Vern J. Ostdiek
Donald J. Bord
Chapter 8
Electromagnetism and EM Waves
(Section 2)
8.2 Interactions Between Electricity and
Magnetism
• Consider the following items that we usually take
for granted:
•
•
•
electric motors in hair dryers, automobiles,
computer disc drives, elevators, and countless
other devices;
generators that produce most of the electricity we
use; speakers, audio and videotape recorders, and
high-fidelity microphones;
and the waves that make radios, wireless
telephones, radar, microwave ovens, medical xrays, and our eyes work
8.2 Interactions Between Electricity and
Magnetism
• What do all of these have in common?
•
They all are possible because electricity and
magnetism interact with each other in basic—and
very useful—ways.
• The word electromagnetic, which appears dozens
of times in this chapter, is perhaps the best
indication of just how intertwined these two
phenomena are.
8.2 Interactions Between Electricity and
Magnetism
• Let’s summarize and
review the key aspects of
electrostatics and
magnetism:
• Electric charges produce
electric fields in the space
around them.
8.2 Interactions Between Electricity and
Magnetism
• An electric field, regardless of its origin, causes a
force on any charged object placed in it.
8.2 Interactions Between Electricity and
Magnetism
• Magnets produce magnetic fields in the space
around them.
8.2 Interactions Between Electricity and
Magnetism
• A magnetic field, regardless of its origin, causes
forces on the poles of any magnet placed in it.
• These statements have been worded in a
particular way because, as we shall see, it is the
electric and magnetic fields that are involved in the
interplay between electricity and magnetism.
8.2 Interactions Between Electricity and
Magnetism
• There are three basic observations of the
interactions between electricity and magnetism:
•
The first of the three observations is the basis of
electromagnets.
Observation 1: A moving electric charge
produces a magnetic field in the space around it.
•
An electric current produces a magnetic field
around it.
8.2 Interactions Between Electricity and
Magnetism
• A single charged
particle creates a
magnetic field only
when it is moving.
• The magnetic field
produced is in the shape
of circles around the
path of the charge.
8.2 Interactions Between Electricity and
Magnetism
• For a steady (DC) current, which is basically a
succession of moving charges, in a wire, the field
is steady, and its strength is proportional to the
size of the current and inversely proportional to the
distance from the wire.
•
The field is quite weak unless the current is large.
8.2 Interactions Between Electricity and
Magnetism
• A current of 10 amperes or more will produce a
field strong enough to be detected with a
compass.
• Reversing the direction of the current in the wire
will reverse the directions of the magnetic field
lines.
8.2 Interactions Between Electricity and
Magnetism
• Most applications of this phenomenon use coils—
•
long wires wrapped in the shape of a cylinder, often
around an iron core
• The magnetism induced in the iron greatly
enhances the magnetic field of the coil.
8.2 Interactions Between Electricity and
Magnetism
• The magnetic field of such a coil (when carrying a
direct current) has the same shape as the field
around a bar magnet.
8.2 Interactions Between Electricity and
Magnetism
• This device is an electromagnet. It behaves just
like a permanent magnet as long as there is a
current flowing.
•
One end of the coil is a north pole, and the other is
a south pole.
• Electromagnets have an advantage over
permanent magnets in that the magnetism can be
“turned off” simply by switching off the current.
•
Large electromagnets are routinely used to pick up
scrap iron.
8.2 Interactions Between Electricity and
Magnetism
• A coil with a length that is much greater than its
diameter is called a solenoid.
• If an iron rod is partially inserted into a solenoid
with a hollow core, the rod will be pulled in when
the current is switched on:
•
The magnetic pole associated with the coil’s field
nearest the rod induces a magnetic field with the
opposite polarity in the rod, thus exerting an
attractive force on it.
8.2 Interactions Between Electricity and
Magnetism
• Solenoids are used in common devices for striking
doorbell chimes, opening valves to allow water to
enter and to leave washing machines, withdrawing
deadbolts in electric door locks, and engaging
starter motors on car and truck engines.
8.2 Interactions Between Electricity and
Magnetism
• Electromagnets are used to produce the strongest
magnetic fields on Earth.
• Two factors contribute to stronger fields:
•
wrapping more coils around the cylinder and using
a larger electric current
• The former suggests the use of thinner wire so
that more coils can fit into the same amount of
space.
•
But smaller wire requires smaller electric current so
the wire does not overheat and melt.
8.2 Interactions Between Electricity and
Magnetism
• This limitation is overcome in superconducting
electromagnets.
8.2 Interactions Between Electricity and
Magnetism
• When the wire used in an electromagnet is a
superconductor, it can carry huge electric currents
with no ohmic heating because there is no
resistance.
• Very small superconducting electromagnets can
generate very strong magnetic fields while using
much less electrical energy than a conventional
electromagnet.
8.2 Interactions Between Electricity and
Magnetism
• Superconducting electromagnets do have
limitations, though.
