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Transcript
Energy & Thermodynamics
Internal Energy:
All objects are made of particles in constant random motion. As these particles interact,
their kinetic and potential energies may change, but the sum of kinetic energy and potential
energy does not change. This total energy is called the internal energy of the object.
An object’s Internal Energy depends on:
• its material composition
• its mass
• its temperature
• and its physical state (solid, gas, etc.)
Increasing Internal
Energy:
Internal energy can be transferred from one object to another if they have different
temperatures. This energy transfer is called HEAT.
Another way to change an object’s internal energy is by doing some work on it.
Example: Rubbing your hands together uses friction to raise their temperature.
So an object’s internal energy can be increased by adding heat, or by doing work on the
object.
Decreasing Internal
Energy:
The reverse is also true – an object’s internal energy can be decreased by removing heat or
by the object doing work.
Thermodynamics:
The science of dealing with the relationships between internal energy, heat, and work is
called thermodynamics.
First Law of
Thermodynamics:
The First Law of Thermodynamics states that when work is done on a system, some of the
work is changed to heat (thermal) energy.
Examples of First
Law:
Example: A car engine heats up as it runs due to friction between the moving parts
Another Example: As a roller coaster moves down from the highest point on the track,
some of its potential energy is changed into kinetic energy, sound energy, and
thermal energy (due to friction). The original energy (potential energy) is equal
to the sum of the kinetic energy, sound energy and thermal energy.
Heat Engines:
Heat Engines convert thermal energy to mechanical energy. All heat engines:
• Absorb thermal energy from a high temperature source.
• Convert some of the thermal energy to work.
• Discard the remaining thermal energy into a low temperature “sink.”
Examples of Heat
Engines:
Examples of Heat Engines:
• Car engines which use the energy from hot gases to turn the wheels and work
other equipment. Excess heat and exhaust are released to the atmosphere.
• The Space Shuttle uses a chemical reaction between hydrogen and oxygen for
energy.
• Your body uses the food you eat for energy.
-2Refrigerators and
Heat Pumps:
Refrigerators and Heat Pumps operate in a cycle that is the reverse of the heat engine.
Refrigerators and Heat Pumps convert work to thermal energy and move heat away from a
cold reservoir to a hot reservoir.
• Refrigerators use a refrigerant (working fluid) to absorb heat from the cold
inside and release that heat to the warmer outside.
• Heat pumps work much the same way to cool in the summer, but can be
reversed to heat in the winter.
Second Law of
Thermodynamics:
The Second Law of Thermodynamics states that when work is done, some energy must be
transferred to the surrounding system, and becomes unavailable for further use. i.e. No
system can convert 100% of the available to energy to usable forms.
Example of Second
Law:
Energy Dissipation:
Examples of Energy
Dissipation:
Example:
As a roller coaster moves down the hill, the gain in kinetic energy will always be less
than the loss in potential energy since some of the energy is converted to sound and
to thermal energy (friction that heats up the track).
Energy is not “used up” in mechanical, fluid, electrical or thermal systems. But some
energy is converted from usable forms to unusable forms.
Example:
When electrical energy flows through a light bulb, some of the energy produces
visible light and some is dissipated as thermal energy that heats the bulb.
Another Example:
When gasoline is burned in a car engine, some of the energy produces motion and
operates equipment, and some of the energy is dissipated as thermal energy that
heats the earth’s atmosphere.
Energy dissipation means there may never be a shortage of energy on the earth, but
someday there may be a shortage of energy in usable forms.