Magnetism

From Experiment Central, published by U·X·L

In this experiment you will create a magnet and then test the effects on the magnet's strength of heat, cold, jarring, and rubbing with another magnet.

One of the most mysterious phenomena we witness every day is magnetism, a fundamental force of nature caused by the motion of electrons in an atom. You put a note on a refrigerator door. You watch the speedometer in a car tell you how fast you are travelling. You listen to a tape of recorded music. All of these depend on magnetism, but how do these things work? How does the simple physics of the magnet make so much possible?

Magnetism is a matter of alignment

What turns an ordinary piece of iron into a magnet? A large iron bar actually contains millions of "mini-magnets," small magnetized areas called domains. Each has a north pole and a south pole. If the poles of the iron's domains are aimed in all different directions, their magnetic forces act against one another and cancel each other out. When all of the domains are facing the same way, the bar becomes a magnet because it now has a single, strong magnetic field, a space in which its magnetic force can be observed.

How can we get all the domains facing the same way? This can be achieved by repeatedly rubbing the bar with one pole of another magnet in the same direction. Once the bar is magnetized, its magnetic field will exert enough force on the domains in nearby iron filings to temporarily magnetize them. Each filing has its own north and south poles, and those poles are attracted to or repelled by the magnet's poles. (Remember that unlike poles attract and like poles repel.)

The position of the domains in such a magnet is not permanent, however. Striking or jarring the bar will literally knock its domains out of alignment, and the bar will lose its magnetism. Even as time passes and the magnet sits in a drawer, it will slowly lose its magnetism as the domains shift back to their original positions. One way to preserve a magnet is to keep it in a magnetic circuit, in which each domain is held in place by the direction of the next domain. Placing a steel plate across the poles of a horseshoe magnet will complete the circuit: all the domains in the circuit will point in the same direction and will tend to remain that way.

Electricity can also produce magnetism

Electrical current flowing through a wire produces a magnetic field. If the wire is wound into a coil, it will produce a stronger magnetic field, similar to that of a bar magnet: each end of the coil will become a magnetic pole. This effect was discovered by Danish physicist Hans Christian Oersted (1777-1851). He noticed that electric current disturbed the normal functioning of magnetic compasses.

Electromagnetism is a form of magnetic energy produced by the flow of an electric current through a metal core. It has many applications in our modern technology. Stereo speakers are one of the most common applications. Electrical signals pass through a coil, creating a varying magnetic field that pushes and pulls on another magnet attached to the speaker. This causes the paper speaker cone to move back and forth to produce sound. Some metals, including iron, can be made into electromagnets strong enough to lift tons of scrap steel. One advantage of electromagnets is that they can be turned on and off with the flip of a switch.

Magnets: How do heat, cold, jarring, and rubbing affect the magnetism of a nail?

Purpose/Hypothesis

In this experiment, you will first test the effect of rubbing a bar magnet on a steel or iron nail. The bar magnet should align the domains in the iron so that the nail becomes magnetized. You will then measure the effect of four actions upon the nail's magnetic strength—-heating, cooling, rubbing with a magnet in the opposite direction, and striking with a hammer. Each of the four actions will be tested on a different magnetized nail. Before you begin, make an educated guess about the outcome of this experiment based on your knowledge of magnetism. This educated guess, or prediction, is your hypothesis. A hypothesis should explain these things:

• the topic of the experiment
• the variable you will change
• the variable you will measure
• what you expect to happen

A hypothesis should be brief, specific, and measurable. It must be something you can test through observation. Your experiment will prove or disprove whether your hypothesis is correct. Here is one possible hypothesis for this experiment: "Rubbing a magnetized nail with the opposite pole of the bar magnet that was used to magnetize it, striking or dropping it, and raising or lowering its temperature will decrease the strength of its magnetic field."

In this case, the variables you will change are the four actions you will take on identically magnetized nails, and the variable you will measure is the resulting strength of the nail's magnetic field. You expect that all four actions will reduce the nail's magnetic strength.

What Are the Variables?

Variables are anything that might affect the results of an experiment. Here are the main variables in this experiment:

• type of metal in the nails
• the size of the nails
• the strength of the bar magnet used
• the number of times the nail is rubbed with the bar magnet
• the direction in which the nail is rubbed with the bar magnet
• the actions performed on the nails (striking, heating, etc.)

In other words, the variables in this experiment are anything that might affect the magnetic strength of the nails. If you change more than one variable for each nail, you will not be able to tell which variable had the most effect on the resulting magnetic strength of the nail.

