How can sunlight produce electricity?

How can sunlight produce electricity?

Materials needed

•  Solar cells
•  Galvanometer
• Light source

Procedure

1. In recent years scientists have been trying to find new sources of electricity to replace our rapidly diminishing fossil fuel resources (coal and oil). A most promising source is solar (sun) energy. Look at your solar cell. When light strikes this cell, a very small amount of electrical energy is produced.

2. Connect your cell to the galvanometer and shine a bright light on it. As you turn the light on and off, observe the needle on the galvanometer. What happens to the needle?

For problem solvers: if you can get some solar cells, learn how to connect them to operate a small light bulb. Perhaps you can find some discarded calculators and use the solar cells from them.

Do some research and see learn about what you can learn about solar energy as a source of electricity. Make a list of all of the things you can find that are powered by solar energy. Will it likely ever be a major source of electricity for our homes and factories?

Can solar energy be used as a power source for electric ears? Write to general motors (ask about their sun racer) and to ford motor company and ask for information about their research on solar batteries and solar powered ears.

Solar energy is becoming increasingly important and common in our lives. Most children have seen solar cells used in calculators, cameras, and other devices that require small amounts of electricity.

The space program has rapidly expanded the development of this energy source. Satellite use electricity produced in this manner to recharge the batteries that provide electrical power.

A major obstacle to wide use of solar power is the limited amount of electric current each cell can produce. Huge areas of solar cells are required to produce significant amounts of electrical energy. The current produced flows in one direction just as in flashlight batteries... also, on the earth, solar cells as primary producers of electrcal energy are limited to day light hours and further inhibited by cloudy days.

Solar cells (often in clusters connected in series) can be obtained from many electronic supply stores.

How can electricity help us communicate?

How can electricity help us communicate?

Materials needed

• Two small light sockets with bulbs 
• Two switches
• Lantern battery 
• Six 1 m (1 yd.) lengths of insulated copper wire.

Procedure

1. Use your materials to construct a circuit like the one in figure 8.22-1.
2. Press one switch. What happened?
3. Release the first switch and press the second one. What happened?
4. Press both switches at once.
5. Can you think of some use for a device like this?
6. If it is available, you could splice more wire into the circuit and take one switch and light into another room.

For problem solvers:

 look up the mores code in an encyclopedia. If you and a friend would care to learn the mores code, you could have a lot of fun sending messages to each other. Or you might prefer to create your own code. Set up your telegraph sets so that you can be in separate rooms, or in separate parts of the room, and make each other's light blink with your switch.

Teacher information

This is a variation of the telegraph. There are several ways to wire the circuit. This one is parallel. The original telegraph sets were wired in series so all keys to but one had to be closed and all messages went through all the sounders in the circuit. Telegraphs offices often followed the railroads and needed only one wire on the poles. The iron rails were used as the second or ground wire. The original telegraph, patented by Morse, used an electromagnet to attract a magnetic material (soft iron) and make a loud clicking sound. Telegraphers were trained to hear combinations of long and short "dots and dashes" to represent letters of the alphabet. This is called the mores code.
The completion of the transcontinental telegraph near the end of the civil war was extremely significant. For the first time, a message could travel across the nation in seconds rather than in weeks. The circuit above uses the electric light, which was not invented until much later.
In addition to the activity suggested in the "for problem solvers" section, your motivated students might enjoy designing and constructing a telegraph using electromagnets.

Where is your centre of gravity?

Materials needed

• · Pencil or other small objects

Procedure

1. Put your pencil on the floor.
2. Standing near the pencil, pick it up without bending your legs or moving your feet.
3. Stand against the wall, with your heels touching the wall.
4. Drop your pencil on the floor near your feet.
5. Bend over and pick up your pencil without moving your feet or bending your legs. You must also not lean against anything or hold onto anything for support.
6. What happened? Why?
7. Repeat steps 1 and 2. Notice your movements as you pick up the pencil. Explain what happened in step 5 in terms of the effect of the centre of gravity.

For problem solvers: 

try this activity with family members and friends.try replacing the pencil with a coin or with a dollar bill. Can you find anyone who can pick up the object without breaking the rules?

Teacher information

Any time we are on our feet, whether we are walking, running, standing, or bending overawe are constantly adjusting to the centre of gravity in order to remain “balanced”. The body makes these adjustments so automatically that we don’t think about them.
The “for problem solvers” section invites students to try this activity with family members. Replacing the pencil with a dollar bill and offering it to the person who can pick it up without breaking the rules will increase interest and effort substantially. The money is as safe as if it were behind lock and key.

