Transcript chapter7-Section3

Vern J. Ostdiek
Donald J. Bord
Chapter 7
Electricity
(Section 3)
7.3 Electric Currents—Superconductivity
• An electric current is a flow of charged particles.
The cord on an electrical appliance encloses two
separate metal wires covered with insulation.
•
When the appliance is plugged in and operating,
electrons inside each wire move back and forth.
7.3 Electric Currents—Superconductivity
• Inside a television picture tube, free electrons are
accelerated from the back of the tube to the
screen at the front.
•
There is a near vacuum inside the picture tube, so
the electrons can travel without colliding with gas
molecules.
7.3 Electric Currents—Superconductivity
• When salt is dissolved in water, the sodium and
chlorine ions separate and can move about just
like the water molecules.
•
If an electric field is applied to the water, the
positive sodium ions will flow one way (in the
direction of the field), and the negative chlorine ions
will flow the other way.
7.3 Electric Currents—Superconductivity
• Regardless of the nature of the moving charges,
the quantitative definition of electric current is as
follows.
Current: The rate of flow of electric charge.
•
The amount of charge that flows by per second.
charge
q
current =
I=
time
t
•
•
The SI unit of current is the ampere (A or amp),
which equals 1 coulomb per second.
Current is measured with a device called an
ammeter.
7.3 Electric Currents—Superconductivity
7.3 Electric Currents—Superconductivity
• Either positive charges or negative charges can
comprise a current.
•
The effect of a positive charge moving in one
direction is the same as that of an equal negative
charge moving in the opposite direction.
• Formally, an electric current is represented as a
flow of positive charge.
•
This is because it was originally believed that
positive charges moved through metals.
• Even after it was discovered that it is negatively
charged electrons that flow in a wire to comprise
the current, the convention of defining the direction
of current flow as that which would be associated
with positive charges was retained.
7.3 Electric Currents—Superconductivity
• If positive ions are flowing to the right in a liquid,
• then the current is to the right.
• If negative charges (like electrons) are flowing to
the right, then the direction of the current is to the
left.
7.3 Electric Currents—Superconductivity
• The ease with which charges move through
different substances varies greatly.
• Any material that does not readily allow the flow of
charges through it is called an electrical insulator.
•
Substances such as plastic, wood, rubber, air, and
pure water are insulators because the electrons are
tightly bound in the atoms, and electric fields are
usually not strong enough to rip them free so they
can move.
• Our lives depend on insulators:
• the electricity powering the devices in our homes
could kill us if insulators, like the covering on power
cords, didn’t keep it from entering our bodies.
7.3 Electric Currents—Superconductivity
• An electrical conductor is any substance that
readily allows charges to flow through it.
•
Metals are very good conductors because some of
the electrons are only loosely bound to atoms and
so are free to “skip along” from one atom to the
next when an electric field is present.
• In general, solids that are good conductors of heat
are also good conductors of electricity.
7.3 Electric Currents—Superconductivity
• Liquids such as water are conductors when they
contain dissolved ions.
•
Most drinking water has some natural minerals and
salts dissolved in it and so conducts electricity.
• Solid insulators can become conductors when wet
because of ions in the moisture.
•
The danger of being electrocuted by electrical
devices increases dramatically when they are wet.
7.3 Electric Currents—Superconductivity
• Semiconductors are substances that fall in
between the two extremes.
• The elements silicon and germanium, both
semiconductors, are poor conductors of electricity
in their pure states, but they can be modified
chemically (“doped”) to have very useful electrical
properties.
•
Transistors, solar cells, and numerous other
electronic components are made out of such
semiconductors.
7.3 Electric Currents—Superconductivity
• The electronic revolution in the second half of the
20th century, including the development of
inexpensive calculators, computers, soundreproduction systems, and other devices, came
about because of semiconductor technology.
7.3 Electric Currents—Superconductivity
• What makes a 100-watt light bulb brighter than a
60-watt bulb?
•
•
The size of the current flowing through the filament
determines the brightness.
That, in turn, depends on the filament’s resistance.
Resistance A measure of the opposition to
current flow.
•
Resistance is represented by R, and the SI unit of
measure is the ohm (W).
7.3 Electric Currents—Superconductivity
• In general, a conductor will have low resistance
and an insulator will have high resistance.
• The actual resistance of a particular piece of
conducting material—a metal wire, for example—
depends on four factors:
Composition. The particular metal making up the
wire affects the resistance.
•
For example, an iron wire will have a higher
resistance than an identical copper wire.
