Circles of Magnetism IV - Two parallel, current-carrying wires exert forces on each other.

 

Circles of Magnetism IV

 

Two parallel, current-carrying wires exert forces on each other.

When an electric current flows through a wire, a magnetic field is created around the wire. If you place two current-carrying wires near each other, the magnetic field around each wire exerts a force on the current flowing in the other wire. These forces can push two current-carrying wires apart, or pull them together. 

 

• One 6-volt lantern battery or an equivalent current supply.
• 2 electrical lead wires with alligator clips at both ends (available at Radio Shack).
• Tinkertoys™ or wood for a stand.
• Masking tape or transparent tape.
• Light aluminum foil.
• Adult help.

 

(15 minutes or less)

Make a stand from wood or Tinkertoys™ (see the photo on page 14 and the diagrams below), or build a stand of your own design from available materials.

Cut a strip of aluminum foil measuring about 2 feet (60 cm) long and 1/2 inch (1.3 cm) wide. Tape one end of the foil strip to your support. Run the strip down and back up to the support, making a loop, then tape the other end in place. Be sure the ends of the strip do not touch.

Attach one clip lead to each battery terminal, but do not attach the other ends of the lead wires to the strip yet. 
 

(15 minutes or more)

Touch the two clip leads to the ends of the foil strip. The descending and ascending portions of the loop will repel each other. The closer you can hang the descending and ascending portions of the loop to each other - without allowing them to touch - the larger the repulsion.

Now hang the foil strip from the support with the two ends overlapping, so they make a good electrical contact. Connect one of the clip leads to these overlapping ends. Separate the two sides of the loop and briefly touch the other clip to the bottom of the loop. Notice that the sides of the loop are attracted to each other when the current flows. (This step requires a little coordination and a delicate touch to clearly demonstrate that it is the current flow in the strips that makes them move together and not forces that you create when you touch the clip to the bottom of the loop.) 
 

A current-carrying wire generates a magnetic field that circles the wire (See the "Circles of Magnetism I" Snack.)

When a current flows in a magnetic field, the field exerts forces on that current. (See the "Motor Effect" Snack.) So each current-carrying wire in this Snack generates a magnetic field at the position of the other wire and thus exerts a force on the current in the other wire. Two parallel wires will either attract or repel each other, depending on the direction of current flow in each wire. If both currents flow in the same direction, the wires will attract; if they flow in opposite directions, they will repel.

The forces produced on the aluminum foil are small. This is because the electrical current flowing through the foil is small, only a couple of amperes. Larger currents produce larger forces. The Exploratorium exhibit, for example, uses wires carrying 400 amperes, which produces forces that are more than 10,000 times stronger than the forces you produce with this Snack.

The ampere, the fundamental unit of electrical current, is defined by the force exerted by one wire on another. The definition of the ampere is as follows: A current of 1 ampere flowing in each of two infinitely long parallel wires separated by 1 meter will produce an attractive force of 2 x 10^-7 newton on each 1-meter length of wire. For comparison, a force of 1 newton is approximately the weight of a quarterpound of hamburger.

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Circles of Magnetism I - You can make a magnetic field that's stronger than the earth's!

 

Circles of Magnetism I

 

You can make a magnetic field that's stronger than the earth's!

Compass needles are little magnets that are free to rotate. Compasses allow us to observe the direction of a magnetic field. Normally, they respond to the earth's magnetic field, orienting themselves parallel to magnetic field lines. If we create a magnetic field that is stronger than the field of the earth - for example, by using electric currents - a compass needle will orient itself parallel to the new field. 

 

• A 6- or 12-volt lantern battery.
• A 1 foot (30 cm) length of heavy wire that is rigid enough to stand by itself. (You can use the wire from a coat hanger.)
• A Tinkertoy™ set for building the stand (or another improvised stand).
• A flat, rigid support surface measuring approximately 6 x 6 inches (15 x 15 cm). (This can be made of posterboard or even a manila file folder.) It should have a hole in the center of it that is large enough for the wire to pass through.
• 4 or 6 small compasses, measuring about 1 inch (2.5 cm) in diameter.
• 2 electrical lead wires with alligator clips at both ends (available at Radio Shack).
• Adult help.

