## CIRCUITS

An electric current flows in a loop, powering bulbs or other electric COMPONENTS. The loop is an electric circuit. A circuit is made up of various components linked together by wires. The current is driven around the circuit by a power source, such as a BATTERY.

Table 26. CIRCUIT DEFINITIONS

 Voltage is the energy given to each unit of charge that flows in a circuit Current is the amount of electric charge flowing past a point in a circuit each second Wattage is the amount of electrical energy a circuit uses each second

## WHAT IS AN ELECTRIC CURRENT?

Electric current is a flow of electric charge (usually in the form of electrons) through a substance. The substance or conductor that an electric current flows through is often metal wire, although current can also flow through some gases, liquids, and other materials.

Circuits can be wired in two ways. In a series circuit, current flows to each bulb in turn. Both bulbs are dimly lit. In a parallel circuit, the current divides and flows directly to both bulbs at the same time. The bulbs are then brighter.

## WHEN DOES CURRENT FLOW IN A CIRCUIT?

Current only flows when a circuit is complete—when there are no gaps in it. In a complete circuit, the electrons flow from the negative terminal (connection) on the power source, through the connecting wires and components, such as bulbs, and back to the positive terminal.

## WHAT MAKES CURRENT FLOW IN A CIRCUIT?

When a wire is connected to battery terminals, electrons flow from negative to positive. Unlike (opposite) charges attract, like (same) charges repel. Electrons have a negative charge—they are repelled from the negative and attracted to the positive.

## BATTERY

A battery is a compact, easily transportable source of electricity. When a battery is connected in a circuit, it provides the energy that drives the electrons along in a current. Batteries contain chemical substances that react together to separate positive and negative charges.

### WHAT IS INSIDE A BATTERY?

A battery is made of one or more sections or cells. Inside each cell, two chemically active materials called electrodes are separated by a liquid or paste called the electrolyte. Small batteries may have just one cell. Large, powerful batteries may have six cells.

### HOW DOES A BATTERY CELL WORK?

Inside a cell the electrolyte reacts with the electrodes, causing electrons to move through the electrolyte from one electrode to the other. One electrode gains a negative charge and the other a positive charge. The two electrodes are the positive and negative terminals.

## COMPONENTS

The different objects that make up a circuit are called components. A circuit must have a power source, such as a battery, and the current flows through a conductor, such as a wire. Bulbs, buzzers, and motors are components that change electricity into light, sound, and movement.

### HEART PACEMAKER

The battery and other components of an artificial heart pacemaker send electric pulses through wires to a patient’s heart to keep it beating steadily. A pacemaker is put in when the heart does not beat steadily by itself.

### WHAT IS A CONDUCTOR?

A material that carries a current well is called a conductor. Metals are good conductors because metal atoms readily release electrons to carry the current. Silver and copper are the best conductors, and most electric wires are made from copper. To prevent electric shocks, wires are covered with an insulator.

### WHAT IS AN A INSULATOR?

Some materials do not carry current well. They are said to resist (oppose) the flow of current. Materials that do this are called insulators. Plastics, glass, rubber, and ceramics are all good insulators. Insulators are used to cover wires and components to prevent electric shocks, and to stop currents from flowing.

### HOW DOES A SWITCH WORK?

Switches are like gates that control the flow of electricity in a circuit. When a switch is open, it creates a gap in the circuit and current will not flow. When it is closed, it completes the circuit, and current flows through it. Switches are used in parallel circuits to turn different parts of the circuit on and off.

### HOW IS ELECTRICITY SUPPLIED?

Most of the electricity we use in our homes and workplaces is produced by machines in power stations called generators. Generators send the electric current through a huge network of circuits and wires and into houses, offices, and other buildings.

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