7.3 Electric Currents—Superconductivity
Length. The longer the wire is, the higher its
resistance.
Diameter. The thinner the wire is, the higher its
resistance.
Temperature. The higher the temperature of the
wire, the higher its resistance.
•
•
The filament of a 100-watt bulb is thicker than that
of a 60-watt bulb, so its resistance is lower.
This means a larger current normally flows through
the 100-watt bulb, so, it is brighter.
7.3 Electric Currents—Superconductivity
• Resistance can be compared to friction.
Resistance inhibits the flow of electric charge, and
friction inhibits relative motion between two
substances.
•
•
In metals, electrons in a current move among the
atoms and in the process collide with them and give
them energy.
This impedes the movement of the electrons and
causes the metal to gain internal energy.
• The consequence of resistance is the same as
that of kinetic friction—heating.
•
The larger the current through a particular device,
the greater the heating.
7.3 Electric Currents—Superconductivity
• In 1911, Dutch physicist Heike Kamerlingh Onnes
made an important discovery while measuring the
resistance of mercury at extremely low temperatures.
•
He found that the resistance decreased steadily as the
temperature was lowered, until at 4.2 K (–452.1F) it
suddenly dropped to zero.
7.3 Electric Currents—Superconductivity
• Electric current flowed through the mercury with
no resistance.
• Onnes named this phenomenon superconductivity
for good reason:
•
mercury is a perfect conductor of electric current
below what is called its critical temperature
(referred to as Tc) of 4.2 K.
• Subsequent research showed that hundreds of
elements, compounds, and metal alloys become
superconductors, but only at very low
temperatures.
•
Until 1985, the highest known Tc was 23 K for a
mixture of the elements niobium and germanium.
7.3 Electric Currents—Superconductivity
• Superconductivity seems too good to be true:
electricity flowing through wires with no loss of
energy to heating.
•
Once a current is made to flow in a loop of
superconducting wire, it can flow for years with no
battery or other source of energy because there is
no energy loss from resistance.
7.3 Electric Currents—Superconductivity
• A great deal of the electrical energy that is wasted
as heat in wires could be saved if conventional
conductors could be replaced with
superconductors.
• But the superconducting state for a given material
has limitations.
•
Resistance returns if the temperature is raised
above the superconductor’s Tc, if the current
through the substance becomes too large, or if it is
placed in a magnetic field that is too strong.
7.3 Electric Currents—Superconductivity
• Practical superconductors were developed in the
1960s and are now widely used in science and
medicine.
•
Most of them are copper oxide compounds that
contain calcium, barium, yttrium, and other rareearth elements.
• Superconducting electromagnets, the strongest
magnets known, are used to study the effects of
magnetic fields on matter and to direct high-speed
charged particles.
7.3 Electric Currents—Superconductivity
• The Large Hadron Collider (LHC), an enormous particle
accelerator located near Geneva, Switzerland, uses
superconducting electromagnets to guide and focus
protons as they are accelerated to nearly the speed of
light.
• An entire experimental passenger train was built that
levitated by superconducting electromagnets.
• Magnetic resonance imaging (MRI) uses superconducting
electromagnets to form incredibly detailed images of the
body’s interior.
7.3 Electric Currents—Superconductivity
• Widespread practical use of these
superconductors is severely limited because they
must be kept cold using liquefied helium.
•
•
Helium is very expensive and requires sophisticated
refrigeration equipment to cool and to liquefy.
Once a superconducting device is cooled to the
temperature of liquid helium, bulky insulation
equipment is needed to limit the flow of heat into
the helium and the superconductor.
• These factors combine to make the so-called low-
Tc superconductors unwieldy or uneconomical
except in certain special applications when there
are no alternatives.
7.3 Electric Currents—Superconductivity
• But hope for wider use of superconductivity
blossomed beginning in 1987 when a new family of
“high-Tc” superconductors was developed with
critical temperatures that now reach as high as
about 140 K.
•
This was an astounding breakthrough because these
materials can be made superconducting through the
use of liquid nitrogen (boiling point 77 K).
7.3 Electric Currents—Superconductivity
• Liquid nitrogen is widely available, is inexpensive
to produce compared to liquid helium, and can be
used with much less-sophisticated insulation.
• However, the new high-Tc superconductors are
handicapped by a couple of unfortunate
properties:
•
they are brittle and consequently are not easily
formed into wires, and they aren’t very tolerant of
strong magnetic fields or large electric currents.
• If these problems can be overcome, a new
revolution in superconducting technology will
occur.