Chapter 7 Electricity (Section 5)

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Chapter 7Electricity(Section 5)
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7.5 Power and Energy in Electric Currents

• Because a battery or other electrical supply must
  continually put out energy to cause a current to
  flow, it is important to consider the power
  output
• the rate at which energy is delivered to the
  circuit.
• The power is determined by the voltage of the
  power supply and the current that is flowing.
• Think of it this way
• the power output is the amount of energy expended
  per unit amount of time.
• The power supply gives a certain amount of energy
  to each coulomb of charge that flows through the
  circuit.

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7.5 Power and Energy in Electric Currents

• Consequently, the energy output per unit time
  equals the energy given to each coulomb of charge
  multiplied by the number of coulombs that flow
  through the circuit per unit time
• energy per unit time energy per coulomb
• number of coulombs per unit time

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7.5 Power and Energy in Electric Currents

• These three quantities are just the power,
  voltage, and current, respectively.
• Consequently, the power output of an electrical
  power supply is
• The units work out correctly in this equation
  also joules per coulomb (volts) multiplied by
  coulombs per second (amperes) equals joules per
  second (watts).
• The power output of a battery is proportional to
  the current that it is supplying
• the larger the current, the higher the power
  output.

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7.5 Power and Energy in Electric CurrentsExample
7.4

• In Example 7.1, we computed the current that
  flows in a flashlight bulb.
• What is the power output of the batteries?
• Recall that the batteries produce 3 volts and
  that the current in the lightbulb is 0.5 amperes.
  
• The power output is
• The batteries supply 1.5 joules of energy each
  second.

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7.5 Power and Energy in Electric Currents

• What happens to the energy delivered by an
  electrical power supply?
• In a lightbulb, less than 5 percent is converted
  into visible light, and the rest becomes internal
  energy.
• Even the visible light emitted by a lightbulb is
  absorbed eventually by the surrounding matter and
  transformed into internal energy. (Interior
  lighting is actually used to heat some
  buildings.)

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7.5 Power and Energy in Electric Currents

• Electric motors in hair dryers, vacuum cleaners,
  and the like convert about 60 percent of their
  energy input into mechanical work or energy while
  the remainder goes to internal energy.
• The mechanical energy is generally dissipated as
  internal energy through friction.
• In a similar way, we can trace the energy
  conversions in other electrical devices and the
  outcome is the same most electrical energy
  eventually becomes internal energy.

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7.5 Power and Energy in Electric Currents

• Ordinary metal wire converts electrical energy
  into internal energy whenever there is a current
  flowing.
• You may have noticed when using a hair dryer that
  its cord becomes warm.
• This heating, called ohmic heating, occurs in any
  conductor that has resistance, even when the
  resistance is quite small.
• The huge cables used to conduct electricity from
  power plants to cities are heated by this effect.
  
• This heating represents a loss of usable energy.

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7.5 Power and Energy in Electric Currents

• The temperature that a current-carrying wire
  reaches from ohmic heating depends on the size of
  the current and on the wires resistance.
  Increasing the current in a given wire will raise
  its temperature.
• Many devices utilize this effect.
• The resistances of heating elements in toasters
  and electric heaters are chosen so that the
  normal operating current is large enough to heat
  them until they glow red hot and can toast bread
  or heat a room.

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7.5 Power and Energy in Electric Currents

• The filament in an incandescent lightbulb is made
  so thin that ohmic heating causes it to glow
  white hot and emit enough light to illuminate a
  room.

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7.5 Power and Energy in Electric Currents

• Ohmic heating is a major consideration in the
  design of sophisticated integrated circuit chips.
  
• Even though the currents flowing through the tiny
  transistors are extremely small, there are so
  many circuits in such a small space that special
  steps must be taken to make sure the heat
  produced is conducted away.

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7.5 Power and Energy in Electric Currents

• Because a superconductor has zero resistance,
  there is no ohmic heating.
• The overall efficiencies of most electrical
  devices could be improved if regular wires could
  be replaced by superconductors.
• Superconducting transmission lines would allow
  electricity to be carried from a power plant to a
  city with no loss of energy.
• The limitations of currently known
  superconductors, however, make such uses
  impracticable.

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7.5 Power and Energy in Electric Currents

• A sufficiently large current in any wire can
  cause it to become very hothot enough to melt
  any insulation around it or to ignite combustible
  materials nearby.
• Fuses and circuit breakers are put into electric
  circuits as safety devices to prevent dangerous
  overheating of wires.
• If something goes wrong or if too many devices
  are plugged into the circuit and the current
  exceeds the recommended safe limit for the size
  of wire used, the fuse or circuit breaker will
  automatically break the circuit and the current
  will stop.

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7.5 Power and Energy in Electric Currents

• A fuse is a fine wire or piece of metal inside a
  glass or plastic case.
• When the current exceeds the fuses design limit,
  the metal melts away, and the circuit is broken.

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7.5 Power and Energy in Electric Currents

• Designers of electric circuits in cars, houses,
  and other buildings must choose wiring that is
  large enough to carry the currents needed without
  overheating.
• They must also include fuses or circuit breakers
  that will disconnect a circuit if it is
  overloaded.
• Most electrical devices are rated by the power
  that they consume in watts.
• The equation P VI can be used to determine how
  much current flows through the device when it is
  operating.

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7.5 Power and Energy in Electric CurrentsExample
7.5

• An electric hair dryer is rated at 1,875 watts
  when operating on 120 volts.
• What is the current flowing through it?
• The wires in the electric cord must be large
  enough to allow 15.6 amperes to flow through them
  without becoming dangerously hot.

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7.5 Power and Energy in Electric Currents

• The highest current that can flow in a particular
  wire without causing excessive heating depends on
  the size of the wire.
• This is one reason why electric utilities use
  high voltages in their electrical power supply
  systems. The electricity delivered to a city,
  subdivision, or individual house must be
  transmitted with wires.
• Because P VI, using a large voltage makes it
  possible to transmit the same power with a
  smaller current.
• If low voltages were used, say, 100 volts instead
  of the more typical 345,000 volts, much larger
  cables would have to be used to handle the larger
  currents.

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7.5 Power and Energy in Electric Currents

• Customers pay for the electricity supplied to
  them by electric companies based on the amount of
  energy they use.
• An electric meter keeps track of the total energy
  used by monitoring the power (rate of energy use)
  and the amount of time each power level is
  maintained.

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7.5 Power and Energy in Electric Currents

• Recall the equation used to define power
• Therefore,
• The amount of energy used is equal to the power
  times the time elapsed.
• If P is in watts and t is in seconds, then E will
  be in joules.