Magnetism
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?
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.
Earth’s Magnetic Field
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.
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.
Magnetic Fields
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.
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 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.
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 1 9.6 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 19.12 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 Galvanometer
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 (1777-1851) 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 19.22 (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 19.11 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 1 9.27 All segments of
the current loop produce a
magnetic field at the center of
the loop, directed out of the
page.
Figure 19.28 (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 19.29 The
magnetic field lines for a
loosely wound solenoid.