 

(30 minutes or less)

Construct a Tinkertoy™ stand (or the equivalent), and lay the flat support surface in position on the stand. (See the photo and the diagram above.)

If the coat hanger wire is painted or varnished, scrape the coating off to expose about 1 inch (2.5 cm) of bare metal at each end.

Insert the wire through the hole in the flat support surface, and support the wire vertically in the stand, as shown in the photo and diagram.

Arrange the compasses in a circle on the support surface as shown in the diagram.

Attach one clip lead to each battery terminal. but donot attach the other ends of the lead wires to the coat hanger wire yet. 
 

Observe the compass needles around the wire as when there is no current passing through the wire. Rotate the support surface. What happens to the compass needles? They will point north, orienting themselves so that they are parallel to the earth's magnetic field. (Note: a few of your compasses may point south! Inexpensive compasses that are exposed to a strong magnet will sometimes become magnetized in the reverse direction. It's nothing to worry about, though - just keep in mind which end of your compasses points north.)

Attach the clip leads to the ends of the coat hanger wire where it has been scraped. Watch what happens to the compass needles as current passes through the wire. If the electrical current is large enough, each compass will point in a direction tangent to a circle centered on the wire.

Rotate the support surface again. What happens to the compass needles this time? The compasses will continue to point in a direction tangent to a circle centered on the wire.

Don't leave the clip leads connected too long, because the electric current will rapidly drain the battery. A few seconds should be long enough to make good observations.

Switch the clip leads to the other terminals of the battery. What happens? The compass needles will reverse direction when the electrical current reverses direction. 
  

Compass needles line up with magnetic fields. Since the earth is a magnet, a compass will normally line up with the earth's magnetic field. Because opposite magnetic poles attract, the magnetic north pole of the compass points toward the magnetic south pole of the earth. (The magnetic south pole of the earth is located in northern Canada! That is not a misprint. The south pole of the earth magnet is near the geographic north pole.)

The electric current passing through the wire creates a magnetic field that is stronger than the earth's field (in a region close to the wire). You can visualize the shape of this new field as a set of concentric circles surrounding the wire. Each of these circles has its center at the wire.

The closer to the wire you are, the stronger the magnetic field. The compass needles align themselves with the total magnetic field at each point, the sum of the earth's field and that of the wire. Since the magnetic field from the wire is significantly larger than that from the earth, each needle ends up pointing essentially in the direction of the magnetic field of the wire.

When you reverse the current, the direction of the magnetic field also reverses, and the needles dutifully follow it.

To find the direction of the magnetic field made by an electrical current, use a technique called the righthand rule.

Place your right hand with the thumb parallel to the wire carrying the current. Point your thumb in the direction of the electrical current in the wire. (Remember: The electric current flows from the plus side of the battery through the wire to the minus side.) Wrap your fingers around the wire. Your fingers will now point in the direction of the magnetic field around the wire. If there are compasses near the wire, they will point in the same direction as your fingers.

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Cheshire Cat - Make a friend disappear, leaving only a smile behind.

Cheshire Cat

 

Make a friend disappear, leaving only a smile behind.

Under most circumstances, both of your eyes receive fairly similar views of the of the world around you. You fuse these views into a single three-dimensional picture. This Snack lets you explore what happens when your eyes receive different images. 

 

• A handheld mirror, approximately 4 to 6 inches (10 to 15 cm) on a side.
• A white wall or other white surface (white posterboard works well).
• A partner.

 

No assembly needed.

(15 minutes or more)

 
Sit so that the white surface or wall is on your right. Hold the bottom of the mirror with your left hand, and put the mirror edge against your nose so that the reflecting surface of the mirror faces sideways, toward the white surface.

While keeping the mirror edge against your nose, rotate the mirror so that your right eye sees just the reflection of the white wall, while your left eye looks forward at the face of a friend who is sitting a couple of feet away (see diagram). Move your hand in front of the white surface as if passing a blackboard eraser over the surface. Watch as parts of your friend's face disappear.

It will help if your friend is sitting very still against a plain, light-colored background. You should also try to keep your own head as still as possible.

