Reference Card, Assestment Card, Enrichment Card And

Reference Card, Assestment Card, Enrichment Card and Reference Card.

Magnetism The ancient Greeks, originally those near the city of Magnesia, and also the early Chinese knew about strange and rare stones (possibly chunks of iron ore struck by lightning) with the power to attract iron. A steel needle stroked with such a "lodestone" became "magnetic" as well, and around 1000 the Chinese found that such a needle, when freely suspended, pointed north-south.

The magnetic compass soon spread to Europe. Columbus used it when he crossed the Atlantic ocean, noting not only that the needle deviated slightly from exact north (as indicated by the stars) but also that the deviation changed during the voyage. Around 1600 William Gilbert, physician to Queen Elizabeth I of England, proposed an explanation: the Earth itself was a giant magnet, with its magnetic poles some distance away from its geographic ones (i.e. near the points defining the axis around which the Earth turns).

The Magnetosphere

On Earth one needs a sensitive needle to detect magnetic forces, and out in space they are usually much, much weaker. But beyond the dense atmosphere, such forces have a much bigger role, and a region exists around the Earth where they dominate the environment, a region known as the Earth's magnetosphere. That region contains a mix of electrically charged particles, and electric and magnetic phenomena rather than gravity determine its structure. We call it the Earth's magnetosphere

Only a few of the phenomena observed on the ground come from the magnetosphere: fluctuations of the magnetic field known as magnetic storms and substorms, and the polar aurora or "northern lights," appearing in the night skies of places like Alaska and Norway. Satellites in space, however,

sense much more: radiation belts, magnetic structures, fast streaming particles and processes which energize them. All these are described in the sections that follow.

More about the magnetosphere

But what is magnetism?

Until 1821, only one kind of magnetism was known, the one produced by iron magnets. Then a Danish scientist, Hans Christian Oersted, while demonstrating to friends the flow of an electric current in a wire, noticed that the current caused a nearby compass needle to move. The new phenomenon was studied in France by Andre-Marie Ampere, who concluded that the nature of magnetism was quite different from what everyone had believed. It was basically a force between electric currents: two parallel currents in the same direction attract, in oposite directions repel. Iron magnets are a very special case, which Ampere was also able to explain.

What Oersted saw...

In nature, magnetic fields are produced in the rarefied gas of space, in the glowing heat of sunspots and in the molten core of the Earth. Such magnetism must be produced by electric currents, but finding how those currents are produced remains a major challenge.

More about magnetism

Magnetic Field Lines

Michael Faraday, credited with fundamental discoveries on electricity and magnetism (an electric unit is named "Farad" in his honor), also proposed a widely used method for visualizing magnetic fields. Imagine a compass needle freely suspended in three dimensions, near a magnet or an electrical current. We can trace in space (in our imagination, at least!) the lines one obtains when one "follows the direction of the compass needle." Faraday

http://www-istp.gsfc.nasa.gov/Education/wmfield.html

http://www-istp.gsfc.nasa.gov/Education/wms1.html

called them lines of force, but the term field lines is now in common use.

Compass needles outlining field lines

Fi eld lines of a bar magnet are commonly illustrated by iron filings sprinkled on a sheet of paper held over a magnet. Similarly, field lines of the Earth start near the south pole of the Earth, curve around in space and converge again near the north pole.

However, in the Earth's magnetosphere, currents also flow through space and modify this pattern: on the side facing the Sun, field lines are compressed earthward, while on the night side they are pulled out into a very long "tail," like that of a comet. Near Earth, however, the lines remain very close to the "dipole pattern" of a bar magnet, so named because of its two poles.

Magnetic field lines from an idealized model.

To Faraday field lines were mainly a method of displaying the structure of the magnetic force. In space research, however, they have a much broader significance, because electrons and ions tend to stay attached to them, like beads on a wire, even becoming trapped when conditions are right. Because of this attachment, they define an "easy direction" in the rarefied gas of space, like the grain in a piece of wood, a direction in which ions and electrons, as well as electric currents (and certain radio-type waves), can

easily move; in contrast, motion from one line to another is more difficult.