A fifth nail will be magnetized and tested without any action performed on it. This control experiment lets us know that any changes we see in magnetism result from the actions and not from some unseen factor.

Level of Difficulty

Easy/moderate.

Materials Needed

• bar magnet
• 5 steel or iron nails about 3 inches (7.5 centimeters) long (iron is preferable; steel is an alloy containing other metals that cannot be magnetized)
  hammer
• 1 cup of hot tap water
• 1 cup of cold tap water with ice added
• 10 staples (separated and unused)
• 10 steel paper clips
• 10 plastic-coated paper clips
• small wooden block
• safety glasses

Approximate Budget

Less than $10 for the magnet. (Try to borrow the hammer and safety glasses, if you do not have them.)

Timetable

About 30 minutes.

How to Experiment Safely

Safety glasses must be worn any time you are striking metal on metal. Do not strike the nail with great force, and be sure to rest the nail on the wooden block so it does not bend or snap when hit. Do not lift the hammer more than 6 inches (15 centimeters) from the block.

Step-by-Step Instructions

1. Rub one pole of the bar magnet lengthwise down one nail fifty times, always in the same direction.
2. Test the nail for magnetism by touching its point to a staple, then to a steel paper clip, then to a coated paper clip.
3. Observe and record on your data chart which objects the nail can lift. Carefully set the nail aside. Keep it several inches away from the other nails.
4. Repeat this procedure with three other nails, rubbing them the same number of times in the same direction with the same pole of the bar magnet. The magnetic strength of the nails should be almost the same. If one is significantly weaker, rub it with the magnet until the strength of its field is similar to the others. Your data chart should look like the illustration.
5. To establish your control experiment, test the remaining nail for magnetism. If this nail picks up any of the test objects, it has somehow been magnetized. Do not be surprised if the nail does have a very weak magnetic field. Just the movements of nails against one another in a box can align a small percentage of the domains in the metal. To prove that rubbing the first four nails with the bar magnet caused them to become magnetized, however, you must see a significant difference between their magnetic strength and that of the control nail.
6. Now rub the control nail the same number of times in the same direction. Check to be sure it is magnetized, record the results, and carefully set it aside away from the other nails.
7. Perform one action on each nail. (Remember not to disturb the control nail.)
1. Place the first nail in hot water and leave it for ten minutes.
2. Place the second nail in the ice water and leave it for ten minutes.
3. Rub the third nail with the same pole of the magnet used earlier, but in the opposite direction, twenty-five times.
4. With everyone present wearing safety goggles, place the shaft of the fourth nail flat on the wooden block and strike it firmly three or four times. (Do not lift the head of the hammer any more than 6 inches [15 centimeters].)
8. Test the magnetic strength of each nail and note any changes on your chart.
9. Finally, check the control nail to make sure that nails do not lose their magnetic strength simply by sitting unused for several minutes. Record the strength of the control nail in the appropriate row on your chart.

Summary of Results

Compare your data from the four tests. Determine which of the actions demagnetized the nails and which did not. Check your findings against the predictions you made in your hypothesis. Which actions did you accurately predict would demagnetize the nails? Which actions did not have the effect you expected? Summarize your results in writing.

Troubleshooter's Guide

This experiment is fairly straightforward. You should encounter little difficulty if you use the listed materials. When you are doing experiments with magnetism, results can be difficult to measure precisely. To compare the strengths of magnets, test their lifting power several times and average the results to achieve a greater degree of accuracy.

Here are some problems that may arise during the experiment, some possible causes, and ways to remedy the problems.

Problem: All of the nails are strongly magnetized to start with.
Possible cause: They may have been exposed to a strong magnetic field prior to the experiment. Demagnetize them by striking each several times with the hammer. (It is not necessary to strike with great force. Remember to wear safety glasses and place the nails flat on a wooden block so they will not bend or snap.)

Problem: The nails will not magnetize.
Possible causes:

1. The nails are made of a metal or alloy that cannot be magnetized. Use iron or steel nails. (Iron is preferable.)
2. Your bar magnet is too weak. Check its strength and replace it if necessary.
3. You are changing the direction of the stroke as you rub the magnet on the nail, or you are accidentally switching poles as you rub the nail. Either mistake will sweep the nail's domains in different directions. M Follow this procedure carefully.

Change the Variables

By altering your variables, you can make this experiment the basis of a series of interesting and informative investigations into magnetism. For example, how fast does magnetic strength weaken? Can we preserve a magnet longer by refrigerating it? Are the effects of demagnetization always reversible, or can domains be put permanently out of order?