PHYSICS PUZZLER Aurora borealis (the northern lights), photographed near Fairbanks, Alaska. Auroras occur when cosmic rays— electrically charged particles, mainly from the Sun—fall into the Earth's atmosphere over the magnetic poles and collide with other atoms, resulting in the emission of visible light. Why is it that only charged particles are trapped by the Earth's magnetic field? Why is the aurora most often observed around the magnetic poles of the Earth? Magnetism

The magnetic field pattern of a bar magnet, displayed by iron filings on a sheet of paper. (b) The magnetic field pattern between unlike poles of two bar magnets, displayed by iron filings. (c) The magnetic field pattern between two like poles.

The geographic north pole corresponds to a magnetic south pole, and the geographic south pole corresponds to a magnetic north pole. In fact, the configuration of the Earth's magnetic field (pictured) very much resembles what would be achieved by burying a bar magnet deep in the interior of the Earth. Earth’s Magnetic Field

map of the lower forty-eight United States showing the declination of a compass from true north.

The magnetic field of the Earth is used to label runways at airports according to their direction. A large number is painted on the end of the runway so that it can be read by the pilot of an incoming airplane. This number describes the direction in which the airplane is traveling, expressed as the magnetic heading, in degrees measured clockwise from magnetic north divided by 10. Thus, a runway marked 9 would be directed toward the east (90° divided by 10), and one marked 18 would be directed toward the magnetic south.

Experiments show that a stationary charged particle does not interact with a static magnetic field. However, when moving through a magnetic field a charged particle experiences a magnetic force. This force has its maximum value when the charge moves perpendicularly to the magnetic field lines, decreases in value at other angles, and becomes zero when the particle moves along the field lines. We shall make use of these observations in describing the magnetic field. Magnetic Fields

If F is in Newtons, q in coulombs, and v in meters per second, the SI unit of magnetic field is the Tesla (T), also called the Weber (Wb) per square meter (that is, 1 T = 1 Wb/m2). Thus, if a 1-C charge moves through a magnetic field of magni­tude 1 T with a velocity of 1 m/s, perpendicularly to the field (sin B = 1), the mag­netic force exerted on the charge is 1 N. We can express the units of B as if Fis in Newtons, q in Coulombs, and v in meters per second, the SI unit of magnetic field is the Tesla (T), also called the Weber (Wb) per square meter (that is, 1 T = 1 Wb/m2). Thus, if a 1-C charge moves through a magnetic field of magni­ tude 1 T with a velocity of 1 m/s, perpendicularly to the field (sin B = 1), the mag­netic force exerted on the charge is 1 N.

In practice, the cgs unit for magnetic field, the gauss (G), is often used. The Gauss is related to the Tesla through the conversion

Figure The right-hand rule for determining the direction of the magnetic force on a positive charge moving with a velocity of v in a magnetic field, B. With your thumb in the direction of v and your four fingers in the direction of B, the force is directed out of the palm of your hand.

This apparatus demonstrates the force on a current-carrying conductor in an external magnetic field. Why does the bar swing away from the magnet after the switch is closed?

Figure 19.8 A segment of a flexible vertical wire partially stretched between the poles of a magnet, with the field (blue crosses) directed into the page. (a) When there is no current in the wire, it remains vertical. (b) When the current is upward, the wire deflects to the left. (c) When the current is downward, the wire deflects to the right.

A Magnetic Pump Figure A simple electro­magnetic pump has no moving parts to damage a conducting fluid, such as blood, passing through. Application of the right-hand rule shows that the force on the current-carrying segment of the fluid is in the direction of the velocity

The GalvanometerGalvanometer A galvanometer is a device used in the construction of both ammeters and voltmeters. Its basic operation makes use of the fact that a torque acts on a current loop in the presence of a magnetic field. Figure 19.15a shows the main components of a galvanometer.

During a lecture demonstration in 1819, the Danish scientist Hans Oersted ( ) found that an electric current in a wire deflected a nearby compass needle. This discovery, linking a magnetic field with an electric current, was the beginning of our understanding of the origin of magnetism

Figure (a) When there is no current in the vertical wire, all compass needles point in the same direction. (b) When the wire carries a strong current, the compass nee­dles deflect in directions tangent to the circle, pointing in the direction of B due to the current

Magnetic Field surrounding a wire carrying a current

In equation form… This result shows that the magnitude of the magnetic field is proportional to the current and decreases as the distance from the wire increases, as one might intuitively expect. The proportionality constant uo, called the permeability of free space

Equation enables us to calculate the magnetic field due to a long, straight wire carrying a current. A general procedure for deriving such equations was pro-posed by the French scientist Andre-Marie Ampere (1775–1836); it provides a relation between the current in an arbitrarily shaped wire and the magnetic field produced by the wire.

Figure All segments of the current loop produce a magnetic field at the center of the loop, directed out of the page.

Figure (a) Magnetic field lines for a current loop. Note that the magnetic field lines of the current loop resemble those of a bar magnet. (b) Field lines of a current loop, displayed by iron filings. (c) The magnetic field of a bar magnet is similar to that of a current loop.

Figure The magnetic field lines for a loosely wound solenoid.