104 Setting Up Circuits-High

Setting Up Circuits FOCUS Question How does electricity flow through a circuit?

Electrifying! You walk into a room, flip a switch and, like magic, the room instantly lights up. But it's not magic. It's electricity and the science of atoms. An atom is the smallest part of an element that can exist by itself. Atoms have three parts— neutrons, protons, and electrons. Neutrons and protons make up the nucleus, or center, of the atom. The negatively charged electrons inside the atom swirl around the nucleus, forming rings. The electrons in the outermost ring can jump from one atom to the next, and when they do, an electric current flows. An electric current is the electricity that runs through power lines, the wires in your house, and even a flashlight.

Powering the Circuit To get from one place to another, an electric current travels through a closed, looped pathway called a circuit. Every circuit starts with a power source to get electrons moving. Many sources can power a circuit, including power plants, batteries, windmills, and solar cells. Electric current runs through a circuit in one of two ways. The simplest is direct current (DC), in which electrons move around the circuit in just one direction. Batteries are a source of direct current, which is good for powering simple toys and gadgets. More complex circuits require alternating current (AC). Instead of racing one way around a circuit like cars around a racetrack, the speedy electrons in an alternating current keep reversing direction. (Don't try this on a real racetrack!)

Go with the Flow In both DC and AC currents, a power source gets the electrons moving, but it's a conductor that moves the electric current along the circuit. Metal wires made of copper, aluminum, or silver make good conductors. In the atoms of these metals, electrons are loosely bound to the outer ring and can easily move to new atoms and create a current. In fact, the electrons move so much that insulators are needed to keep the current from exiting the wires. In insulators, such as rubber or plastic, the atoms have tightly-bound electrons that block the flow of electricity. Next, the circuit transfers electricity to a device, or a load, that uses the energy. Washing machines, computers, and lightbulbs are loads that use the energy in a circuit. Loads convert electrical energy into other forms of energy. Lightbulbs, for example, change electricity into light and heat. Radios are loads that convert electrical energy into sound energy.

Components of Circuits A simple circuit is made up of a power source, a conductor, and a load. More complex circuits have additional parts. 1 Capacitors temporarily store a charge. In some circuits, they block direct current while allowing alternating current to pass by. They help stabilize power flow in circuits. 2 A resistor is a part of a circuit that reduces the flow of current. Resistors keep loads from becoming too hot and allow people to control the amount of current flowing through a circuit. 3 A transistor is a device that is used as a switch to open or close the circuit or to amplify (strengthen) the electrical signal in a circuit.

Circuit parts such as transistors, resistors, and capacitors have a number of real-world functions. Transistors are often used in mobile phones, televisions, and hearing aids. They help make sounds louder and transmit radio waves. Transistors inside machines called pacemakers help regulate the heartbeat of patients with abnormal heart rhythms. Resistors help avoid an interruption to the current, called a short circuit. They control the volume in TVs and radios. Resistors even allow us to light our houses the way we want by regulating dimmer switches on lights that have multiple settings. Capacitors help radios tune into a specific frequency. They also keep machines like alarm clocks working even if the rest of the power to a household is interrupted. Do You Know? Cameras use capacitors to create a flash.