transistors, computer chips and motors. Wires, made of metals called conductors that have a low resistance to current, connect the components. Copper and aluminum are the most common conductors. Gold, because of its resistance to corrosion, is often used for attaching wires to tiny electronic chips.

­In an incandescent bulb, the current flows through a thin tungsten wire or a metallic filament that offers high resistance to current flow. When the electrons bump into the atoms, the friction, or loss of kinetic energy, produces heat. If the temperature of the filament is high enough, it starts to glow and give off light. This is incandescence. Typical filament temperatures for light bulbs are around 4,600 degrees F (2,550 degrees C). Unfortunately, 90 to 95 percent of the energy supplied to a light bulb is lost in the form of heat rather than light, so incandescent bulbs are very inefficient.­

Fluorescent lights produce light by having electrons pass through a tube filled with mercury vapor and neon or argon gas. As the electrons bump into the mercury atoms, they cause electrons in the atoms to absorb some of their energy. As these electrons return to their normal state, they radiate bundles of light energy called photons. Fluorescent lights are four to five times more efficient than incandescent bulbs.

On the next page, we'll look at closed circuits, open circuits, short circuits, series circuits and parallel circuits.