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Thermodynamics is the study of the
connection between heat and work and the conversion of
one into the other. This study is important because many
machines and modern devices change heat into work (such
as an automobile engine) or turn work into heat (or
cooling, as in a refrigerator).
There are two laws of thermodynamics
that explain the connection between work and heat. But
first it must be shown how mechanical energy can be
equivalent to heat energy.
As a curious person, questions you may
have include:
-
What is the mechanical equivalent of
heat?
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What are the laws of thermodynamics?
-
What are some applications of the
subject?
Mechanical equivalent of heat
Experiments showed that the amount of
heat created is proportional to the work done. This
relationship is called the mechanical equivalent of heat
and can be expressed by the equation:
W = J* H,
where W equals the work done, H equals
the heat created from the work, and J equals the
constant 4.18 joules/calorie.
Using this equation you could calculate
the amount of heat generated from the work required to
stop a moving car. The way you do this is to calculate
the kinetic energy (KE) of a car from its mass and
velocity in Joules (KE = � mv2). Since the work required
to stop a moving car equals its kinetic energy, the
total amount of heat generated in the brakes and tires
to stop the car would be:
H = (KE) / 4.18.
You could also calculate how much work
could be done from eating food that was rated at some
given calories of heat.
Laws of thermodynamics
There are two major laws concerning
thermodynamics.
The First Law of Thermodynamics is the
law of conservation of energy. It states that energy
cannot be created or destroyed. Instead it is converted
from one form to another, such as from work to heat,
from heat to light, from chemical to heat, or such.
One example of that is how the kinetic
energy of a moving car is converted into heat energy at
the brakes and tire surfaces.
Another example is when chemical energy
is released in burning and is converted into light and
heat energy.
The Second Law of Thermodynamics has
several variations, which will be explained below.
One version of the Second Law of
Thermodynamics states that some heat is wasted when
converting heat into mechanical energy. In other words,
in a car engine, not all of the heat created from the
exploding gasoline is used in turning the engine or
moving the car. Some of the heat simply heats the
engine. The percentage of heat turned to work is called
the thermal efficiency of the engine.
The Second Law of Thermodynamics also
states that heat normally flows from high temperature to
low temperature. For example, when you heat the end of a
metal rod, the heat will gradually travel to the cool
end and heat it up.
Another example of this part of the
Second Law of Thermodynamics concerns waterbeds.
The reason waterbeds use heaters to warm
the water is because otherwise the heat from your body
(at 98.6 degrees F) will flow to the cooler water (at
room temperature of 72 degrees F). Since there is so
much water in a waterbed, it would take much energy from
your body to heat the water to body temperature. Thus,
you can feel chilled from the loss of body heat.
An object that absorbs heat like this a
is sometimes called a heat sink.
A third variation on the Second Law of
Thermodynamics states that the energy available for work
in the universe is continually decreasing.
This is also stated as: the entropy of
the universe is continually increasing. Entropy is the
measure of the disorder of a system. In other word, in
any closed system, objects are getting more and more
mixed. Mixtures do not "unmix" by themselves.
Applications of energy conversion
Following are two applications of
thermodynamics or the conversion of energy.
An application of the conversion of
energy is the type of engine used in a car, an internal
combustion engine. The way this engine works is that
gasoline and air is mixed and exploded in a cylinder.
That explosion is the internal combustion, changing
chemical energy to heat energy.
Since gases want to expand when they are
heated, they exert pressure on the piston in the
cylinder, causing it to move and turn a shaft. Thus, the
heat energy is converted into mechanical energy.
Another application of energy conversion
is the refrigerator. Electrical energy is converted into
mechanical energy in an electric motor. This motor
operates a pump.
This is what the pump does in a
refrigerator -- it expands the gas, causing it to become
cold. This is converting mechanical energy into heat
(cold) energy.As you recall, when a gas in a cylinder is
compressed it heats up. The pump in the refrigerator
compresses a special gas, condensing it into a liquid at
a higher temperature. The liquid is held in a tube
called a condenser. In most refrigerators, a fan forces
air across the condenser, transferring the heat to the
surrounding air.
If a gas in a cylinder expands, it cools
off. The liquid refrigerant is then expanded through a
restriction device into an evaporator inside the
refrigerator where it becomes a gas again. It is this
expansion that absorbs heat from inside the
refrigerator, thus cooling the contents of the
refrigerator. Another fan spreads the cold air through
the refrigerator by convection.
The First Law of Thermodynamics is the
law of conservation of energy. The Second Law of
Thermodynamics also states that heat normally flows from
high temperature to low temperature. The refrigerator is
an application of these heat laws. |
