Since the ancient Greeks, mankind has known about magnetism. It was known that a particular kind of rock, called magnetite, possessed the property that if two chunks of it were brought together facing the correct way, the rocks would stick together. If one of the rocks was turned around, the two chunks would repel each other. By the Middle Ages, it was discovered that a piece of magnetite, attached to a piece of wood and set afloat on water, would always point in the same direction. This property was used for navigation and ushered in an era of exploration. The following demonstrations and activities deal with magnetism.
It was found early on that only certain materials were attracted to magnets. To test this, we simply need a magnet and various test materials (ex metal strips from previous experiments, pencils, nails, etc.). Touch the magnet to the objects. If they stick, they are magnetizable. If not, they are nonmagnetic. Do you notice any similarity between the magnetizable objects?
Evaluation: This experiment worked well. It allows students to be very inquisitive inside the bounds of a very safe experiment.
As we saw in the previous experiment, iron is magnetizable. We can use this fact to view what the magnet field for a magnet looks like. For this, we need a magnet, a clear piece of plastic sheeting, and iron filings. Place the magnet underneath the plastic. Sprinkle the iron filings on top of the plastic. What do the field lines look like? If you want to display this to the classroom, put the demo on top of an overhead projector and focus.
Magnetic fields can form due to electrons spinning about some axis. In most atoms, the spins of all of the electrons are random, thus causing a cancellation of the magnetic fields. Iron, however, has two electrons that have their spin fixed in the same direction. Because of this, iron is magnetizable. All that is required is that most of the magnetic fields due to the atoms in a chunk of iron line up in the same direction for a bar magnet to exist. We can test this with a bar magnet, iron filings, test tubes, and a compass.
Fill the test tube with the iron filings (shredded steel wool will also work) and seal with a stopper. Bring the test tube near the compass. What happens? Pass a bar magnet by the test tube (Always have the same pole facing the test tube.) 10 times. Again, place the tube near the compass. Repeat this procedure until you have passed the magnet by the tube 100 times. Does the tube have a greater effect on the compass? Now shake the test tube for about a minute. Place the tube near the compass. What happens now? Explain what went on.
Evaluation: This is a nice demonstration of what goes on in a permanent magnet. The shaking clearly shows that randomized magnets produce no net field.
In the previous experiment, we found that one way to create a magnet is to stroke a magnetizable substance with a magnet. The magnet causes the magnetic moments of the atoms in the material to begin to align themselves in a preferred direction. Another way to create a magnet is to heat and cool a substance in the presence of a strong magnetic field. When the object is heated, the atoms in the substance have a greater kinetic energy and are vibrating much faster. If placed in a magnetic field, the atoms more easily align themselves with the field. As the substance cools, the magnetic moment of the atoms "freezes" in place, creating a permanent magnet. It should be noted that the Earth itself has a magnetic field. Therefore, there is the possibility that if a substance is heated and cooled with no other magnets present, it might become a permanent magnet.
To test this, we will need several large steel nails, a compass, a magnet, steel paper clips, and a pair of tongs or pliers. First take one nail and place it, unheated, against the magnet. Then, using the tongs to hold it, place a nail in the Bunsen burner flame until it glows. Place this nail also along the magnet. When the nail has cooled, remove both nails from the magnet. Pass them by the compass to see if either or both have a magnetic field. Test the magnetic strength of the nails by seeing how many paper clips both can hold.
Using the compass to determine a N-S line, heat two nails in the flame until both glow. Place one nail on a ceramic surface so that it is facing N-S; the other, so that it is facing E-W. After the two have cooled, see if they have any effect on the compass. Test their strength by seeing how many paper clips they will hold.
Evaluation: This experiment gave the participants some trouble. Placing the hot nails on the magnets definitely gave a stronger permanent magnet than just placing cold ones on them. However, placing two hot magnets in different orientations in the Earth's magnetic field gave a poor result. The nails placed in the E-W orientation seemed to acquire a N-S magnet field. Quite possibly, all of the other permanent magnets on the table were distorting the induced field.
If the Earth is a giant magnet, is it possible to shield yourself from its field? To test this, we will need a compass, two magnets, and several iron cans (lids cut off both ends) of different radii. Place the compass on a table far from any magnets and note its direction. On the E-W sides, about 5-10 cm away, place two magnets so that one magnet has N pointing toward the compass and the other has S. Now note the direction the compass is pointing. Place one can over the compass. Now what happens? Place as many cans as you can over the compass and note the direction the compass is pointing. Why does this occur?