What is centre of gravity?

What is centre of gravity?

Materials needed

• Meter stick (or yardstick)
• String
• Chair
• Various books

Procedure

1. Balance the meter stick on the back of the chair. It will balance at its "centre of gravity," which should e at or very near the 50-cm (18-in) mark. The part of the chair where the meter stick rests is the fulcrum.
2. Get two identical books and tie a string around each one.
3. Make a loop in the other end of each string and slide the loops over opposite ends of the meter stick. Leaving the books supported at the ends of the meter stick, where is the centre of gravity (where the fulcrum has to be to balance the books)?
4. Replace one of the books with a smaller book. With the books still suspended at the ends of the meter stick, where is the centre of gravity?
5. Replace the other book with a larger one. Where is the centre of gravity now?
6. What can you say about the centre of gravity when a large object is balanced with a small object? Consider the teeter-totter as you explain your answer.

For problem solvers: 

do you know what a mobile is? Build one? then explain why it is important to know about centre of gravity when constructing mobiles? If you do not know what a mobile is, ask your teacher, a parent, or a friend to help you get started.

What happens when you burn a candle at both ends?

What happens when you burn a candle at both ends?

Materials needed

• Candle
• Match
• Toothpicks
• Water glasses

Procedure

1. Prepare a candle so the wick may be lighted at both ends.

2. Insert round toothpicks into the candle and balance it on the water glasses as shown in the illustration. It doesn't have to balance perfectly.

3. Predict what will happen if you light body ends of the candle.

4. Light both ends of the candle. Observe for several minutes. What happened?

5. Think about it. And explain what happened the best you can. Share your ideas with others in your group.

Teacher information

Caution: activities with fire or heat should be done only under close adult supervision or as a teacher demonstration.

                When the candle is lighted at both ends, the end tilting downward will burn wax away more rapidly and become lighter. When it tilts up, the other end will be down, and it will burn wax away more rapidly. As this process continues to reserve, the candle will rock back and forth, often quite vigorously.

How does gravity affect heavy and light objects?

How does gravity affect heavy and light objects?

Materials needed

• Large book
• Small book
• Wadded paper
• Pencil
• Eraser
• Paper clip
• Paper

Procedure

1. Take the large book in one hand and the small boo in other. Hold the two books at exactly the same height.
2. Drop both books at the same time, but before you drop them, predict which one will fall faster. Have someone watch to see which book hits the floor first.
3. Repeat the book drop three times to be sure of your results.
4. Which book falls faster, the large one or the small one?
5. Compare the pencil and the paper in the same way. First predict which you think will fall faster.
6. Compare the various objects, two at a time. In each case predict which will fall faster, and then drop them together three times to test your prediction.
8. Explain how the force of gravity compares with objects that are large, small, heavy, and light, according to your findinding, how do the falling speeds compare?
9. Compare the falling speed of the wadded paper with that of a flat sheet of paper dropped horizontally.
10. Compare the falling speeds of two flat sheets of paper, one dropped vertically and the other horizantly.
11. Compare the falling speed of the wad of paper with that of a flat sheet of paper dropped vertically.
12. Discuss your observations with your group.

For problem solvers: 

go to encyclopedias and other resources and do some research about gravity, can you find out what really causes gravity? How large does an object have to be in order for it to have a gravitational attraction for other things? How much do scientists know about gravity?

Teacher Information

The force of gravity pulls all objects to the earth at the same rate, regardless of the size or weight of the object. Air resistance can slow the rate of fall, so the flute paper held in horizontal position will fall more slowly. Except for the factor of air resistance, however, the rate of fall is equal. A rock and a feather will fall at the same speed if placed in a vacuumed chamber.
Scientists are still trying to figure out exactly what gravity is. They have learned a great though the force is too weak to really notice unless the objects are huge, as with planets and stars.

How can magnet do work?

How can magnet do work?

Materials needed

• · Magnet
• · Steel ball

Procedure

1. Place the magnet on on the table.
2. Place the steel ball on the table about 2--3 cm (1 in.) from the end of the magnet.
3. Let go of the steel ball.
4. What happened?
5. What is work, and how was work done in step 3?

For problem solvers:

 find a variety of magnets. Predict which ones are strongest and weakest, and lay they out in order from strongest to weakest, according to your predictions. Then continue with the above activity, comparing the strength of these magnets. Which one seems to attract the steel ball from the farthest distance?
Were your predictions accurate? Compare size with strength. Are larger magnets always stronger than smaller magnets?

