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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
    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).

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.
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