The video compared life on a late nineteenth century farm to that on a present day farm. Over a hundred years ago, most farms supplied their own energy sources. Woodlots supplied fuel for heating and cooking. Pastures provided energy for horses, mules, or oxen. These animals powered treadmills, plows, grain grinders, and farm wagons. Human labor and hand tools helped do other work like feeding, milking, cutting, and threshing.
By the 1930's, many farms started to use electricity. At first barnyard "wind chargers" provided the electricity. A wind charger was a windmill connected to a generator. The wind charger changed the energy of the wind-turned blades into electricity. The farm family used this electricity for the home and some farm tools. Cheap and plentiful supplies of coal, petroleum, and natural gas helped electric power networks spread across the country. Most windmills were discarded in favor of the inexpensive, steady, and convenient supply of electricity from power plants.
Most likely you have seen flashes of lightning during a summer thunderstorm. Lightning is electricity that can be dangerous to people. Even though electricity can be useful, you must be careful whenever you use it.
People have been interested in electricity for thousands of years. Today, people have many ways of using and controlling electricity. In this unit you will learn how we control and use electricity.
How Do Electric Charges Act?
You might get a shock when you walk across a rug and touch some metal. Perhaps you even notice a spark. You might have to pull apart a shirt and a sock after you take them out of the clothes dryer. Maybe you hear or see a spark and feel its shock.
Sparks often happen after two objects are rubbed together. The rubbing gives an electric charge to both objects. For example, if you rub a balloon with wool cloth, both objects become electrically charged. When you bring them near each other, they pull on -- or attract -- each other. You might notice that sometimes your hair is attracted to a comb or brush after passing either one through your hair several times. Your hair and the comb or brush become electrically charged.
Where Do Charges Come From?
By observing charged objects, people have learned that charge comes in only two kinds. One kind is positive. The other kind is the opposite -- negative. Two objects with a positive charge repel each other. So do two objects with a negative charge. Like charges repel each other. But an object with a positive charge and an object with a negative charge attract each other. Unlike charges attract each other.
You can cut a piece of paper into smaller and smaller pieces. Finally, you reach a piece too small to be cut. Matter comes in even smaller pieces that are too tiny to be seen. These particles of matter are atoms. Scientists conclude that everything around us is made up of these basic units of matter called atoms. Atoms are incredibly small. If 200,000,000 atoms were placed in a line, they would stretch less than a single inch! Inside every atom are even smaller particles that carry positive and negative charges.
Study the diagram model of the atom. Notice that most of an atom is empty space. At the center of an atom is the nucleus. The nucleus has two kinds of particles. Each particle with a positive charge is a proton. Each particle with no charge is a neutron. Swirling around the nucleus at high speed are electrons. The electron has a negative charge. A proton has the same amount of charge as an electron, but the charges are opposites.
Most of the time, objects have the same number of protons as electrons. So the positive charges balance the negative charges, and the object is uncharged.
The electric force attracts the electrons and protons to each other. Electrical charges hold all these particles together, preventing the atom from falling apart. This force holds all matter together.
Electrons can move about more freely than protons or neutrons. Rubbing two objects together takes electrons away from one object and moves them to the other. The drawings show what happens. The object with extra electrons now has a negative charge. The object with more protons than electrons has a positive charge.
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balloons diagram
Sometimes many extra electrons build up on an object. The electrons repel each other because they have the same kind of charge. They move as far away from each other on the object as they can.
If a charge keeps building up on an object, the electrons become crowded. All at once, some of them jump to a nearby object, and you see a spark. All sparks happen in this way. Lightning, which is just a big spark, results from charge building up on clouds.
Think About It:
1. How will two objects with negative charges act?
2. What kind of charges do the particles in an atom have?
3. Challenge What do you think happens to two oppositely charged balloons after they touch each other for a while?
What Is Electric Current?
Sparks show that charges move. A flow of charged particles is an electric current. Just as air has wind currents, matter can have electric currents. A spark is really a burst of electric current in the air.