Evaluation: Not done.
Permanent magnets are not the only types of magnets, though. In the 1820, H. C. Oersted noticed that when he ran a strong current through a wire, a compass nearby was deflected from north. After placing several compass around the wire, he found that a current running through a wire produced a magnetic field that ran in circles around the wire. Hence, the connection between electricity and magnetism was made. Let us test this principle with the following experiments.
Experiment 1: For this experiment, we will need a 9 V power supply or battery, wires, aluminum foil, and two pencils. Cut two strips of aluminum that are 1 x 10 cm long. Place the pencils on a table parallel to each other 6 cm apart. Put the strips on the pencils in a perpendicular fashion .5 cm apart with the broad sides facing one another. Attach the wire leads to the power supply or battery. Attach the wires to the strips so that the currents will run in the same direction. What happens? Re-attach the wires now so that the currents run in the opposite direction. What happens? Why does this occur?
Evaluation: This demonstration failed miserably. The foil was too thick and the current to small to show any kind of effect.
Experiment 2: For this experiment, we will need a variable power supply, one long wire, two wire leads, tape, a steel or iron rod, and paper clips. Coil the wire about the rod such that there is one coil per 1 cm and tape in place. Attach the wire leads to the power supply and then to the wire coil. Turn the power supply on until it read 1 amp. See how many paper clips the end of the rod can hold before they start falling off. Increase the current to 3 and 5 amps and repeat at both settings. Now, change the number of coils to one every .2 cm and repeat. Then change the number of coils until they are touching (note the number of coils per centimeter) and repeat.
1 amp3 amps5 amps1 coil/cm5 coils/cm__ coils/cm
Does there appear to be a pattern?
Evaluation: This is a good experiment for students. In a classroom, you might want to expand the table to more coil and amparage settings. Plotting the results of the strength versus the two different variables shows the dependence. An improvement to the experiment is to twist one paper clip like a hook, attach it to the electromagnet, and then hang all the other paper clips to it. This gets rid of the variation of how each paper clip is attached to the electromagnet.
Oersted's discovery was a major breakthrough. It meant that one could create magnets with electricity. Therefore, if one placed a permanent magnet near a wire coil and then turned on the current to create an electromagnet, then the coil would either be attractive or repulsive to the permanent magnet. In either case, motion would be a result. Therefore, electromagnets could take electrical energy and produce kinetic energy. This is what we call a motor. However, the problem was that we had now way of creating currents in large supply other than through chemical batteries. What many began to wonder is if you could reverse the process: If a current could create a magnet, could a magnet create a current? Michael Faraday answered this question by discovering that passing a magnet through a loop of wire created a current. This discovery was to change the world, as it allows for electrical energy to be created from kinetic energy.
To study this, we will need a permanent magnet, a long wire, a piece of pipe, and an ammeter. Wrap the wire around the pipe, connecting the ends of the wire to the ammeter. Slowly push the magnet through the pipe. What happens to the ammeter? Now, turn the pipe on its end and drop the magnet through the pipe. What is the reading on the ammeter?
Joseph Henry discovered after Faraday that the current produced around any closed loop of wire is proportional to the rate at which the magnetic flux was changing through the loop. The slower the magnet moves, the slow the change in the flux, and, thus, the lower the current produced. The faster the magnet moves, the stronger the current. Thus the wire loop is transferring kinetic energy (magnet movement) to electrical energy.
A nice demonstration of conservation of energy is the following activity. For this, we will need a meter long section of copper pipe and a very strong magnet that will fit through the copper pipe. Holding the pipe vertically, drop the magnet ON THE OUTSIDE of the pipe and measure how long it takes it to drop the one meter. Now, drop the magnet down the inside of the pipe. Depending on the strength of the magnet, it should take considerably longer. Why is this, considering that we showed earlier that copper was not magnetic?
As the magnet begins to move down the pipe, the magnetic flux through any loop around the copper pipe begins to change. Therefore, a current is produced, i.e., energy is produced. Since energy is conserved, this energy must come from the magnet, which means it slows down. Hence, it takes the magnet a much greater time to make it down the pipe.
Evaluation: This demonstration works nice, but it requires a very strong magnet. The best for this is some sort of neodymium magnet (order through supply catalogs).