# Chapter 7 Electricity (Section 3) - PowerPoint PPT Presentation

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## Chapter 7 Electricity (Section 3)

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Title: Chapter 7 Electricity (Section 3)

1
Chapter 7 Electricity (Section 3)
2
7.3 Electric CurrentsSuperconductivity
• 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.

3
7.3 Electric CurrentsSuperconductivity
• 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.

4
7.3 Electric CurrentsSuperconductivity
• 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.

5
7.3 Electric CurrentsSuperconductivity
• 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.
• 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.

6
7.3 Electric CurrentsSuperconductivity
7
7.3 Electric CurrentsSuperconductivity
• 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.

8
7.3 Electric CurrentsSuperconductivity
• 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.

9
7.3 Electric CurrentsSuperconductivity
• 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, didnt keep it from entering our
bodies.

10
7.3 Electric CurrentsSuperconductivity
• 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.

11
7.3 Electric CurrentsSuperconductivity
• 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.

12
7.3 Electric CurrentsSuperconductivity
• 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.

13
7.3 Electric CurrentsSuperconductivity
• The electronic revolution in the second half of
the 20th century, including the development of
inexpensive calculators, computers,
sound-reproduction systems, and other devices,
came about because of semiconductor technology.

14
7.3 Electric CurrentsSuperconductivity
• 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 filaments
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).

15
7.3 Electric CurrentsSuperconductivity
• In general, a conductor will have low resistance
and an insulator will have high resistance.
• The actual resistance of a particular piece of
conducting materiala metal wire, for
exampledepends 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.

16
7.3 Electric CurrentsSuperconductivity
• 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.

17
7.3 Electric CurrentsSuperconductivity
• 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 frictionheating.
• The larger the current through a particular
device, the greater the heating.

18
7.3 Electric CurrentsSuperconductivity
• 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.

19
7.3 Electric CurrentsSuperconductivity
• 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.

20
7.3 Electric CurrentsSuperconductivity
• 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.

21
7.3 Electric CurrentsSuperconductivity
• 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 superconductors Tc, if the current
through the substance becomes too large, or if it
is placed in a magnetic field that is too strong.

22
7.3 Electric CurrentsSuperconductivity
• 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
rare-earth 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.

23
7.3 Electric CurrentsSuperconductivity
• 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 bodys interior.

24
7.3 Electric CurrentsSuperconductivity
• 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.

25
7.3 Electric CurrentsSuperconductivity
• 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
• This was an astounding breakthrough because these
materials can be made superconducting through the
use of liquid nitrogen (boiling point 77 K).

26
7.3 Electric CurrentsSuperconductivity
• 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 arent very tolerant
of strong magnetic fields or large electric
currents.
• If these problems can be overcome, a new
revolution in superconducting technology will
occur.