Teacher information

Work was dined in this section's "to the teacher” as moving something (a force acting through a distance). The magnet should cause the steel ball to roll toward it. If this did not happen, try putting the steel ball a bit closer to the magnet or find a stronger magnet.
A paper clip can be used in the place of the steel ball if necessary.

How can the energy of sound cause something to move?

How can the energy of sound cause something to move?

Materials needed

• · Two guitars

Procedure

1. Be sure the two guitars are tuned alike.
2. Stand the two guitars face to face, about 5-10 cm (2-4 in.) part.
3. Strum the strings of one guitar. After two or three seconds, silence the strings of the guitar you strummed by putting your hand on them.
4. Listen carefully to the other guitar.
5. What do you hear? How did it happen?

Teacher information

This activity shows that sound can actually do work. It can make something move. Energy is transferred from one guitar to other by sound waves. And the strings of the second guitar vibrate. The two guitars should be tuned alike so the vibrating frequency is the same for the two sets of strings.

How does a nail change as it is driven into a board?

How does a nail change as it is driven into a board?

Materials Needed

• · Hammer
• · Nail, at least 5cm (2 in.) long
• · Board, at least 4 cm (1 1/12 in.) thick
• · Pounding surface

Procedure

1. Place the board on a good pounding surface such as another board, a stack of newspapers, or concrete.
2. Pound the nail at least 2.5 cm (1 in.) into the board. Do not pound it all the way in.
3. As soon as you stop pounding, feel the nail. What difference do you notice in the nail?
4. Pull the nail out of the board with the hammer.
5. As soon as you get the nail out of the board, feel it again.
6. What difference do you notice in the nail by feeling it? What can you say about this?

For problem solvers: 

rib your hands together, hard and fast. Do you feel a temperature change? How is this similar to what you experienced with the nail in the above activity?

Teacher information

As the nail is pounded into the board, some of the energy from the hammer is changed to heat energy due to friction between the nail and the board. As the nail is removed from the board, friction again changes some of the energy to heat. If the nail is pulled out quickly, the heat might be even more noticeable than when it was pounded in.

Teacher information

As the nail is pounded into the board, some of the energy form the hammer is changed to heat energy due to friction between the nail and the board. As the nail is pulled out quickly, the heat might be even more noticeable than when it was pounded in.

What happens to gases as they are heated and cooled?

What happens to gases as they are heated and cooled?

Materials needed

• Narrow necked jar with one whole stopper
• Balloon stick (available from cart or party supply stores)
• water

Procedure

1. Put a small amount of water in the bottom of the jar.
2. Insert the plastic tube through the stopper
3. Place the stopper in the jar. The lower and of the tube must be in the water.
4. Notice the water level in the tube.
5. Place the jar in a window in direct sunlight.
6. Check the water level in the tube every three or four minutes for at least one half hour.
7. What happened to the water level as the air warmed in the sunlight? Why?
8. Remove the jar from the sunlight and place it in cool place.
9. Again check the water level in the tube every few minutes.
10. What happened to the water level as the air cooled? Why?
11. What can you say about the effect of temperature change on gases?

For the problems solvers: 

blow up a balloon and measure the distance around it with a string. Mark the string to show the length required to reach around the balloon. Place the balloon over heat vent in front of a heater for a few minutes. Use the same string to measure the distance around the balloon again. Is there a difference? Can you explain why?
Learn what you can about hot balloons. Why do they rise into the air? Why do they come down again? Do balloon pilots usually fly their ships in the cool air of morning or in the heat of the afternoon? Why?

Teacher Information

This activity is very similar to activity 2.11, but this time the changes in water level in the tube are caused by expansion and contraction of air within the jar instead of expansion and contraction of liquid. As the air in the jar warms in the sunlight, it will expand, forcing water up the tube and very likely spilling it out the top of the tube, demonstrating that as the temperature of gas increases, the gas expands. At the air cools, it will contract and the level of the water in the tube will drop.
If food coloring is available, add a few drops to the water to make the water level in the tube more visible. This is a type of thermometer.

What happens to liquids as they are heated and cooled?

What happens to liquids as they are heated and cooled?

