Electric Current

A flow of electric charge through a conductor is called electric current. Energy is associated with the flow of current. As current flows through electric devices, this energy may be converted to useful forms. For example, electric energy is converted into heat by an electric range and into light by a light bulb.

Direct and alternating current. Current that flows steadily in one direction is called direct current (DC). A battery produces direct current. Sometimes current flows back and forth, changing direction rapidly. It is then called alternating current (AC). The current in household wiring is alternating current. In the United States and Canada, household current reverses direction 120 times per second, completing 60 full cycles.

Sources of current. By itself, a conductor does not have electric current flowing in it. But if a positive charge is applied to one end of the conductor, and a negative charge to the other end, then electric charge will flow through the conductor. Because positive and negative charges attract, some type of energy must be supplied to separate the charges and keep them at opposite ends of the conductor. The energy may come from chemical action, motion, sunlight, or heat.

Batteries produce electric energy by means of chemical action. A battery has two structures called electrodes, each made from a different chemically active material. Between the electrodes, the battery contains a liquid or paste called an electrolyte, which conducts electric current. The electrolyte helps promote chemical reactions at each electrode. As a result of the chemical reactions, a positive charge builds up at one electrode and a negative charge builds up at the other. Electric current will then flow from the positive electrode, through a conductor, to the
negative electrode.

In a flashlight battery, the flat end is the negative electrode. The end with a bump connects to the positive electrode. When a wire links the electrodes, a current flows. The electric energy is converted to light if it passes through a flashlight bulb. Chemical reactions in the electrolyte keep the electrodes oppositely charged and so keep the current flowing.

Eventually, the chemical energy runs out and the battery can no longer produce electric energy. Some worn-out batteries must be discarded. Others, called rechargeable batteries, can be charged again by passing electric current through them.

Generators change mechanical energy into electric energy. In a generator, a source of mechanical energy spins coils of wire near a magnet to produce electric current. A generator works because moving a conductor near a magnet produces a current in the conductor. Most generators produce alternating current.

Generators furnish most of the electric energy people use. In a car, a small generator called an alternator is turned by the engine and produces electric energy that recharges the car's battery. A large generator in an electric power plant can provide enough electric energy for a city of 2 million people. Electric current from the generator reaches homes, factories, and offices through vast networks of power lines.

Solar cells, also called photovoltaic cells, convert sunlight into electric energy. Solar cells power most artificial satellites and other spacecraft as well as many handheld calculators. Photovoltaic cells are made from semiconducting materials, usually specially treated silicon. Energy from the sun forces negative and positive charges in the semiconductor to separate. The charges will then flow through a conductor.

Piezoelectric crystals are nonmetallic minerals that develop electric charge along their surfaces when stretched or compressed. Quartz is the most common piezoelectric crystal. Some microphones use piezoelectric crystals to convert sound energy into electric energy for recording or radio broadcasting. Modern gas ranges have piezoelectric crystals instead of pilot lights. The crystals produce electric sparks that ignite the gas.

Electric circuits

To use electric energy, an electric device must be connected to an energy source. A complete path must be provided for electric current to flow from the energy source to the device and back again. Such a path is called an electric circuit.

A simple circuit. Suppose you want to make a battery-powered light bulb shine. Electric current will only flow if there is a complete circuit that leads from the battery to the bulb and back to the battery. To make the circuit, connect a wire from the positive terminal of the battery to the light bulb. Then, connect another wire from the bulb back to the negative terminal. Electric current will then flow from the battery's positive terminal, through the light bulb, to the battery's negative terminal.

Inside the bulb is a thin wire called a filament. The filament is made from a material with greater resistance than the wires linking the battery and bulb. The moving electrons that make up the current collide with atoms in the filament and give up most of their energy. The released energy heats the filament, which glows and gives off light.

Series and parallel circuits. A single battery or generator often powers more than one electric device. In such cases, circuit designs called series circuits and parallel circuits are necessary.

A series circuit has only one path. The same current flows through all parts of the path and all electric devices connected to it. Flashlights, some Christmas tree lights, and other simple devices use series circuits. In a parallel circuit, the current splits to flow through two or more paths. Parallel circuits enable a single energy source to provide current to more electric devices than a series circuit could. Household lights and appliances are connected in parallel circuits.

Many circuits include some parts that are series and some that are parallel. An extremely complex circuit, like that in a computer or TV, has millions of parts connected in various series and parallel combinations.

Electric fields. When most people think of an electric current, they think of moving electrons carrying charges through a wire. Actually, most of the energy flows through the space around the wire in the form of electric and magnetic fields. Energy from the fields enters the wires and replaces energy the electrons lose through resistance. The battery, generator, or other energy source continually restores energy lost from the fields.

In DC circuits, electrons flow from one battery terminal, through the circuit, to the other terminal. But the energy of the electric and magnetic fields flows at the same time from both terminals to the electric device. In AC circuits, individual electrons move back and forth in the wires and do not travel the entire circuit. Nevertheless, electric energy flows from the energy source to the device in the form of the fields.

Controlling electric current. The simplest way to stop a current flowing through a circuit is with a switch. A basic switch consists of two electric conductors that can be moved apart to create a gap in a circuit. When the switch is off, the gap is open, and no current flows. When the switch is on, the conductors are connected, and current flows.

Wires and electric devices become dangerously hot if too much current flows through them. Switches called fuses and circuit breakers protect the wiring in most buildings. If too many electric devices are plugged into an outlet, a fuse or circuit breaker will shut off the current. Many individual electric devices also contain fuses.

Sometimes people need to vary the strength of current, rather than merely turn it on or off. One way to adjust current strength is to vary resistance within the circuit. For example, turning the volume knob on a radio operates a variable resistor. This device adjusts resistance to the flow of current through the radio, making the sound louder or softer.

Switches and variable resistors cannot change currents quickly. Tiny semiconductor devices called transistors can be used to adjust current more rapidly. Transistors act as high-speed switches that turn on and off billions of times each second. Some devices contain millions of transistors on a single tiny chip of silicon, called an integrated circuit or simply a chip. Integrated circuits form the heart of computers, calculators, video games, and many other devices.

Electrically powered devices are said to be electronic if they carry electrical signals that can be varied in some way to represent information. Electronic devices include transistors, diodes, capacitors, inductors, and integrated circuits. Signals may represent sounds, pictures, numbers, letters, computer instructions, or other information. In the amplifier of a compact disc player, for example, transistors provide a continuous range of currents that strengthen electrical signals representing the sounds being played.