A map of the magnetic field lines of the magnetosphere, like the one displayed above (from a mathematical model of the field), tells at a glance how different regions are linked and many other important properties.

More about magnetic field lines

Electromagnetic Waves

Faraday not only viewed the space around a magnet as filled with field lines, but also developed an intuitive (and perhaps mystical) notion that such space was itself modified, even if it was a complete vacuum. His younger contemporary, the great Scottish physicist James Clerk Maxwell, placed this notion on a firm mathematical footing, including in it electrical forces as well as magnetic ones. Such a modified space is now known as an electromagnetic field.

Today electromagnetic fields (and other types of field as well) are a cornerstone of physics. Their basic equations, derived by Maxwell, suggested that they could undergo wave motion, spreading with the speed of light, and Maxwell correctly guessed that this actually was light and that light was in fact an electromagnetic wave.

Heinrich Hertz in Germany, soon afterwards, produced such waves by electrical means, in the first laboratory demonstration of radio waves. Nowadays a wide variety of such waves is known, from radio (very long waves, relatively low frequency) to microwaves, infra-red, visible light, ultra-violet, x-rays and gamma rays (very short waves, extremely high frequency).

Radio waves produced in our magnetosphere are often modified by their environment and tell us about the particles trapped there. Other such waves have been detected from the magnetospheres of distant planets, the Sun and the distant universe. X-rays, too, are observed to come from such sources and are the signatures of high-energy electrons there.

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INTRODUCTION TO THE MYSTERY OF MAGNETISM

After reading this section you will be able to do the following:

Identify a number of common items that rely on magnetism to work.

Each time you turn on a light, listen to your stereo, fly in an airplane, or watch TV, you are depending on the principles of magnetism to work for you. Take a look at the pictures below. All of the items in these pictures have something to do with magnetism.

Hydroelectric Dam

Video Cassette Tape

Fan-

Magnetic Particle Inspection Unit

Airplane Navigational Panel

Do you know how your life might be different without these? What do you think magnetism has to do with each of these things? Think about

these questions as you explore these materials on magnetism. We will revisit this page later.

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http://www.ndt-ed.org/EducationResources/HighSchool/Magnetism/magnetismintro.htm

MAGNETIC PROPERTIES

After completing this section you will be able to do the following:

Explain how small magnets can be while retaining its magnetic properties.

Questions

1. What is happening when you cut the magnet?2. How small do you think you can make a magnet before it

no longer acts like a magnet?

What is happening when you cut a magnet?

A magnet can be cut into smaller and smaller pieces indefinitely, and each piece will still act as a small magnet. Thus, the cause of magnetism must be from a property of the smallest particles of the material, the atoms. So what is it about the atoms of magnets, or objects that can be magnetized (ferromagnetic materials), that is different from the atoms of other material? For example, why is it that copper keys or aluminum soda cans cannot be magnetized?

AGNETIC PROPERTIES


Explain how small magnets can be while retaining its magnetic properties.

Questions

1. What is happening when you cut the magnet?2. How small do you think you can make a magnet before it

no longer acts like a magnet?

What is happening when you cut a magnet?

A magnet can be cut into smaller and smaller pieces indefinitely, and each piece will still act as a small magnet. Thus, the cause of magnetism must be from a property of the smallest particles of the material, the atoms. So what is it about the atoms of magnets, or objects that can be magnetized (ferromagnetic materials), that is different from the atoms of other material? For example, why is it that copper keys or aluminum soda cans cannot be magnetized?

REVIEW OF THE ATOM


Discuss the origin of magnetism. Discuss why some materials can be magnetized while

others cannot.