Materials needed

• · Narrow-necked jar with a one whole rubber stopper.
• Balloon stick (available from craft or party supply stores)
• · Marker, rubber band, or masking tape
• · Water

Procedure

1. Fill the jar completely with cold water.
2. Insert the plastic tubing through the rubber stopper.
3. Place the stopper in the jar. As you press the stopper into place, three should be no air space beneath the stopper, and water should be force part way (not more than half way) up the tube above the stopper.
4. Mark the tube at the water level with a marker, or by a putting a rubber band or tape around it.
5. Place the jar in a window in direct sunlight.
6. Check the water level in the tube every few minutes for at least two hours.
7. What happened to the water level as the water warmed in the sunlight?
8. Remove the jar from the sunlight and place it in a cool place.
9. Again check the water level in the tube every few minutes.
10. What happened to the water level as the water cooled?
11. What can you say about the effect of temperature change on liquids?

For problems solvers:

if a motorist goes to the filling station on a hot day and fills the fuel tank clear to the brim, then sparks the vehicle in the sun for two or three hours, sometimes tank will overflow and spill fuel onto the ground. Think about the above activity and see if you can explain why the tank overflows. The manager at your local filling station would probably be glad to discuss to it with you if you have any questions or if you‘d just like to find out if your explanation is correct. Any experienced truck driver could also discuss it with you.

Teacher information

The ideal tube for this activity is a balloon stick, available at party supply outlets.
As the jar of water warms in the sunlight, the water will expand and the water level will expand and the water level will rise in the tube, demonstrating that as the temperature of liquid increases the liquid expands.
As the water cools it will contract, and the level of the water in the tube will drop.
If food coloring is available, have students add a few drops to the drops to the water. This makes the water level in the tube easier to see, and the change is more evident.
This device can become a thermometer if you have students attach a card to the tube and mark the card at different temperatures, by taking temperature readings from a commercial thermometer. Water evaporation in the tube will eventually destroy the accuracy of it as a thermometer and it will need to be re calibrated.
Filling stations store gasoline in large tanks beneath the ground. Thus, the fuel is cool. if a motorist fills the tank, then parks the vehicle in the sun for a time, the fuel will expand from the heat and will sometimes overflow onto the ground.

What happens to solids as they are heated and cooled?

What happens to solids as they are heated and cooled?

Materials needed

• ·         Wire, about  1 m (1 yd.) long
•               Large nail or small bolt

• ·            Candle

• ·             Match

Procedure

1.   Wrap one end of the wire around the nail and anchor the other end to a support. Adjust the wire so the nail swings freely but barely missies the table or floor.

      2.       Light the candle and heat the wire. 
      3.       Observe the nail. What happened?
      4.       Remove the candle and allow the wire to cool.          
      5.   Observe the nail. What happened?

6.       What can you say about the effect of heat on solids?

For problems solvers: 

did you ever notice how hard it is to remove the ring from a jar of fruit? Try running hot water over the lid, and then remove it. What do you think makes the difference?

Why are sidewalks made with joints every few feet? See what you can learn about expansion joints. See if you can find expansion joints as you drive across over passes or bridges on the highway. Why are these joints built into the bridge? Try to find expansion joints in large buildings.

If you know an automobile mechanic, ask him or her why wheel bearings are sometimes installed on axles when they are very hot.

How do molecules behave when heated?

How do molecules behave when heated? 

Materials needed

• Chalk or masking tape

Procedure

1. Have several students stand in a group.
2. Mark a border on the floor around the group with chalk or tape. Leave a few inches between the group and the border all the way around.
3. Ask students to move around slowly. Everyone should move constantly, but no one should move fast and there should be no pushing and shoving. They are to try to stay within the border marked on the floor.
4. Now instruct those in the group to move a bit faster. They are still to try to stay within the border.
5. Continue speeding up the movement of the group until they can no longer remain within the line marked on the floor.
6. Discuss what happened as those in the group increased their speed. Ask how this relates to the movement of molecules as temperature is increased.

What other type of energy accompanies light from the sun?

What other type of energy accompanies light from the sun?

 Materials needed

• Two magnifying glasses 
•  One sheet of paper 
• Glass bowl

Procedure

1. Ask your teacher where you should do this experiment. You will need to be in bright sunlight with no wind.
2. Put the paper in the bowl.
3. Hold the magnifying glass between the paper and the sun so a beam of light focuses on the paper.
4. Notice that as you move the magnifying glass closer to and farther from the paper, the point of light changes in size. Notice also that it gets brighter as it gets smaller.
5. Adjust the distance between the paper and the magnifying glass to make the point of light very small and bright. 

6. Pull the magnifying glass back about 1 cm (1/2 in). And watch the paper.
7. Do you see anything happening to the paper? If so what, and why do you think it is happening? What kind of energy is causing this to happen?
8. What do you might happen if you used two magnifying glasses focused on the same spot on the paper? Try it.