If you have trouble seeing your friend's face disappear, one of your eyes might be stronger than the other. Try the experiment again, but this time switch the eye you use to look at the person and the eye you use to look at the wall.

Individuals vary greatly in their ability to perceive this effect; a few people may never succeed in observing it. You may have to try this several times. Don't give up too soon! Give yourself time to see the effect. 
 

Normally, your two eyes see very slightly different pictures of the world around you. Your brain analyzes these two pictures and then combines them to create a single, three-dimensional image.

In this Snack, the mirror lets your eyes see two very different views. One eye looks straight ahead at another person, while the other eye looks at the white wall or screen and your moving hand. Your brain tries to put together a picture that makes sense by selecting bits and pieces from both views.

Your brain is very sensitive to changes and motion. Since the other person is sitting very still, your brain emphasizes the information coming from the moving hand, and parts of the person's face disappear. No one knows how or why parts of the face sometimes remain, but the eyes and the mouth seem to be the last features to disappear. The lingering mouth gives rise to the name of this exhibit.

The name for this exhibit derives from the Cheshire Cat in Lewis Carroll's story Alice's Adventures in Wonderland. The cat disappears, leaving behind only its smile.

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Charge and Carry - Store up an electric charge, then make sparks!!!

Charge and Carry

 

Store up an electric charge, then make sparks.

Are you tired of electrostatic experiments that just won't work? This experiment will produce a spark that you can feel, see, and hear. You rub a Styrofoam plate with wool to give it a large electric charge. Then you use the charged Styrofoam to charge an aluminum pie pan. The entire apparatus for charging the aluminum plate is called an electrophorus, which is Greek for charge carrier. An even larger charge can be stored up in a device called a Leyden jar, made from a plastic film can. 

 

For the Electrophorus:

• A Stryofoam dinner plate (Acrylic plastic sheets also work well, as will old LP records)
• A piece of wool cloth (Other fabrics may work, but wool will definitely work.)
• A disposable aluminum pie pan
• A Styrofoam cup
• Hot glue gun or masking tape

For the Leyden jar:

• A plastic 35 mm film can
• A nail slightly longer than the film can
• Some aluminum foil.
• Tap water
• Optional: A neon glow tube (available from Radio Shack)

 

(15 minutes or less)

 

Electrophorus:

Tape or hot-glue the Styrofoam cup to the middle of the inside of the pie plate. (Most household glues won't work because they dissolve Styrofoam.) Place the pie pan on top of the upside-down Styrofoam plate or a piece of acrylic plastic.

Leyden Jar:

Push the nail through the center of the lid of the film can. Wrap aluminum foil around the bottom two-thirds of the outside of the film can. You may tape the aluminum foil in place. Fill the film can almost full with water. Snap the lid onto the can. The nail should touch the water.

(30 minutes or more)

Rub the Styrofoam plate with the wool cloth. If this is the first time you are using the Styrofoam in an electrostatic experiment, rub it for a full minute.
To charge the pie pan follow the next steps exactly:
1. Place the pie pan on top of the charged Styrofoam plate.
2. Briefly touch the pie pan with your finger. You may hear a snap and feel a shock.
3. Remove the pie pan using only the insulating Styrofoam cup (see photo). You may have to hold the Styrofoam plate down with your other hand.

The pan is now charged.

Discharge the pan by touching it with your finger. You will hear a snap, feel a shock, and, if the room is dark, see a spark. To make the largest spark, have the pie plate at least one foot away from the Styrofoam plate. You can also discharge the pie pan through a neon glow tube. Hold one of the two metal leads of the tube in your fingers and touch the other lead to the pie pan. The electric spark will go through the neon and make a flash that is easily visible. After charging the Styrofoam once, you can charge the pie pan several times. The pie pan is portable and can be used for many electrostatic experiments.

Charge the Leyden jar by touching the charged pie pan to the nail while holding the Leyden jar by its aluminum foil covering. You can make several charge deliveries by recharging the pan before touching it to the nail. Discharge the jar by touching the aluminum foil with one finger and the nail with another. Watch for a spark. 
 