The study of atoms, electrons, neutrons, and protons is so complex that throughout history scientists have developed several models of the atom. From the early Greek concept of the atom, about 2400 years ago, to today's modern atomic model, scientists have built on and modified existing models, as new information was discovered. There are still concepts on which scientists do not fully agree on. In an attempt to simplify the concept and describe how some materials become magnetized, (for this <../Review/atom.jpg> exercise), we are using a simplification of the Niels Bohr Model of the atom. Niels Bohr was a Danish scientist and made his model in 1913. In his model he depicted that electrons spin and orbit the nucleus of an atom much like planets in a solar system. It is not the only model of the atom, and it is just one theory of how atoms are structured.

In our exercise, the electron appears to orbit in the same path around the nucleus, but the Bohr Model shows that electrons do not really orbit in the same path, but change their orbit with each revolution. There are also later models of the atom that have built on Bohr's model to help explain more complex atoms.

As the electrons circle the nucleus of the atom, they also spin, similar to the way the Earth spins on its axis.

Questions

1. What are the two types of motion that the electrons of an atom exhibit?

2. What scientist's model of the atom do we generally use?

What is the origin of magnetism?

The origin of magnetism is a very complicated concept. In fact, there are some details about magnetism on the atomic scale that scientists still do not fully agree on. To begin to understand where magnetism originates and why some materials can be magnetized while others cannot, requires a fair amount of quantum theory. Quantum theory is the study of the jumps from one energy level to another as it relates to the structure and behavior of atoms. However, explaining quantum theory is well beyond the scope of this material, so this subject will be reserved for high school and college chemistry and physics classes. The basic scientific principles of magnetism can be explained, nevertheless, a few generalizations and simplifications are made.

What does matter consist of?

First, you must recall that all matter is made up of atoms. Atoms have a positively charged center called the nucleus. A nucleus contains one or more protons and neutrons and is orbited by one or more negatively charged particles called electrons. A simplified animation of the center of an atom is what you observed. You should have concluded

that the electrons spin as they orbit the nucleus (which contain protons and neutrons) much like the earth spins as it orbits the sun. As the electrons spin and orbit the nucleus, they produce a magnetic field. A. M. Ampere first suggested the theory that magnetic fields were due to electric currents continually circulating within the atom in the early 1800s. Ampere's insight was pretty amazing considering it was not known for sure whether atoms existed in the early 1800s and the electron would not be discovered for another 75 years.

ELECTRON PAIRING


Explain what paired and unpaired electrons are. Explain how paired electrons affect the magnetic

properties of a material.

Questions

1. What are paired electrons?2. How do paired electrons affect the magnetic properties of

a material?

Since all matter is made up of atoms and all atoms have electrons that are in motion, do all atoms have magnetic fields?

The answer to this question is yes and no. All the electrons do produce a magnetic field as they spin and orbit the nucleus; however, in some atoms, two electrons spinning and orbiting in opposite directions pair up and the net magnetic moment of the atom is zero. Remember that the direction of spin and orbit of the electron determines the direction of the magnetic field. Electron pairing occurs commonly in the atoms of most materials. In the experiment you observed a helium atom showing two electrons spinning and orbiting around the protons and neutrons of the nucleus. The two electrons are paired, meaning that they spin and orbit in opposite directions. Since the magnetic fields produced by the motion of the electrons are in opposite directions, they add up to zero. The overall magnetic field strength of atoms with all paired electrons is zero.

In general, materials that have all paired electrons in the atoms and thus have no net magnetic moment are called diamagnetic materials; yet, there are some exceptions. When placed in the magnetic field of a magnet, diamagnetic materials will produce a slight magnetic field that opposes the main magnetic field. Both ends of a bar magnet will repel a diamagnetic material. If a diamagnetic material is placed in a strong external magnetic field, the magnetic field strength inside the material will be less than the magnetic field strength in the air surrounding the material. The slight decrease in the field strength is the result of realignment in the orbit motion of the electrons. Diamagnetic materials include zinc, gold, mercury, and bismuth.

