Electricity

Watch a bolt of lightning flash across the sky. Flip a switch and light up your bedroom. Click the remote and see the TV come on. What do all of these things have in common? Electricity.

WHAT IS ELECTRICITY?
Electricity is a powerful force of nature. Electricity is everywhere in the universe. Electrical forces hold water, metals, and all other kinds of matter together. You can walk and run because electric signals go through your nerves from your brain to your muscles. The signals tell your muscles where to move. Electricity makes many machines work. Electricity makes bulbs light up and runs motors in saws, fans, hairdryers, and other appliances. The computer you are using works because of electricity.

WHERE DOES ELECTRICITY COME FROM?
Electricity starts with atoms. Atoms are tiny bits of matter much too small for you to see. Everything in the universe is made up of atoms.Atoms have two main parts: a center or nucleus, and electrons that orbit or go around the nucleus. Electricity comes from electrons. You cannot see electrons and you cannot see electricity. You can see what electricity does because of electric charge and electric energy.

WHAT IS ELECTRIC CHARGE?
Electric charge comes from the parts inside atoms. There are two kinds of electric charge called positive charge and negative charge. Positive charge comes from the nucleus of an atom. Negative charge comes from electrons. Atoms do not normally have any overall charge because their positive and negative charges cancel each other out. Charge comes when electrons move away from an atom. Positive charge is just the opposite of negative charge. Positive and negative charges pull toward each other. The pull of positive and negative charges makes two kinds of electricity—static electricity and electric current.

WHAT IS STATIC ELECTRICITY?
Did you ever get a shock after walking across a carpet and touching a metal doorknob? That shock came from static electricity. Huge amounts of static electricity cause lightning. Electrons that move away from their atoms cause static electricity. You can make static electricity by rubbing certain materials together. Run a plastic comb through your hair. Be sure your hair is clean and dry. Electrons jump from your hair to the comb. This gives the comb a negative electric charge. Your hair loses electrons. This gives your hair a positive electric charge. Hold the
comb above your head and watch some of your hairs stand on end. Your hair stands on end because the positive and negative charges are pulling toward one another.

Static electricity also causes lightning. The pull of positive and negative charges between clouds and the ground creates a huge spark. The spark is actually the charges moving very quickly toward each other. Lightning can also be caused by opposite charges inside one cloud, between two clouds, or between clouds and the air.

WHAT MAKES LAMPS LIGHT UP?
Electric current makes lamps and all other electric devices work. Electric current is actually electrons moving in a big loop. Something must give the electrons a push to get them moving.Batteries can start electrons flowing. Batteries are a source of electric energy. A battery, two wires, and a light bulb can make an electric circuit. The current starts flowing from the battery through a wire to the light bulb. The other wire carries the electric current back to the battery. If you cut the wire, the electric current stops. Switches on an electric circuit turn the current on and off. This is how a wall switch works to turn lights on and off in your home. The electric energy in your home does not come from batteries. You plug appliances into electric outlets in your walls. The electric energy in the outlets comes from electric power plants.

HOW DO POWER PLANTS WORK?
Huge electric power plants generate or make electricity. Steam or falling water in dams make big machines called turbines turn. The turbine drives another machine called an electric generator. The generator makes electricity. Long power lines carry electricity from power plants to your home. Wires inside your home bring the electric energy to light bulbs, TVs, microwaves, and your computer.

WHO DISCOVERED ELECTRICITY?
For thousands of years people knew that a material called amber mysteriously pulled on some materials. The ancient Greeks called amber elektron. Scientists in Europe in the 1600s and early 1700s called the materials that amber attracted electrics. Benjamin Franklin, an American printer, patriot, and inventor, experimented with electricity. He thought lightning and electricity
were the same thing. He did a dangerous experiment in the mid-1700s to find out. Franklin flew a kite during a thunderstorm. He attached a metal key to the kite string. An electric charge ran down the wet kite string to the key. The charge made a spark when it hit the key.

This showed Franklin that lightning was electricity. He was lucky he was not killed. Many other scientists have experimented with electricity since Benjamin Franklin. They learned how to make electricity with batteries. They found that electricity would go through wires. An American inventor named Thomas Alva Edison invented many things that use electricity, including the electric light bulb.

Lighthouses

Lighthouses are tall buildings near seacoasts. They shine flashing lights at night or in foggy weather. These lights guide ships that sail close to the coast. Many lighthouses also have foghorns to guide ships in foggy weather. Lighthouses are built at places on a coast that are important to ships. They warn ships of hard-to-see dangers such as rocks or strong currents. They also mark entrances to harbors or rivers. Most lighthouses are painted white so thatthey can be seen easily during the day.

WHEN WERE LIGHTHOUSES FIRST BUILT?
Lighthouses have been protecting sailors for thousands of years. Lighthouses were built on the coast of the Mediterranean Sea at least as long as 2,600 years ago. One of the so-called Seven Wonders of the World was an enormous lighthouse called the Pharos, built at Alexandria in Egypt. Ancient lighthouses were simple structures. Fires burned on top of them as signals to ships.

LIGHTHOUSE KEEPERS
Until fairly recently, many lighthouses had lighthouse keepers living in them. The job of the lighthouse keeper was to make sure the lights were kept in working order at all times. Sometimes the lighthouse keeper’s family lived in the lighthouse, too. It was lonely living in a lighthouse in an out-of-the-way place. The lighthouse keepers might not see any other people for weeks. These days, lighthouse keepers are no longer needed. The lighthouses work automatically.