In a wire an electric current is a flow of electrons. Every atom has one or two electrons which wander about in the spaces between the atoms. When you turn on a light, the electrons move in one main direction as a current. These motions happen very quickly. The light shines almost immediately. But if you had to wait for one electron to make its way along the wire, you would wait several hours for the light to shine!
Electric current passes through different materials differently. For example, current passes easily through many metals. It is much harder for current to pass through air, rubber, glass, and plastic.
Resistance is a measure of how well an electric current passes through a material. Current passes easily through materials with low resistance. These materials are conductors. A conductor is a material that electricity can flow through easily. Most metals, especially gold, silver, copper, and aluminum, are good conductors.
Current moves with more difficulty or not at all through materials with high resistance. These materials are insulators. An insulator is a material that electricity cannot flow through easily. Rubber, glass, and plastic are good insulators.
How Do People Put Resistance to Good Use?
People use both conductors and insulators. In your home, electric current moves through metal wires to reach many appliances. Insulators cover the wires to keep the current away from you and your home.
Current reaches the light bulb in the diagram through copper wires. But, inside the bulb, the current must pass through very thin wires. They are made of a metal that has a higher resistance than copper. Current makes these wires heat up because it is harder for the current to pass through them. The wires become so hot that they give off light.
Think About It:
1. What is electrical resistance?
2. Compare a conductor with an insulator.
3. Challenge Use the idea of resistance to explain how the wires in a toaster toast bread.
Distributing Electricity
Electricity from a turbine-generator cannot be sent directly to your house. This is because electricity flows through a wire much like water flowing through a garden hose. Unless there is pressure pushing the water through the hose, it will not come out the other end. To get electricity through the wires to your home, it must be pushed under pressure. The word voltage describes the pressure in an electrical system. To increase the voltage of electricity, it is sent from generators to transformers just outside the power plant. Transformers increase the voltage, giving the electricity an extra push. This increased voltage helps the electricity travel long distances to reach homes and factories.
In the diagram below, you can learn how electricity travels from a power plant to your home. You probably have seen large metal towers that hold up power lines. The lines carrying high voltage electricity are transmission lines. When the electricity reaches your town, or city, its voltage is too high to use in your home. A substation reduces this voltage. A substation contains many transformers. These transformers reduce the electricity to a more useable voltage. The electricity leaves the substation along wires called distribution lines. These are the familiar wires and poles that you see along streets in most neighborhoods. Distribution lines also may be placed underground. Before the electricity is useable in your house, its voltage must be lowered again. The voltage is reduced one more time by a pole transformer. Locate the pole transformer in the diagram. Notice how distribution lines then carry electricity to the electric meter for your house.
How Does Current Move in a Circuit?
The electricity we get from electric power plants is very easy to use. At the flip of a switch, electric current reaches your lamp or toaster. But, to be used, electric current must follow a closed path called a circuit. Your house is part of a circuit that includes the power plant, transmission lines, transformers and the electric wires inside your house.
The picture shows a dry cell. A dry cell or a battery is a source of electrical energy. One pole of the dry cell has a positive charge. The other pole has a negative charge. The dry cell "pushes" current from the negative pole through a circuit and back to the positive pole. This electrical "push" is like the push of the water pump shown. The pump does work on the water to send the water back up the pipe. The dry cell does work on the electrons to send current through the circuit.
Notice in the first circuit diagram how each part of the circuit is connected. A wire leads from the dry cell to the bulb. Another wire connects the bulb back to the dry cell. Current follows the path from the dry cell through the bulb and back to the dry cell. The lighted bulb shows that current is flowing. The circuit is closed because the path for current to follow is unbroken.
Notice that the bulb in the other circuit diagram is not lighted. The circuit is open because one of the wires is not connected. Current cannot flow in this open circuit because the path has been broken. Current can flow only in a closed circuit.