Another key concept in magnetism is that diamagnetic materials will oppose an applied magnetic field. Both ends of a magnet will repel diamagnetic materials.

Are all materials that have unpaired electrons magnetic?

Most materials with one or more unpaired electrons are at least slightly magnetic. Materials with a small attraction to a magnet are called paramagnetic materials, and those with a strong attraction are called ferromagnetic materials. Aluminum, platinum, and manganese are some paramagnetic materials. Iron, cobalt, and nickel are examples of ferromagnetic materials.

MAGNETIC DOMAIN


Define a magnetic domain. Explain one way an object can be magnetized.

A magnetic domain is region in which the magnetic fields of atoms are grouped together and aligned. In the experiment below, the magnetic domains are indicated by the arrows in the metal material. You can think of magnetic domains as miniature magnets within a material. In an unmagnetized object, like the initial piece of metal in our experiment, all the magnetic domains are pointing in different directions. But, when the metal became magnetized, which is what happens when it is rubbed with a strong magnet, all like magnetic poles lined up and pointed in the same direction. The metal became a magnet. It would quickly become unmagnetized when its magnetic domains returned to a random order. The metal in our experiment is a soft ferromagnetic material, which means that it is easily magnetized but may not retain its magnetism very long.

Click and drag the magnet across the metallic strip. The arrows represent the alignment of the atoms in the metallic strip.

Questions

1. What happened to the piece of metal when you rubbed a strong magnet across it the first time? The second time?

2. What do the arrows in the material represent?3. Why do they become lined up when the magnet is

brought in contact with the metal?4. If you wanted to turn a paper clip into a magnet, how do

you think you could do it?

How can you turn a paper clip into a magnet?

You can turn a paper clip into a magnet by rubbing a strong magnet several times over the surface of the paper clip. The more you drag the magnet over the paper clip, the stronger the paper clip will become magnetized. The same thing happened with the metal in the experiment. When we rubbed the magnet over the surface of the metal, some of the magnetic domains aligned and the metal became partially magnetized. When we rubbed the magnet over the metal a second time, more of the magnetic domains became aligned and the metal became a stronger magnet.

What is different about ferromagnetic materials that make them strongly magnetic?

In ferromagnetic materials, the magnetic moments of a relatively large number of atoms are aligned parallel to each other to create areas of strong magnetization within the material. These areas, which are approximately a millimeter in size, contain billions of aligned atoms and are called magnetic domains. Magnetic domains are always present in ferromagnetic materials due to the way the atoms bond to form the material. However, when a ferromagnetic material is in the unmagnetized condition, the magnetic domains are randomly oriented so that the magnetic field strength in the piece of material is zero.

In the unmagnetized condition, the material will be attracted to a magnet but will not act as a magnet. That is to say, two unmagnetized pieces of ferromagnetic material will not be attracted to each other. When a ferromagnetic material is magnetized, the magnetic domains align parallel to each other to produce a large net field strength in the material and the material becomes magnetic.

THE TWO ENDS OF A MAGNET


Explain what a compass is and how it is affected by a magnet.

Understand how a compass helps us to navigate on the earth.

Experiment 1

Experiment 2

Experiment 3

Questions

1. What happened to the blue pole of the compass arrow when it was brought close to the north pole of the magnet?

2. What happened to the blue pole of the compass arrow when it was brought close to the south pole of the magnet?

3. What is a compass and what direction does it always point?

4. What would you expect to happen if a magnet is suspended by a string and allowed to hang freely?

5. From your observations, what can you conclude about the earth's magnetic properties?

What is important about the two ends of a magnet?

What you have been observing is the behavior of the north and south poles of a magnet. One end of any bar magnet will always want to point north if it is freely suspended. This is called the north-seeking pole of the magnet, or simply the north pole. The opposite end is called the south pole. The needle of a compass is itself a magnet, and thus the north pole of the magnet always points north, except when it is near a strong magnet. In Experiment 1, when you bring the compass

near a strong bar magnet, the needle of the compass points in the direction of the south pole of the bar magnet. When you take the compass away from the bar magnet, it again points north. So, we can conclude that the north end of a compass is attracted to the south end of a magnet.

