Homework Helpers: Physics

7 Magnetism

At the risk of sounding completely old and out of touch, I have to say that I don”t think children spend enough time playing with magnets these days. I have fond memories of the hours that I spent as a child, playing with various magnets. Two bar magnets could keep me busy for hours, as I designed and carried out simple activities to test the limits of their mysterious powers. I was fascinated by the fact that if you played with a magnet and a nail for long enough, the nail would start to exhibit magnetic properties of its own. My father once helped me construct a powerful electromagnet. I loved to power it up and pick up a bunch of nails, and then shut off the power and watch most of the nails fall, as much of the magnet”s power disappeared with the electric current. I even had this great toy where you would use a magnetic wand to add iron-filing “hair” to a cartoon bald guy. To this day, I still feel like a child again when I have some magnets to play with. Now, before this starts, to sound any more like my grandfather”s “When I was young we had to whittle all of our toys out of wood!” speech, let”s move on to the lessons on magnets.

The word magnet is derived from the Greek word magnetite. Magnetite is a naturally magnetic ore that the Greeks found in an area of Turkey known as Magnesia. I sometimes wonder what it would have been like to see or even carry out demonstrations with these natural magnets, using them to attract small pieces of iron in an age where the advances of technology had not overshadowed the wonder invoked by forces that can act over a distance.

If you have ever played with magnets, then you know that they don”t attract all materials. Of the elements found in nature, only iron, nickel, and cobalt are strongly attracted to magnets, and are called ferromagnetic. Alloys of these metals are used to create strong bar magnets, like the ones that you may find in a science lab. It is believed that the magnetic properties of these elements are a result of their atomic structures. Magnetic properties are exhibited by charges in motion, and all of the electrons in atoms are charges in motion. Therefore, all moving electrons should act like tiny magnets. Why aren”t all atoms magnetic? The magnetic fields produced by individual electrons are very weak, but when many tiny magnetic fields overlap in the proper orientation in areas called magnetic domains, they can act as a bigger magnetic field. In most elements, the tiny magnetic fields work against each other as often as they work together. In natural magnets, enough of the tiny fields work together to produce noticeable magnetic properties.

You can induce magnetic properties in a nail because exposing it to a magnetic field will cause its domains to line up in the proper orientation to allow the smaller magnetic fields to overlap and produce a bigger one. Stroking a nail repeatedly with one pole of a bar magnet, moving in the same direction, will speed up this process. Dropping or heating the nail can make it lose its magnetic properties, because the domains can be knocked out of alignment.

Magnetic Poles

If you spent some time playing with bar magnets, you probably made certain discoveries about them fairly quickly. First, you might notice that they are strongest near the ends, or poles. If the poles of your bar magnets are labeled “north” and “south,” or “+” and “−,” then you would quickly notice that the like poles repel each other and the opposite poles attract each other. If you broke one of your magnets, you would find that instead of separating the two poles, you end up with two bar magnets, each with its own north and south poles. If you hung a bar magnet from a string, you would find that it would rotate and orientate itself in the same direction, provided it was not interfered with by other objects.

If you cover a bar magnet with a sheet of paper or thin glass, you can “map” out the magnetic field surrounding the magnet by sprinkling iron filings on the covering. The iron filings line up along the lines of force and give us a two-dimensional picture of a magnetic field.

Figure 7.1

Combining this technique with a compass used to determine the direction of the field lines, we could get a clearer picture of the magnetic field around the bar magnet. The differences between the field between two like poles and the field between two unlike poles can be shown with iron filings. The north pole of your compass would point to the south pole of the bar magnet. It is the accepted convention to draw the arrows of the magnetic field diagram originating, or coming out of the north pole of the magnet, and going into the south pole of the magnet.

Earth”s Magnetic Field

One potentially confusing aspect concerning the magnetic field of Earth is that the north pole of a compass needle points toward Earth”s geographic North Pole. So, the south pole of Earth”s magnetic field is actually located near the Earth”s geographic North Pole.

When I say that Earth”s magnetic south pole is located near the geographic North Pole, I mean that the two poles are separated by more than 1,000 miles. In fact, the actual locations of Earth”s magnetic poles change on a daily basis. Following the compass needle directly wouldn”t necessarily lead you to Earth”s geographic North Pole. Depending on your longitude and latitude, there will be a difference between the lines drawn to the geographic North Pole and the magnetic south pole. We call this difference the magnetic declination. Unless you make a correction to your path equal to the magnetic declination, your compass will lead you to Earth”s magnetic south pole, rather than its geographic North Pole.

Lesson 7–1 Review

1. A _______________ is a microscopic region in which the magnetic fields of atoms are aligned in the same direction.

2. ________________ materials, such as iron, nickel, and cobalt, are strongly attracted to magnets.

3. Which pole (north or south) of the Earth”s magnetic field is located near the Earth”s geographic South Pole?