1. Week# 13
Magnetic field and magnetic forces: Magnetism, magnetic fields,
magnetic field lines & magnetic flux, motion of charged particle in a
Book: “University Physics” by Young & Freedman 13th edition, page: P-883”
Physics-II (Ph-1002), 4 (3, 3)
Magnetic resonance imaging (MRI) makes it possible to see details of
soft tissue (such as in the foot shown here) that aren’t visible in x-ray
images. Yet soft tissue isn’t a magnetic material (it’s not attracted to a
2. 27.1: Magnetism, p-883
Magnetic phenomena were first observed at least 2500 years ago in
fragments of magnetized iron ore found near the ancient city of
Magnesia (now Manisa, in western Turkey). These fragments were
examples of what are now called permanent magnets.
The earth itself is a magnet.
Its north geographic pole is close to a magnetic south pole, which is
why the north pole of a compass needle points north. The earth’s
magnetic axis is not quite parallel to its geographic axis (the axis of
rotation), so a compass reading deviates somewhat from geographic
north. This deviation, which varies with location, is called magnetic
declination or magnetic variation.
Also, the magnetic field is not horizontal at most points on
the earth’s surface; its angle up or down is called magnetic
inclination. At the magnetic poles the magnetic field is
27.3: A sketch of the earth’s magnetic field. The field, which is caused by currents in
the earth’s molten core, changes with time; geologic evidence shows that it reverses
direction entirely at irregular intervals of 104 to 106 years.
4. The Earth’s
magnetic field is
similar to that of
a bar magnet.
Note that the
Pole” is really a
pole, as the north
ends of magnets
are attracted to it.
The first evidence of the relationship of magnetism to moving charges
was discovered in 1820 by the Danish scientist Hans Christian Oersted.
He found that a compass needle was deflected by a current-carrying
wire. Similar investigations were carried out in France by André
A few years later, Michael Faraday in England and Joseph Henry in the
United States discovered that moving a magnet near a conducting
loop can cause a current in the loop.
Magnetic Poles Versus Electric Charge
The concept of magnetic poles may appear similar to that of electric
charge, and north and south poles may seem analogous to positive and
negative charge. But the analogy can be misleading. While isolated
positive and negative charges exist, there is no experimental evidence
that a single isolated magnetic pole exists; poles always appear in
pairs. If a bar magnet is broken in two, each broken end becomes
a pole. The existence of an isolated magnetic pole, or magnetic
27.2: Magnetic Field, p-885
We represented electric interactions in two steps:
1. A distribution of electric charge at rest creates an electric field in
the surrounding space.
2. The electric field exerts a force F=qE on any other charge that is
present in the field.
We can describe magnetic
interactions in a similar way:
1. A moving charge or a current
creates a magnetic field in the
2. The magnetic field exerts a
force F on any other moving
charge or current that is present
in the field.
Application: Spiny Lobsters and Magnetic Compasses
Although the Caribbean spiny lobster (Panulirus argus) has a
relatively simple nervous system, it is remarkably sensitive to
magnetic fields. It has an internal magnetic “compass” that
allows it to distinguish north, east, south, and west. This
lobster can also sense small differences in the earth’s
magnetic field from one location to another, and may use
these differences to help it navigate.
26. This frequency is independent of the radius of the path. It is called
the cyclotron frequency; in a particle accelerator called a cyclotron,
particles moving in nearly circular paths are given a boost twice
each revolution, increasing their energy and their orbital radii
but not their angular speed or frequency.
The orbit of a charged
particle in a uniform
magnetic field. A charge
moving at right angles to a
uniform B field moves in a
circle at constantspeed
because F and v are always
perpendicular to each other.
27.19 A magnetic bottle. Particles near either end of the region
experience a magnetic force toward the center of the region.
This is one way of containing an ionized gas that has a temperature of
the order of 106 K, which would vaporize any material container.
27.20 (a) The Van Allen radiation belts around the earth. Near the
poles, charged particles from these belts can enter the atmosphere,
producing the aurora borealis (“northern lights”) and aurora australis
(“southern lights”). (b) A photograph of the aurora borealis.
27.21 This bubble chamber
image shows the result of a
high-energy gamma ray
(which does not leave a track)
that collides with an electron in
a hydrogen atom. This electron
flies off to the right at high
speed. Some of the energy in
the collision is transformed
into a second electron and a
positron (a positively charged
electron). A magnetic field is
directed into the plane of the
image, which makes the
positive and negative
particles curve off in different
Example 27.3: Electron motion in a magnetron
A magnetron in a microwave oven emits electromagnetic waves
with frequency f=2450 MHz What magnetic field strength is required
for electrons to move in circular paths with this frequency?