This can be a little confusing since it would seem that what we call the North Pole of the Earth is actually its magnetically south pole. Remember that a compass is a magnet and the north pole of a magnet is attracted to the south pole of a magnet. This situation is also seen in Experiment 1 & 2. In Experiment 2, when you move the north pole of a magnet toward the south pole of the other magnet, the two magnets attract. However, in Experiment 3, when you move the south pole of a magnet toward the south pole of another magnet, the two magnets repel each other and you cannot move them together. The rule for magnetic poles is that like poles repel each other and unlike poles attract each other.

Use of a Compass

Since the north seeking pole of a compass always wants to point north, then the compass could be useful in helping us navigate. With a compass we can always tell which direction is north and if you know north, then you know all of the other directions. A compass and a map are essential tools when hiking in the woods. Since the north seeking pole of the compass needle is always attracted to the north, then the earth must be like a huge magnet with a magnetic pole at each end. This is exactly the case but magnetic north is slightly different from the north axis of rotation of the earth. Scientists believe that the movement of the Earth's liquid iron core and other things are responsible for the magnetic field around the earth.

MAGNETIC LINES OF FORCE


Explain how magnetic lines of force enter and exit a bar magnet.

Discuss how magnetic lines of force act upon other ferromagnetic objects.

Questions

1. What happened when you placed the circular piece of metal in the magnetic lines of flux? Outside the lines?

2. What do the lines around the bar magnet indicate?3. If the earth is like a huge magnet, with a magnetic pole at

the north end and another magnetic pole at the south end, what might these imaginary lines look like around the earth?

What do the lines around the bar magnet indicate?

The lines that we have mapped out around the magnet, called the magnetic lines of force, indicate the region in which the force of the magnet can be detected. This region is called the magnetic field. If an iron object is near a magnet, but is not within the magnetic field, the object will not be attracted to the magnet. When the object enters the magnetic field, the force of the magnet acts, and the object is attracted. The pattern of these lines of force tells us something about the characteristics of the forces caused by the magnet. The magnetic lines of force, or flux, leave the north pole and enter the south pole.

How is the earth like a magnet?

Since the earth is a huge magnet with a magnetic north and south pole, the lines of magnetic force around the earth look like there is a huge vertical bar magnet running through the center of the earth. We will see in the next experiment how the magnetic lines of flux around a magnet can be seen. The next page will tell you more about how you can observe the magnetic field of a magnet and what you can learn from reading the patterns of the magnetic lines of force.

MAGNETIC FIELDS


Explain how magnetic lines of flux are affected by other magnetic fields.

In each of the following pictures a magnet is put onto a piece of paper. Then a light dusting of iron filings is sprinkled around the magnet. The lines around the magnets in the following pictures are produced by the iron filings gathering together around the field lines.

Box A

This picture demonstrates what occurs when one magnet is placed on paper, and iron filings are sprinkled around it.

Box B

Pictured here are two magnets placed on a piece of paper with their like poles facing each other, and iron filings are sprinkled around them.

Box C

Lastly, this picture has two magnets placed on a piece of paper with their opposite poles facing each other, and iron filings are sprinkled around them.

Questions

1. What is happening when iron particles are sprinkled over and around the magnets?

2. Do you see any differences in the patterns in each of the three situations? If so, what differences do you see?

3. What do the patterns indicate in each situation?4. Can you tell by these patterns where the magnetic forces

might be the strongest? The weakest? 5. Can you tell by these patterns where the magnetic forces

are attracting? Repelling?

What does the pattern made by the iron particles indicate?

