Magnetic Force

As children, we discovered that magnets “stick” to certain metals. Iron, nickel, a few other elements, and alloys or solid mixtures containing any of them constitute ferromagnetic materials. Magnets exert force on these metals. Magnets do not exert force on other metals unless those metals carry electric currents. Electrically insulating substances never “attract magnets” under normal conditions.

When you bring a magnet near a piece of ferromagnetic material, the atoms in the material line up to some extent, temporarily magnetizing the sample. This alignment produces a magnetic force

between the atoms of the sample and the atoms in the magnet. Every atom acts as a tiny magnet; when they act in concert with one another, the whole sample behaves as a single large magnet. Magnets always “stick” to samples of ferromagnetic material.

If you place two magnets near each other, you observe a stronger magnetic force than you see when you bring either magnet near a sample of unmagnetized ferromagnetic material (an iron nail, say). The mutual force between two rod-shaped or bar-shaped magnets manifests as attraction if you bring two opposite poles close together (north-near-south or south-near-north) and repulsion if you bring two like poles into proximity (north-near-north or south-near-south). Either way, the force increases as the distance between the ends of the magnets decreases.

Whenever the atoms in a sample of ferromagnetic material align to any extent rather than existing in a random orientation, a “region of influence” called a magnetic field surrounds the sample. A

magnetic field can also result from the motion of electric charge carriers. In a wire, electrons move in incremental “hops” along the conductor from atom to atom. In a permanent magnet, the movement of orbiting electrons occurs in such a manner that an effective current arises.

Physicists and engineers describe magnetic fields in terms of flux lines, also called lines of flux.

The intensity of the field depends on the number of flux lines passing at right angles through a region having a certain cross-sectional area, such as a square centimeter or a square meter. The flux lines aren’t material things, of course, but you can see their effects by doing a simple experiment.

Have you seen the classical demonstration in which iron filings lie on a sheet of paper, and then the experimenter holds a permanent magnet underneath the sheet? The filings arrange themselves in a

pattern that shows, roughly, the “shape” of the magnetic field in the vicinity of the magnet. A bar magnet has a field whose lines of flux exhibit a characteristic pattern, as shown in Fig. 1-9.

FIGURE 1-9 Magnetic flux around a bar magnet.

Another experiment involves passing a current-carrying wire vertically through a sheet of paper oriented horizontally. The iron filings bunch up in circles centered at the point where the wire passes through the paper. This experiment shows that the lines of flux around a straight, current-carrying wire form concentric circles in any plane passing through the wire at a right angle. The center of every

“flux circle” lies on the wire, which serves as the path along which the charge carriers move (Fig. 1-10). A magnetic field has a specific orientation at any point near a current-carrying wire or a

permanent magnet. At any point, the magnetic flux lines always run parallel with the direction of the magnetic field’s “flow.”

FIGURE 1-10 Magnetic flux produced by an electric current traveling in a straight line.

Did You Know?

Scientists consider any magnetic field to begin, or originate, at a north pole, and to end, or terminate, at a south pole. These poles don’t correspond to the earth’s magnetic poles, however. They’re the opposite! The earth’s north magnetic pole is actually a magnetic south pole because it attracts the north poles of compasses such as the ones hikers and hunters use. Similarly, the earth’s south magnetic pole is really a magnetic north pole because it attracts the south poles of compasses.

Quick Question, Quick Answer

• How do the flux lines in a magnetic field differ from the flux lines in an electric field, such as the sort that surrounds an electrically charged particle?

• A charged particle hovering all by itself in space produces electric flux lines that aren’t closed; they radiate away from the particle in all directions, like infinitely long spikes. But magnetic flux lines always form closed loops. In the vicinity of a magnet, you can always find a starting point (the north pole) and an ending point (the south pole). You can’t have a magnetic north or south pole anywhere all by itself; it must have a “mate” of the opposite polarity nearby.

You might suppose that the magnetic field around a current-carrying wire, such as the one shown in Fig. 1-10, arises from a single, isolated magnetic pole. Or, you might imagine that no magnetic poles exist at all! The concentric flux circles don’t seem to originate or terminate anywhere. You can get around this problem by means of a mind game. You can “invent” an originating point and a

terminating point anywhere you want on one of the flux circles, thereby defining a pair of opposite magnetic poles in close proximity.

In a magnetic field, the lines of flux always connect the two magnetic poles. Some flux lines appear straight in a local sense, but in the larger sense, they always form curves. The greatest magnetic field strength around a bar magnet occurs near the poles, where the flux lines converge or diverge. As you move away from the poles, the magnetic field grows less intense. Around a current-carrying wire, the greatest field strength exists near the wire, and the intensity diminishes as you move away from the wire.