When you rub the Styrofoam plate with a wool cloth, you charge it negatively. That's because the Styrofoam attracts electrons from the cloth. Often, a plate fresh from the package will start with a positive charge. If it does, you will have to rub the plate long enough to cancel this initial charge before you can begin building a sizable negative charge. By using an electroscope (such as the one you can build with the Electroscope Snack, you can determine whether the Styrofoam is positively or negatively charged. Styrofoam is an insulator; it will hold its charge until it is discharged by current leaking into the air or along a moisture film on the surface of the Styrofoam.

When you place the pie pan on the Styrofoam, the electrons on the Styrofoam repel the electrons on the pan. Since the electrons can't leave the pie pan because it is completely surrounded by insulating air and Styrofoam, the pan retains its neutral charge. If you touch the pie pan while it is near the Styrofoam, the mobile electrons will be pushed off the pan and onto you. The electrons make a spark as they jump a few millimeters through the air to reach your finger. The air in the spark is ionized as the moving electrons knock other electrons off air molecules. The ionized air emits light and sound. You can also feel the flow of electrons though your finger.

After the electrons leap to your finger, the pan has a positive charge. Physicists say the pan has been charged by induction. You can carry the positively charged pan around by its handle and carry the positive charge to other objects. If you bring the positive pan near your finger again, or near any object that can be a source of electrons, the pan will attract electrons, creating a second spark.

The low-pressure neon gas in a neon glow tube is easier to ionize than air that is at atmospheric pressure. If you discharge the pan through a neon glow tube, the spark will make a bigger flash of light.

When you touch a positively charged pie pan to the nail on the Leyden jar, electrons from the nail flow onto the pie pan. The resulting positive charge on the nail attracts electrons from your body through your hand onto the aluminum foil of the jar. The Leyden jar will then have a positive center separated from a negative foil outside by the insulating plastic of the film can. If you touch one finger to the foil and bring another finger near the nail at the center of the Leyden jar, a spark will jump as the negative charges are attracted through you to the positive nail. The beauty of the Leyden jar is that it can store charges from several charged pie pans, thus building up to a larger, more visible, more powerful (and more painful) spark.

The Leyden jar is the forerunner of the modern-day capacitor. It was invented in 1745 at the University of Leyden by Pieter Van Musschenbroeck. Early Leyden jars were larger than a plastic film can and could hold more charge. The inventor discharged one through himself and wrote, "My whole body was shaken as though by a thunderbolt." At another time, a Leyden jar was discharged through 700 monks who were holding hands. The charge caused them to simultaneously jump slightly off the ground.

To give the Styrofoam plate a positive charge, try rubbing it with a plastic bread bag. Try rubbing it with other cloths, too. Try charging the Leyden jar in reverse. That is, while holding the nail, touch the aluminum foil with the pan.

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Center of Gravity - How to balance a checkbook using the physics method !

Center of Gravity

 

How to balance a checkbook using the physics method.

Here is an easy way to find the center of gravity of a long, thin object, even if the object's weight is unevenly distributed. 

 

• A meterstick, cane, or any stick of similar length.
• Clay or weight.
• Masking tape.

 

(5 minutes or less)

First try the experiment with just the stick itself. Then tape the clay or weight somewhere on the stick and try again.

(5 minutes or more)

Support the stick by resting each of its ends on a finger. Slowly slide your fingers together until they meet. Your fingers will meet under the stick's center of gravity. Attach the weight or a piece of clay to some point on the stick. Again support the stick on two fingers, and then slide your fingers together to locate the new center of gravity. Move the weight or piece of clay to some new place on the stick. Repeat the experiment. Your fingers will always meet right under the center of gravity.

The stick's center of gravity is the place where you could balance the stick on just one finger. When you first support the stick with two fingers, in general one finger (the one that is closer to the center of gravity) will be holding a little more of the weight than the other. When you try to move your fingers closer together, the one that is carrying less weight will slide more easily. This finger will continue to slide more easily until it gets closer to the center of gravity than the other finger, at which point the situation will reverse and the other finger will begin to slide faster. Your left and right fingers simply alternate moving until they meet at the center of gravity, where both fingers support equal weight.

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