MAGNETIC PROPERTIES
Questions
- What is happening when you cut the magnet?
- 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?
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. Nevertheless, the basic scientific principles of magnetism can be explained if 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 an atom is what you observed above. You should have concluded that the electrons spin as they travel around 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. In the early 1800s, A. M. Ampere first suggested the theory that magnetic fields were due to electric currents continually circulating within the atom. Ampere's insight was pretty amazing considering the electron would not be discovered for another 75 years.
CREATION OF MAGNETIC FIELDS
- In the previous experiment you saw how the electron spins and travels around the nucleus of an atom. What does that cause to happen?
A magnetic field describes a volume of space where there is a change in energy. Later, you will see a simple way to detect a magnetic field with a compass. As Ampere suggested, a magnetic field is produced whenever an electrical charge is in motion. The spinning and orbiting of the nucleus of an atom produces a magnetic field as does electrical current flowing through a wire. The direction of the spin and orbit determine the direction of the magnetic field. The strength of this field is called the magnetic moment.
The motion of an electric charge producing a magnetic field is an essential concept in understanding magnetism. The magnetic moment of an atom can be the result of the electron's spin, which is the electron orbital motion and a change in the orbital motion of the electrons caused by an applied magnetic field.
THE TWO ENDS OF A MAGNET
Experiment 1
Experiment 2
Experiment 3
- What happened to the blue pole of the compass arrow when it was brought close to the north pole of the magnet?
- What happened to the blue pole of the compass arrow when it was brought close to the south pole of the magnet?
- What is a compass and what direction does it always point?
- What would you expect to happen if a magnet is suspended by a string and allowed to hang freely?
- From your observations, what can you conclude about the earth's magnetic properties?
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
- What happened when you placed the circular piece of metal in the magnetic lines of flux? Outside the lines?
- What do the lines around the bar magnet indicate?
- 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?
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
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.
- What is happening when iron particles are sprinkled over and around the magnets?
- Do you see any differences in the patterns in each of the three situations? If so, what differences do you see?
- What do the patterns indicate in each situation?
- Can you tell by these patterns where the magnetic forces might be the strongest? The weakest?
- Can you tell by these patterns where the magnetic forces are attracting? Repelling?
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.
ELECTROMAGNETS
- Describe what happens when the power is turned on in this activity.
- How can turning on the electricity allow the iron crane to pick up the car?
- Do you think electricity and magnetism are somehow related?
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
- What happens to the compass needle as the compass moves around the wire carrying electrical current?
- Why do you think this happens?
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.
MAGNETISM AND THE DIRECTION OF CURRENT FLOW
Questions
- What happens each time you reverse the electrical current in the wire?
- 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.
- From what you have observed in this experiment here, can you explain how an electric motor works?
- Why is important that alternating current is supplied to our houses?
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
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 calledmagnetic 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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