•
The superconducting state is lost if the
temperature, electric current, or magnetic field
strength exceeds certain values.
• Most superconducting electromagnets now found
in laboratories throughout the world must be kept
cold with liquid helium (T = 4 K).
•
The added cost of the liquid helium system is offset
by the high magnetic fields achieved and the great
reduction in use of electric energy compared to
conventional electromagnets.
8.2 Interactions Between Electricity and
Magnetism
• The polarity of an electromagnet is reversed if the
direction of the current is reversed.
8.2 Interactions Between Electricity and
Magnetism
• An alternating current in a coil will produce a
magnetic field that oscillates:
•
It increases, decreases, and switches polarity with
the same frequency as the current.
• Such an oscillating magnetic field will cause a
nearby piece of iron to vibrate.
• The oscillating magnetic field of a coil with AC in it
is used in many common devices.
8.2 Interactions Between Electricity and
Magnetism
• This first interaction not only explains how
electromagnets work, but also gives us new
insight into permanent magnets as well.
• Because electrons in atoms are charged particles
in motion about the nucleus, they produce
magnetic fields.
8.2 Interactions Between Electricity and
Magnetism
• The electrons have their
own magnetic fields
associated with their
spin.
•
In any unmagnetized
material, the individual
magnetic fields of the
electrons are randomly
oriented and cancel
each other out.
8.2 Interactions Between Electricity and
Magnetism
•
In ferromagnetic materials, these fields can be
aligned with one another by an external magnetic
field;
•
The material then produces a net magnetic field.
• So we can conclude that moving electric charges
are the causes of magnetic fields even in ordinary
bar and horseshoe magnets.
8.2 Interactions Between Electricity and
Magnetism
• This brings us back to a statement made at the
beginning of Chapter 7:
•
electric charges are the cause of both electrical and
magnetic effects
• We might regard electricity and magnetism as two
different manifestations of the same thing—
charge.
8.2 Interactions Between Electricity and
Magnetism
• The second observation helps us understand how
things such as electric motors and speakers work.
Observation 2: A magnetic field exerts a force on
a moving electric charge.
•
Therefore, a magnetic field exerts a force on a
current-carrying wire.
8.2 Interactions Between Electricity and
Magnetism
• A stationary electric charge is not affected by a
magnetic field, but a moving charge usually is.
• Note that this second observation is a logical
consequence of the first:
•
Anything that produces a magnetic field will itself be
affected by other magnetic fields.
8.2 Interactions Between Electricity and
Magnetism
• A curious characteristic of
electromagnetic phenomena
is that the effects are often
perpendicular to the causes.
•
The direction of the
magnetic field from a
current-carrying wire is
perpendicular to the
direction the current is
flowing.
8.2 Interactions Between Electricity and
Magnetism
• Similarly, the force that a magnetic field exerts on a
moving charge or on a current-carrying wire is
perpendicular to both the direction of the magnetic
field and the direction the charge is flowing.
• For example, if a horizontal magnetic field is
directed away from you and a wire is carrying a
current to your right, the force on the wire is
upward.
• If the direction of the current is reversed, the
direction of the force is reversed (downward).
• An alternating current would cause the wire to
experience a force that alternates up and down.
8.2 Interactions Between Electricity and
Magnetism
• Electric motors—like those in hair dryers and
elevators—exploit this electromagnetic interaction.
•
The simplest type of electric motor consists of a coil
of wire mounted so that it can rotate in the magnetic
field of a horseshoe-shaped magnet.
8.2 Interactions Between Electricity and
Magnetism
• A direct current flows through the coil, the
magnetic field causes forces on the sides of the
coil, and the coil rotates.
•
•
•
Once the coil has completed half of a rotation, a
simple mechanism reverses the direction of the
current.
This reverses the force on the coil, causing it to
rotate another half turn.
This process is repeated, and the coil spins
continuously.
• Motors designed to run on AC can exploit the fact
that the direction of the current is automatically
reversed 120 times each second (60-cycles-persecond AC, with two reversals each cycle).
8.2 Interactions Between Electricity and
Magnetism
• Liquid metals, such as the molten sodium used in
certain nuclear reactors, can be moved through
pipes using an electromagnetic pump that has no
moving parts.
•
•
If the metal has to be moved in a pipe that is
oriented north–south, for example, a large electric
current can be sent across the pipe—east to west
perhaps.
Then, if a strong magnetic field is directed
downward through the same section of pipe, the
current-carrying metal will be forced to move
southward.
8.2 Interactions Between Electricity and
Magnetism
• Several large-scale devices used in experimental
physics make use of the effect of magnetic fields
on moving charged particles.
•
•
High-temperature plasmas cannot be kept in any
conventional metal or glass container because the
container would melt.
Because plasmas are composed of charged
particles, magnetic fields can be used to contain
them in what is known as a magnetic bottle.