In theory, the flux field around any magnet, or around any current-carrying wire, extends into space indefinitely. In practice, the effects “wear off” at a certain distance from any magnet or wire because the field simply gets too weak to influence anything in the real world.

Did You Know?

Magnetic fields can arise from the motion of electrically charged subatomic particles through space, as well as from the motion of charge carriers through a conductor. The sun constantly ejects protons (which carry a positive charge) and electrons (which carry a negative charge). These particles produce electrical currents as they travel through space. The currents produce vast, fluctuating magnetic fields, just as a steady current in a wire generates a small-scale, stable magnetic field in a situation of the sort shown in Fig. 1-10. Conversely, fluctuating or unstable magnetic fields induce electric currents in electrical conductors, such as utility wires and radio antennas.

Fact or Myth?

We’ve all heard news reports from time to time, warning us that an eruption has taken place on the sun, and that we should prepare for possible disruptions to our communications or utility infrastructures. Are these warnings exaggerated? To some extent, maybe so; but when a solar flare occurs, the sun ejects far more charged particles than usual. As these particles approach the earth, their magnetic fields, working together, disrupt our planet’s magnetic field, spawning a geomagnetic storm. Such an event can temporarily wipe out “shortwave radio” communications.

In addition, people who live at high latitudes witness aurora borealis (“northern lights”) and aurora australis (“southern lights”) at night. If a big enough geomagnetic storm occurs, it can interfere with wire communications and electric power transmission at the surface. No one really knows (as of this writing, anyway) whether or not a massive solar flare will ever cause a

worldwide power blackout lasting for years. But people have already witnessed dramatic effects.

All the way back in the year 1859, a geomagnetic storm produced a so-called electromagnetic pulse (EMP) strong enough to generate currents in telegraph wires that set some stations on fire.

Electromagnets

The motion of electrical charge carriers always produces a magnetic field. This field can reach

considerable intensity in a tightly coiled wire having a lot of turns and carrying a lot of current. When you place a ferromagnetic rod called a core inside a wire coil, as shown in Fig. 1-11, the magnetic

lines of flux concentrate in the core, making the core sample into a powerful temporary magnet: an electromagnet.

FIGURE 1-11 A simple electromagnet.

Most electromagnets have rod-shaped cores. When you wind a wire into a coil around a rod-shaped object, you get a solenoid. A solenoid’s length-to-diameter ratio can vary from extremely low (like a fat pellet) to extremely high (like a thin stick). Regardless of the length-to-diameter ratio, however, the flux produced by current in the solenoid’s coil temporarily magnetizes the core that runs through it.

You can build a DC electromagnet by wrapping insulated wire around a large iron bolt. You can find these items in any good hardware store. You should test the bolt for ferromagnetic properties while you’re still in the store, if possible. (If a permanent magnet “sticks” to the bolt, then the bolt is ferromagnetic.) Ideally, the bolt should measure at least 3/8 inch (approximately 1 centimeter) in diameter and at least 6 inches (roughly 15 centimeters) in length. You must use insulated wire, preferably made of solid, soft copper. Don’t use bare wire!

Wind the wire at least 100 times around the bolt. You can layer two or more windings if you like, as long as the wire always keeps going around in the same direction. Secure the wire in place with electrical or duct tape. A large “lantern battery” can provide plenty of DC to operate the

electromagnet. You can connect two or more such batteries in parallel to increase the current delivery.

Never leave the coil connected to the battery for more than a few seconds at a time.

Warning!

experiment! The near short-circuit produced by an electromagnet can cause the acid from this type of battery to boil out, resulting in serious injury to you, not to mention possible damage to objects in the vicinity.

All DC electromagnets have well-defined north and south poles, exactly as permanent magnets

have. However, an electromagnet can, at least in theory, get much stronger than any permanent

magnet. The magnetic field exists only as long as the coil carries current. When you remove the power source, the magnetic field nearly vanishes. A small amount of residual magnetism remains in the core after current stops flowing in the coil, but this field is usually weak.

Some commercially manufactured electromagnets operate from 60-Hz utility AC. These magnets

“stick” to ferromagnetic objects. The polarity of the field reverses every time the current reverses, producing 120 magnetic-field “pulses” every second, assuming a 60-Hz AC line frequency. The instantaneous intensity of the magnetic field varies along with the AC cycle, reaching alternating-polarity peaks at 1/120-second intervals and nulls of zero intensity at 1/120-second intervals.

Some electromagnets produce fields so powerful that no human can pull them apart if they get

“stuck” together, and no human can push them all the way together against their mutual repulsive force. Industrial workers sometimes use huge electromagnets to carry heavy pieces of scrap iron or steel from place to place. Other electromagnets can provide sufficient repulsion to suspend one object above another, an effect known as magnetic levitation.

Warning!

Experiments You Can Do at Home (McGraw-Hill, 2010) offers instructions for doing it.