You learned in a previous experiment that no matter how many pieces you cut a magnet into, each piece is still a magnet. Even if you shred a magnet into particles the size of sand, each tiny grain is a magnet with a north pole and a south pole. When these magnetized particles are sprinkled over the magnet in Box A, the resulting pattern shows the magnetic field around a single magnet. We can see that the force of the magnet is the strongest at the two ends because more iron particles are concentrated in these areas. The magnetic lines of flux flow from one end to the other.

How do you explain what is occurring?

To understand what is happening, recall from a previous experiment that a magnet allowed to stand freely, like a compass needle, will point to the north in response to the earth’s magnetic field unless it is near a strong magnetic. If the compass is near a strong bar magnet, the opposite poles of the magnets are attracted to each other. We can use this knowledge to identify the magnetic field of a magnet by placing a compass at various locations around the bar magnet and observing where the compass needle points. If the compass is far away from the bar magnet the compass will always point north because it is not in the bar magnet’s magnetic field. As it gets closer to the magnet, the

compass begins to point more and more toward the magnet as a result of the force, or the magnetic field, of the magnet. The compass needle aligns itself with the magnetic flux lines of the magnet.

What if...

Let's say that instead of using one compass to move around the bar magnet, we place thousands of tiny compass needles all around the bar magnet and watch which direction they point and what pattern they make. That is what is happening in our experiment with the iron filings. Each tiny magnetic iron filing is a tiny magnet with a north and south pole, just like a tiny compass. When the iron filings are sprinkled, those very close to the magnet, where the magnetic force is the strongest, will cling to the magnet.

Those filings a little farther away, where the magnetic force is less strong, will align themselves with the magnetic flux lines, but they will not be drawn to cling to the magnet. Those filings even farther away, outside the magnetic force, will point north in response to the earth’s magnetic field. These patterns formed by the direction of the tiny compasses can tell us something about where the magnetic force is the strongest, where it is an attracting force, and where it is a repelling force. In Box B, this pattern indicates a repelling force because the tiny magnets are moving away from the ends of the larger bar magnets. Looking at the pattern in Box C, you see that the two ends of these magnets are attracted because the tiny magnets appear to be lined end to end, attracting to one another and also attracting to the ends of the larger bar magnets.

FERROMAGNETIC MATERIALS


Explain the differences between a permanent magnet and a temporary.

Explain why some materials have magnetic properties only when a permanent magnet is near them.

Questions

1. What is happening in this experiment?2. What conclusions can you draw about magnets and

magnetism from this experiment?

How does an object become a magnet?

We have determined in previous discussions that magnets can be permanent or temporary. A permanent magnet is more difficult to magnetize but will retain the properties of magnetism indefinitely. A temporary magnet is generally made of soft iron and will remain magnetized only as long as the magnetizing cause is present. From previous experiments you saw how the difference in magnetized and unmagnetized material depends on the motion and arrangement of the material's molecules. Bringing a ferromagnetic object, like a nail, into the magnetic field of a strong magnet can cause the molecules of the iron material to line up and the nail to become a temporary magnet. As long as it is in the magnetic field of the bar magnet, the nail acts like a magnet and picks up other ferromagnetic materials. In this case it is the paper clip. Then, the paper clip becomes a magnet and can pick up another paper clip, and so forth.

ELECTROMAGNETS


Discuss the relationship between electricity and magnetism that is demonstrated.

Questions

1. Describe what happens when the power is turned on in this activity.

2. How can turning on the electricity allow the iron crane to pick up the car?

3. Do you think electricity and magnetism are somehow related?

How can electricity be used to make a magnet?

In this experiment you used electricity to make a temporary magnet, called an electromagnet. As long as the electric current was on, the

iron crane was a magnet and could pick up ferromagnetic objects. When the electricity was turned off, the magnetizing cause was no longer present, so the object was not attracted to the iron crane. So, let's see how electricity is able to make a magnet.

ELECTRICITY AND MAGNETISM


Discuss what happens to a compass when a wire with electrical current is near.

Describe the relationship between electricity and magnetism.