• This is one approach being employed in the
attempt to harness nuclear fusion as an energy
source.
8.2 Interactions Between Electricity and
Magnetism
• In the absence of other forces, a charged particle
moving perpendicularly to a magnetic field will
travel in a circle:
•
the force on the particle is always perpendicular to
its velocity and is therefore a centripetal force.
• An electron, proton, or other charged particle can
be forced to move in a circle by a magnetic field
and then gradually accelerated during each
revolution.
•
Particle accelerators used for experiments in
atomic, nuclear, and elementary particle physics, as
well as those used for producing radiation for
cancer treatments at some large hospitals, operate
on this principle.
8.2 Interactions Between Electricity and
Magnetism
• The world’s highest-energy particle accelerator is
the Large Hadron Collider (LHC) located along the
border between France and Switzerland near the
city of Geneva.
•
•
This device comprises a circular tunnel with a
diameter of 8.6 kilometers (5.3 miles) buried 50 to
175 meters beneath Earth’s surface in which two
counter-rotating beams of charged particles travel
in a vacuum guided by superconducting magnets.
The head-on collisions between the particles in
these oppositely moving beams yield information
about the fundamental forces and interactions in
Nature.
8.2 Interactions Between Electricity and
Magnetism
• The third observed interaction between electricity
and magnetism is used by electric generators.
•
•
Recall that the first observation tells us that moving
charges create magnetic fields.
The third one is a similar statement about moving
magnets.
Observation 3: A moving magnet produces an
electric field in the space around it.
•
A coil of wire moving through a magnetic field has a
current induced in it.
8.2 Interactions Between Electricity and
Magnetism
• The electric field around a moving magnet is in the
shape of circles around the path of the magnet.
This circular electric field will force charges in a
coil of wire to move in the same direction—as a
current.
8.2 Interactions Between Electricity and
Magnetism
• The process of inducing an electric current with a
magnetic field is known as electromagnetic
induction.
• All that is required is that the magnet and coil
move relative to each other.
•
If the coil moves and the magnet remains
stationary, a current is induced.
• If the motion is steady in either case, the induced
current is in one direction.
•
If either the coil or the magnet oscillates back and
forth, the current alternates with the same
frequency—it is AC.
8.2 Interactions Between Electricity and
Magnetism
•
Electromagnetic induction is used in the most
important device for the production of electricity:
the generator.
•
•
The simplest generator is basically an electric
motor.
When the coil is forced to rotate, it moves relative to
the magnet, so a current is induced in it.
8.2 Interactions Between Electricity and
Magnetism
• We might call this device a “two-way energy
converter.”
• When electrical energy is supplied to it, it is a
motor.
•
It converts this electrical energy into mechanical
energy of rotation.
• When it is mechanically turned (by hand cranking,
by a fan belt on a car engine, or by a turbine in a
power plant), it is a generator.
•
It converts mechanical energy into electrical
energy.
8.2 Interactions Between Electricity and
Magnetism
• This motor–generator duality is used in dozens of
pumped-storage hydroelectric power stations.
• During the night when surplus electrical energy is
available from other power stations, the motor
mode is used to pump water from one reservoir to
another that is at a higher elevation.
•
Most of the electrical energy is converted into
“stored” gravitational potential energy.
8.2 Interactions Between Electricity and
Magnetism
• During the peak time of electric use the next day,
water flows in the opposite direction, and the
generator mode is used as the moving water turns
the pumps (now acting as turbines) that turn the
motors (now acting as generators), thereby
producing electricity.
•
You might say that the system functions like a
rechargeable gravitational battery.
8.2 Interactions Between Electricity and
Magnetism
• Another application of this technology is
regenerative braking, which is used in electric and
hybrid vehicles.
• While accelerating and cruising, electric motors
turn the wheels using electricity from batteries.
• During braking, the motors function as generators:
•
The wheels turn them, and the electricity that is
generated can partially recharge the batteries.
• Instead of all of the vehicle’s kinetic energy being
converted into wasted heat—the case with
conventional friction brakes—some of it is saved
for reuse.
8.2 Interactions Between Electricity and
Magnetism
• In summary, when electric charges or magnets are
in motion, electricity and magnetism are no longer
independent phenomena.
• The three observations given here are statements
of experimental facts that illustrate this
interdependence.
•
They can be demonstrated easily using a battery,
wires, a compass, a large magnet, and a sensitive
ammeter.
8.2 Interactions Between Electricity and
Magnetism
• The fact that electricity and magnetism interact
only when there is motion (and then the effects are
perpendicular to the causes) is somewhat startling
when compared to, say, gravitation and
electrostatics.
•
Gravitational and electrostatic forces are always
toward or away from the objects causing them, and
they act whether or not anything is moving or
changing.
• These basic yet surprising interactions between
electricity and magnetism are crucial to our
modern electrified society.
Concept Map 8.2