Energy in Thermal Systems

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Energy in Thermal Systems

• 5.4

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• Internal Energy
• A. The total energy of the sum of kinetic and
  potential energies of all the particles in an
  object.
• B. Depends on
• 1. material composition
• 2. mass
• 3. temperature
• 4. physical state

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C. Internal energy changes because of heat
transfers due to a temperature difference D.
Internal energy changes because of friction
(work done on a system) E. Thermodynamicsthe
science dealing with the relationships between
internal energy, heat, and work
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II. The First Law of Thermodynamics A. The law
of conservation of energy B. ?U Q W C.
Change in the internal energy of a system net
heat input to the system work done by the
system 1. Heat is positive if it enters the
system and negative when it leaves. 2.
Work is positive when the system does the work
and negative if work is done on the system.
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D. Adiabatic processthere is no heat transfer
to or from a system 1. isolate the system
from its surroundings (insulation) 2.
work quickly enough that there is no time for
heat transfer to take place.
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III. Heat Engines A. Device that converts
thermal energy into mechanical energy. B.
Automobiles, nuclear reactions, human body C.
Every heat engine 1. Absorbs thermal energy
from a high- temperature source 2.
Converts some of the thermal energy into
work. 3. Discards the remaining thermal
energy into a low-temperature sink.
(reservoir)
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D. Four-stroke gasoline engine operates in a
cycle 1. intake stroke 2. compression
stroke 3. ignition 4. power stroke 5.
exhaust stroke E. Gasoline engine must 1.
overcome friction 2. keep the engine
operating 3. operate equipment 4. move the
vehicle
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IV. Refrigerators and Heat Pumps A. Cycle is
reverse of a heat engine (net heat input is
negative, so work is negative) B. Steps 1.
Refrigerant enters the compressor as a
low-pressure gas. A piston compresses the gas, so
temperature and pressure is increased. 2. The
compressed gas flows into a condenser, where it
is cooled and changed from a gas to a liquid.
(releases thermal energy) Heat is transferred
away from the condenser to the high-temp.
reservoir using air- or water-cooling of the
condenser.
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3. The refrigerant leaves the condenser as a
high-pressure liquid. The liquid flows through
an expansion valve, decreasing the pressure.
Some liquid becomes gas. 4. The remaining
liquid is vaporized in an evaporator. The liquid
absorbs thermal energy, and is transferred back
to the compressor. C. Heat Pump acts as an air
conditioner in the summer by extracting heat from
the interior of a house and exhausting it
outside. In winter, the system reverses.
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• The Second Law of Thermodynamics
• A. Entropy of an isolated system which is not in
  equilibrium will tend to increase over time,
  approaching a maximum value at equilibrium.
• B. Entropya measure of the disorder of a
  systemengines cannot be 100 efficient because
  some energy is lost (unusable)
• C. Carnot efficiencythe maximum efficiency of
  a heat enginedepends on the absolute
  temperatures (Kelvin scale) of the hot and cold
  reservoirs
• D. Absolute zero 0 K or -273C

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VI. Energy Dissipation A. Energy no longer
available to do work B. Occurs in all
processes C. Every time energy is used, some is
transformed into unusable energy.