Questions

1. What happens to the compass needle as the compass moves around the wire carrying electrical current?

2. Why do you think this happens?

Why does the compass respond when it is near an electrical wire with current flowing through it?

We can conclude from this experiment that an electric current causes a magnetic field around it just like a magnet causes a magnetic field. When you moved the compass near a bar magnet, the needle pointed toward the magnet's magnetic field and not toward the north. When you put the compass near the electrical wire with current flowing through it, the compass did not point north; instead, the compass needle pointed in the direction of the current's magnetic field.

What would happen if we put a ferromagnetic object into the magnetic field?

Now we have established that a conductive wire with a current flowing through it has a magnetic field. If we put a ferromagnetic object in this magnetic field, the object will concentrate the strength of the field and cause the object to become magnetic. Once the current flow in the line stops, the magnetic field disappears and the object stops acting like a magnet. However, the magnetic field of one wire is small and does not have much strength, so it can only make temporary magnets from small objects. But, let’s say that we take a wire and coil it several times to form a long coiled piece of electrical wire, and then we turn on the current. We would have a magnetic field much bigger and stronger than we would without the coiled piece of wire, and we could magnetize even larger objects.

An iron bar placed through the center of the coiled wire would become a temporary magnet, called an electromagnet, as long as the electric current is flowing through the wire.

Warning: Current may need to be restricted to prevent overheating the wire and to prevent damaging the battery.

You can also make an electromagnet by passing the electric current directly through the ferromagnetic object.

MORE ON ELECTRICITY AND MAGNETISM


Explain what a galvanometer in and how it is used. Begin to discuss how magnetism can be used to create

electrical current.

In this demonstration you will use an instrument called a galvanometer. It is an instrument with a bar of iron wrapped with an electrical wire, and a magnet. It detects electrical current. If a wire is connected to this instrument, it can detect if electricity is flowing through the wire. If there is no current through the wire, the needle will move to the left. Similarly, if there is a current flowing through the wire, the needle will move towards the right of the scale. This demonstration should help to give you an idea of the relationship between magnetism and electricity.

Questions

1. What happens to the galvanometer needle when the magnetic lines of force from the magnet interact with the electrical wire?

2. When the galvanometer needle moves to the right, what does it tell you is happening in the wire?

3. Why does the galvanometer needle return to zero when the magnet stops moving?

If electricity produces magnetism, can magnets produce electricity?

What you have just discovered in this experiment is that electricity can be generated by moving a wire through a magnetic field. This process is called electromagnetic induction. When an electrical wire cuts across magnetic lines of force, a current is produced in the wire. We know this because the current is detected by watching the needle on a galvanometer, which is an instrument that can measure electric current in wires. The same result is obtained when a magnet is moved in and out of coils of wire. It does not matter if the magnet is moved or if the coils of wire are moved. The important thing is that there is motion within the magnetic field, and that the magnetic lines of force are cut.

Why is electromagnetic induction important to us?

The discovery of electromagnetic induction is very important in our lives because it is the principle by which electric generators can make electricity. Through the use of magnets, a generator can convert mechanical energy to electrical energy and provide electricity that we need for so many things. Remember that energy is the ability to do work and that mechanical energy is the energy caused by moving objects. For example, when you move your legs to peddle a bicycle, you cause energy that moves the bicycle wheels and runs the bicycle. When a rushing wave of water hits a boat and turns it over, the moving water causes the energy that moves the boat to overturn it. When there is a way to turn this moving energy into electricity that can light a light bulb, we can get light in our home.

How does a magnet help a generator convert mechanical energy into electrical energy?

A generator works very much the same as you saw in the experiment. Inside a generator is a magnet, some electrical wire, and a source of mechanical energy. The mechanical energy moves the wire into the magnetic field of the magnet so that the wire cuts through the magnetic lines of force. As a result, electric current is produced. Electric generators can come in all sizes. Some electric generators are very big and contain huge magnets so they can produce a lot of electricity. On the other hand, some generators contain small magnets and are small enough to hold in your hand. These small generators may produce only enough electricity to light one small light bulb.