LIGHTING A LIGHTHOUSE
Lighthouses of the past were lit by burning coal or wood. In the late 1700s and 1800s, oil lamps became popular. Many lighthouses burned whale oil, especially lighthouses in the United States. In the late 1800s, lighthouses began to burn gas in their lamps. Lighthouse keepers were needed to keep fires burning and clean up soot the fires created. Today, most lighthouses use high-power electric lights that rotate. Reflecting mirrors and lenses make the light beam stronger. The lights work much like the lamp on top of a police car, but they are much bigger and more powerful.

X Rays

Imagine that you could see right through your own skin. You could see the bones inside your body. You could watch food go down your throat when you swallow it. Imagine looking inside someone’s suitcase to see what’s inside. Does that sound impossible? Not when you know about X rays!

WHAT ARE X RAYS?
X rays are very powerful light rays that your eyes can’t detect. These light rays can slip through objects that visible light bounces off. We use X rays as a powerful tool to detect and discover things our eyes can’t see.

HOW WERE X RAYS DISCOVERED?
X rays were discovered by accident. In 1895, a man named Wilhelm Roentgen was experimenting with electricity in vacuum tubes in a black cardboard box. He noticed that a special screen he had nearby glowed when electricity went through the tubes. He experimented more and determined that invisible light rays from the tubes caused the screen to glow. These rays went right through the cardboard box! He named the invisible light rays he had found X rays.

WHAT ARE X RAYS USED FOR?
Just a few years after X rays were discovered, doctors were already using them to find bullets inside people who had been shot. Doctors later began to use X rays to find out if people are sick or have broken bones. Dentists use X rays to check up on people’s teeth. An X-ray device called a CAT scan rotates around a person and creates a 3-D picture of the person’s insides on a screen. This device gives doctors clear views inside any part of the person’s body. Scientists who study matter and energy often use X rays in their research. X rays help them see what things are made of. Many chemical elements were discovered using X rays. Industries use X rays to test products and materials for flaws such as cracks in an airplane wing. X rays are also used to tell whether gems and works of art are real or fake. Border guards use X rays to look inside cars and containers. The X rays can find goods that are being smuggled from one country to another. Airports use low energy X rays to see inside luggage and check for dangerous items.

DOCTORS’ X RAYS
When a doctor takes an X ray of you, the X-ray machine shoots X rays at you. Most of the rays go through you and into a special film, which catches them. Some of the X rays that hit your bones, however, don’t make it through you. Bones absorb X rays more than other parts of your body. Because X rays absorbed by your bones never make it to the film, lighter areas appear on the film where your bones are! These lighter areas provide a picture of the bones. X rays can be harmful. Doctors use X rays to kill cells that are harmful to people, such as cancer tumors. Because too many X rays can be harmful, doctors warn that X rays should be used only
when necessary.

Color

The world is full of beautiful color. You see a violet dress, a blue car, a yellow flower, and a green tree. The colored leaves on trees in autumn mean that winter is coming. All kinds of colors are everywhere.

COLOR COMES FROM LIGHT
White light, including sunlight and light from a light bulb, is actually made of all the colors of the rainbow. Have you ever seen sunlight that hits a piece of crystal? Rays of blue, purple, orange, yellow, and red seem to shoot out from the crystal in all directions. The crystal spreads the colors of light apart a bit so you can see them separately. Scientists show the colors of light in a bar called a spectrum. A rainbow is a spectrum. Its colors go from red through orange, yellow, green, blue, indigo, and violet. Violet is purple and indigo is a deep purplish-blue. It’s easy to remember the colors of the spectrum because the first letter of each color makes up a name:
Roy G. Biv.

HOW DO WE SEE COLOR?
White light, such as sunlight or light from a light bulb, lets you see things. Dresses, cars, and all other things we see absorb (soak up) some of the colors of white light. The colors that don’t get absorbed bounce off of things. Red light bounces off a red dress. The dress soaks up other colors. Your eyes see the red light but not the other colors. Your eyes send this message to your brain. Your brain tells you that you are seeing a dress that looks red. Something very special happens when you see a red dress or green grass. Objects themselves don’t actually have color. What they have is the ability to reflect (bounce back) different types of light. When this reflected light enters your eyes, your brain interprets the different types of light as color. Your eyes and brain work together to translate the different types of light into different colors. COLORS OF PAINT Artists call three special colors the primary colors of paint. These colors are magenta (purplish-pink), yellow, and cyan (light greenish-blue). You can make other colors of paint by mixing the primary colors together. But you cannot mix other colors of paint to make a primary color. Suppose you want to paint a picture of an apple tree. You can make whatever colors you want to use with just four jars of paint: magenta, yellow, cyan, and white. You mix yellow and cyan to make green paint for the leaves. For the apples, mix magenta and yellow paint to make the color red. To paint the sky light blue, you must use some white paint. White makes other colors lighter. Mix magenta and cyan to make a deep blue. Then add some white paint to the blue paint until the blue becomes light enough for the sky. White paint mixed with blue or another color is called a tint. A light blue tint will make a color like a clear sky. If you mix all three colors together you get black paint. You can make a color darker by mixing it with black paint. Colors mixed with black paint are called shades. When you mix black and white
together, you get gray. By mixing together different amounts of magenta, yellow, cyan, and white paint, you can make lots of different colors. Everything you need to make beautiful paintings.