F-2 MAGNETS AND MAGNETIC FIELDS

1. PURPOSE

(1) To experimentally plot magnetic field lines from various configurations of magnets.

(2) To qualitatively study some other properties of magnets.

2. APPARATUS

Various bar, horseshoe and ring magnets.
Unmagnetized pieces of soft iron.
Iron rod.
Carpenter's hammer
Magnetic dip needle
Glass sheet and iron filings.
Small 1/2 inch diameter needle compass.

Read the discussion of magnetic fields and potentials in any good textbook.

4. GENERAL PROCEDURE

Some situations to investigate:

• Field maps: Lay a glass plate over an arrangement of magnets, then sprinkle iron filings over the glass. Tap the glass until the filings reach an equilibrium pattern.
• Field maps: Use a small compass to indicate the field direction at points near a configuration of magnets. Transfer the needle direction to a paper sheet underneath, and when finished, draw line segments on the paper where the needle was, in the direction the needle pointed.
• Magnetization by induction: Use a magnetic dip needle to determine the direction of the Earth's magnetic field in your laboratory. Obtain a soft iron rod, unmagnetized. Check it with a compass needle to be sure it is unmagnetized, and make sure it doesn't attract other unmagnetized metal objects. Hold the unmagnetized bar in the direction of the Earth's field, and hit its end forcibly many times with the hammer. Test it again. Is it now magnetized?
• Now hold the bar perpendicular to the field, and hit it with the hammer in the same manner. Now test it. Has it lost much of its magnetism?
• Keepers: Inexpensive bar magnets tend to demagnetize themselves over time, because their own field exerts force on their aligned domains in a direction to destroy the alignment. To prevent this, soft iron "keepers" are placed across the poles. Investigate the field of a magnet, and then of the same magnet with a keeper in place.

5. ANALYSIS:

Field maps: Trace all of the field lines fully, all the way to the edges of the paper.

The next section includes analysis questions.

6. QUESTIONS

(1) Is it possible to make a magnet with an odd number of poles? If so, why? If not, what would be the relative strengths of the poles?

(2) Is it possible to have a magnet with no poles? Explain. If someone handed you a magnet claimed to have no poles, how could you test that it really is a magnet?

(3) You are given two identical iron bars. One is magnetized, the other is not. By Newton's third law, each exerts a force on the other, and these forces are equal and opposite. Is there any way you could experimentally determine which is magnetized? If not, why not. If so, describe and explain as many different ways as you can discover.

(4) What do electric and magnetic fields have in common? What is different about them?

(5) Older books on elementary physics often used the concept of magnetic pole quantitatively, writing F = (constant)P1P2/R2 for the force two poles exert on each other when they are distance R apart. Unfortunately, poles aren't located at points, but are spread over space. Still, you can check the formula approximately from the maps you made. [When physicists were just beginning to understand magnets, they made very long rod and needle shaped magnets, so the poles were so far apart, the size of the poles was small compared to the dimensions of the magnet, and also so they could study the effect of just one pole of one magnet on just one pole of the other magnet without the other two poles influencing the measurement.]

Text and drawings © 1995, 2004 by Donald E. Simanek.