MAGNETISM AND THE DIRECTION OF CURRENT FLOW


Discuss what happens to the electromagnet when the current is reversed.

Questions

1. What happens each time you reverse the electrical current in the wire?

2. What would happen if we used alternating current (AC) instead of direct current (DC) in the wire?

In the previous experiment, you saw how magnetism makes it possible to convert mechanical energy into electrical energy, allowing electric generators to make electricity. In this experiment and the next experiment, we see that magnetism can also let us do the opposite; that is, we can convert electricity into mechanical energy. In the experiment you just finished, when you turn the current on, the current flows through the wire and temporarily magnetizes the bar of iron material. One end of the magnet becomes a north pole and the other end becomes the south pole.

When you reverse the direction of the current flowing in the wire, the north and south poles are also reversed. When you reverse the current again, the north and south poles reverse again. In fact, each time the current is reversed, the north and south poles will exchange places. Direct current (DC) flows in only one direction through a wire. So, in order to change the direction of flow change, there needs to be a reversing switch. As you will see in the next experiment, alternating current (AC), on the other hand, is constantly changing its direction of flow, so a reversing switch is not necessary.

THE ELECTRIC MOTOR AND MAGNETISM


Discuss why magnetism is important to the operation of an electric motor.

Questions

1. From what you have observed in this experiment here, can you explain how an electric motor works?

2. Why is important that alternating current is supplied to our houses?

How does magnetism make an electric motor operate?

An electric motor converts electric energy into mechanical energy that can be used to do work. In the experiment we first use DC current to flow through the wire. Remember that DC current flows in only one direction unless there is a switch to reverse its direction. When the current is first turned on, the like magnetic poles are near each other. Recall from past experiments that like magnetic poles repel each other, and they are forced to move away from each other.

Since the electromagnet is free to move, its south pole moves away from the south pole of the fixed magnet. However, as it rotates it moves closer to the north pole of the fixed magnet and is pulled toward it by an attracting force because unlike magnetic poles attract each other. When we reverse the direction of the current flow, the location of the poles change places, and again, you have two like poles near each other. This arrangement causes the electromagnet to rotate again as the like poles are forced away from each other and the unlike poles attract each other. Then, again, the movement stops until the current is reversed and the magnetic poles in the electromagnet change places another time.

We can conclude that each time the current flow is reversed in the wire, the electromagnet moves in response to the repelling force of like poles and the attracting force of unlike poles. This movement of the electromagnet, in turn, rotates the shaft to which it is connected-and mechanical energy is created. The rotating shaft can be connected to various other components to create moving parts that can do work. AC current, by nature, is constantly changing the direction of flow and does not need a reversing switch. So, when AC current is run through the wire, the electromagnet continues to rotate without stopping. This happens because the locations of the magnetic poles are continually changing places and attracting or repelling the magnetic poles of the fixed permanent magnet.

THE USE OF MAGNETISM IN NDT


Explain how magnetism is used in nondestructive testing to find cracks in magnetic materials, such as steel pipes.

Another way magnetism is used, is to inspect material for flaws. You may recall from the introduction that nondestructive testing (NDT) is the use of special equipment and methods to learn something about an object without harming the object. One of the NDT methods commonly used is called magnetic particle inspection. The reason we use this test is to find small defects in objects before they become bigger defects and cause serious problems.

In magnetic particle inspection, a magnet or electrical current is used to establish a magnetic field in the object. Iron filings are then dusted on to the surface of the object. The filings should align along the magnetic lines of force. If a crack or other defect is present, the magnetic fines of force will be disrupted and the magnetic particles will cluster along the edges of the flaw.

That concludes this lesson on magnetism. For more information on magnetism review the material on electricity, if you have not already. As you now know, the two are closely related.

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