Chapter 7 Electricity (Section 3)

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Chapter 7Electricity(Section 3)
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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.

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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.

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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.

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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.

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7.3 Electric CurrentsSuperconductivity
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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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.