Using Electricity In Your Home Appliances
You probably use many different kinds of electric appliances in your home. These appliances can range in size from an electric hair dryer to the size of an electric range. As you use more and more appliances to make your work easier, you also use more electricity. This makes your home's electric bills higher. Even though each small appliance uses very little electricity per use, the combined total can greatly increase your electric bills. Look at the graph below. It gives a breakdown of electricity usage in a typical home. Which of these items do you have in your home? What appliances use the most electricity? What items use the least electricity?
Proportion of electricity usage in a typical house
Among small devices, heating appliances such as electric fryers, irons, and hair dryers use the most electricity. If your electricity costs 8 cents per kilowatt-hour, it costs about 10 cents to use a hair dryer for an hour. An electric frying pan would cost about 8 cents for one hour of use. For 8 cents you could open about 400 cans with an electric can opener.
Did you know that many home appliances use electricity even when the control switch is off? Televisions, VCR's, stereos, microwave ovens, and other appliances continue to use small amounts of electricity even when not being used and switched off. TVs and VCRs use the most power when switched off. Many of these continue to use 10 Watts of power. In one month this amounts to 7kilowatt-hours of energy. At 8 cents per kilowatt-hour, this totals $.56 While 10 watts is a tiny amount of power, there are reasons you should be aware of the fact that power is being used when some appliances are switched off. Most homes have several of these appliances. They easily add up to $10 per month.
What is this power used for? There are several different uses. Televisions must keep some circuits working in order to provide "instant on features." Some circuits operate to receive signals from a remote control unit. Other features of televisions, stereos, VCRs, and microwave ovens are clocks and digital displays.
The graph below shows the average power demand for home appliances while they are switched "off." How many of these appliances do you have in your home?
Average power demand for appliances while switched off
How Do We Use Electricity Safely?
Electrical energy can provide heat and light for our homes and energy to run appliances. People use so much electrical energy that they often take it for granted. However, you must never take for granted the safe use of electrical energy.
The electrical wiring in a house is kept out of the way because electric current can be dangerous to people. The wiring is insulated to keep the current in the circuit. A small amount of current can shock or burn you. Current that gets into paper or wood can start a fire by heating that material.
People must be able to control electric current so they use it only when needed. Switches serve this purpose. A switch opens and closes the circuit to let current reach a certain appliance.
Your home is wired with several circuits. Plugging in a lamp or flipping a wall switch connects the lamp into one of these circuits. Too many appliances on one circuit allow too much current to flow. Since heat from too much current can start a fire, you should not use too many appliances at once.
Most homes have devices that stop too much current from flowing through circuits. Circuit breakers are switches. A metal piece in a fuse melts apart when too much current passes through it. Both devices open the circuit when too much current passes through it. When this happens, some of the appliances must be unplugged. Only an adult should reset the circuit breaker or replace the fuse to close the circuit. Then, fewer appliances should be used at the same time.
Think About It:
1. What is a closed circuit? An open circuit?
2. How do transformers help distribute electricity?
3. What is a fuse? A circuit breaker?
4. Explain how appliances might use electricity even when they are switched off.
5. Challenge Make a list of items around your home or school that use electricity even when they are switched off. As a further challenge, calculate the cost per month these appliances use when switched off.
Where Do Charges Come From?
1. They will repel each other.
2. Protons have a positive charge, electrons have a negative charge, and neutrons have no charge.
3. As some of the extra electrons move from the negatively charged balloon to the positively charged balloon, the charges may become neutral.
What is Electric Current?
1. Electrical resistance is a measure of how well an electric current passes through a material.
2. Conductors allow currents to pass more easily than insulators do.
3. The wires in the toaster heat up because they have a higher resistance than other metals. The heat toasts the bread.
How Do We Use Electricity Safely?
1. A closed circuit has an unbroken path. An open circuit has a broken path.
2. Transformers lower the voltage in the electrical wires so that it is safe enough to use in the house.
3. A fuse is a device with a metal piece that melts apart and breaks a circuit when too much current passes through it. A circuit breaker is a switch which opens a circuit.
4. Televisions use power even when turned off because they must keep current running through some parts of the appliance to allow certain features to work.
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