Thermodynamics
And Relationships between
heat and work
What is Internal Energy?
• Internal energy is defined as the energy associated
with the random, disordered motion of molecules.
• It is separated in scale from the macroscopic ordered
energy associated with moving objects; it refers to the
invisible microscopic energy on the atomic and
molecular scale. (Hyperphysics - University of Georgia)
• For example, a room temperature glass of water sitting
on a table has no apparent energy, either potential or
kinetic . But on the microscopic scale it is a seething
mass of high speed molecules traveling at hundreds of
meters per second.
• If the water were tossed across the room, this
microscopic energy would not necessarily be changed
when we superimpose an ordered large scale motion on
the water as a whole. (Hyperphysics -University of Georgia)
Heat, Work and Internal Energy
• Internal energy can be used for
work.
Example #1: Friction forces generated through
pulling a nail from wood, increase the nail’s
temperature. The energy can be transferred to
the surrounding air. (The work
is done by the friction forces.)
Serway/Faughn Physics – pg 332
Heat, Work and Internal Energy
Internal energy can be used for work.
Example #2: Consider a flask of water with a balloon
placed over the opening. Heating the water cause it to
boil. The water vapor expands the balloon. The balloon
expansion provides a force that does work on the
atmosphere. The steam does the work.
Heat & Work are Energy Transferred
to or from a System
• Objects contain internal energy, but are not
said to have heat or work. The heat or work is
transferred to or from a substance.
Serway/Faughn Physics – pg 332
• The coffee cup feels hot as it is transferring
heat energy to your hand.
Energy transfer to or from a system
• A balloon, flask, water, steam can be thought of as a
system. A burner transfers energy to this system. The
system internal energy is increased.
• When the expanding balloon
does work on the surroundings,
the system’s internal energy is
decreased.
• Some of the energy
transferred into the system as
heat is transferred to the
surroundings.
Serway/Faughn Physics – pg 332
is
For Thermodynamic systems work is
defined in terms of pressure and volume
change.
• Thermo – thermal energy – heat
• Dynamic – changing
• Therefore thermodynamic involves
changes in heat/energy.
Example – Gas expanding and pushing a piston
within a cylinder does positive work the piston.
As the gas is compressed, the work done on the
piston is negative. Serway/Faughn Physics – pg 332
First Law of Thermodynamics
• Energy cannot be created or destroyed, but
transferred or converted from one form to
another.
Another way to look at the first law:
Sample 1st Law Calculation
• A total of 135 J of work is done on a gaseous
refrigerant as it undergoes compression.
If the internal energy of the gas increases by 114 J
during the process, what is the total amount of energy
transferred as heat?
W = -135J
U = 114J Q = ?
U = Q – W so Q = U + W
Q = 114J + (-135J) = -21J
A cyclical process.
• In a cyclical process, the system’s properties at
the end of the process are identical to the
system’s properties before the process took
place.
• (The final and initial values of internal energy are the
same, and the change in internal energy is zero.)
Refrigerators and Heat engines (Cyclical processes)
• A refrigerator performs mechanical work to
create temperature difference between its
closed interior and its environment (the air in
the room).
• This is accomplished
in a cyclical process
of compression and
expansion or
refrigerant, and
transferring thermal
energy.
Heat engines
• A heat engine is a device that uses heat to do
mechanical work.
• A heat engine does work by transferring energy
from a high-temperature substance to a lowertemperature substance.
A sterling engine
is driven by
thermal energy
transfer
Internal Combustion Engines
• Internal combustion engines are examples of heat
engines. Potential energy of chemical bonds in fuel is
converted to kinetic energy of particle products from
combustion.
• These gaseous products push against a piston to do
work. Only part of the internal energy leaves the
engine as work done on the environment (pistons).
Most of the energy is removed as heat.
Four-Cycle Gasoline Engine
Intake stroke – An air-fuel mixture is drawn into
the cylinder through the intake valve as the
piston moves downward. (The exhaust valve
stays closed.)
• Compression stroke – work is done by the
piston as the air-fuel mixture is compressed
in the cylinder. (Both valves are closed at this
time.)
• Power stroke – The compressed, hot gases are
ignited and combustion takes place. The combustion
of gases cause the piston to move downward in the
cylinder. A great deal of this energy, in the form of
heat, is transferred to the surrounding environment.
(Both valves
remain closed)
• Exhaust Stroke – The piston moves up through
the cylinder and pushes the combustion
products back out the cylinder through an
exhaust valve.
The Second Law of Thermodynamics
• With regards to a heat engine: “No cyclic process
that converts heat entirely into work is possible.
Some energy is always transferred as heat into the
surroundings.” Serway/Faugh Physics pg 348
• How much heat energy is converted to work, instead
of being lost to the surroundings refers to the
efficiency of that engine.
• Perpetual motion machines do not exist
because energy must be supplied to the
system continually.
• Since energy is always lost, or transferred out
of the system, energy must be continually
supplied to the system to keep it going.
Entropy – tendency toward disorder
• “In thermodynamics, a system left to itself
tends to go from a state with a very ordered
set of energies to one where there is less
order.” Serway/Faugh Physics pg 351
• All systems tend toward more
disorder and randomness.
• The measure of a system’s disorder is
called the entropy of the system. The
greater the entropy, the greater the
disorder. Greater disorder means there is
less energy to do work.
New Calculations Suggest Universe May be Closer to Heat Death
Universe has more entropy than once thought.
• An analysis by Chas Egan of the Australian National University in
Canberra and Charles Lineweaver of the University of New South
Wales in Sydney indicates that the collective entropy of all the
supermassive black holes at the centers of galaxies is about 100 times
higher than previously calculated.
• Because supermassive black holes are the largest contributor to
cosmic entropy, the finding suggests that the entropy of the universe
is also about 100 times
larger than previous estimates,
the researchers reported
online September 23 at
arXiv.org.