First Law of Thermodynamics

First Law of Thermodynamics

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First Law of Thermodynamics

The first law of thermodynamics is shared with most of science; it is
one of the fundamental principals that have shaped our understanding
of the working world.

IS CONSERVED, Brings back that long established idea that nothing can
be created or destroyed. How do we know this? This is an empirical
law, which means that we know that energy is conserved because of many
repeated experiments by scientists. It's been observed that you can't
get any more energy out of a system than you put into it.

Latent heat

Latent Heat is defined as the heat which flows to or from a material
without a change to temperature. The heat will only change the
structure or phase of the material. E.g. melting or boiling of pure

One very good illustration of latent heat in action is observed when
we reduce ice to water. If we imagine a bucket of ice on the floor in
an average temperature room (about 30 degrees Celsius) .The ice
doesn't instantly liquidize, nor does the room instantly freeze.
Instead the temperature of the ice rises until it reaches zero degrees
Celsius whereupon it begins to melt. During the entire melting process
the contents of the bucket remain at zero degrees, however the room
temperature would drop indicating that it was putting heat energy into
the melting process. This heat energy is described as latent heat.

Specific heat

The specific heat capacity of a solid or liquid is defined as the heat
required to raise unit mass of substance by one degree of temperature.

Some substances have more resistance to temperature change than others
so more energy is required to alter the temperature. This can be very
useful particularly in cooling systems.

Imagine trying to fry an egg in a frying pan with a steel handle, the
handle would conduct the heat very nicely to your hand. However the
high carbon plastic sheath on most modern pans is very resistant to

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temperature change and protects your hands.

The following equation describes SPECIFIC HEAT CAPACITY.

[IMAGE]Q= Heat supplied to substance,
m= Mass of the substance,
c= Specific heat capacity,
[IMAGE]T= Temperature rise.


Enthalpy is a measure of heat and energy in the system. Scientists
figure out the mass of a substance when it is under a constant
pressure. Once they figure out the mass, they measure the internal
energy of the system. All together, that energy is the enthalpy. They
use the formula "H = U + PV." H is the enthalpy value, U is the amount
of internal energy, and P and V are Pressure and Volume of the system.
This system works really well for gases.
There are things that affect the level of enthalpy in a system. The
enthalpy is directly proportional to the amount of substance you have.
Chances are if you have more of a substance, you have more energy.
More energy means higher enthalpy.
Another thing to remember is that the value for H (enthalpy) changes
sign when the reactions or values are reversed. When a reaction moves
in one direction, the sign is positive. When a reaction is moves in
the opposite direction, the value is negative.
Finally we have Hess's law. If a process happens in stages or steps,
then the enthalpic change for the overall system can be determined by
adding the changes in enthalpy for each step.


This term describes a system that changes and there is no transfer of
heat in or out. If a system expands ADIABATICALLY, then the internal
energy of the system usually decreases. It's as if you have a cup of
water just out of the tap. You let the cup sit and the water settles
down. Less energy and no transfer of heat.

You can probably see the word "VOLUM" in there. "ISO" usually stands
for constant. Put them together and you get a system that changes, but
the volume stays constant. These types of changes do not produce any
work on the environment. The amount of energy changes, but the heat
just stays inside the system.

You've seen the prefix "ISO," the suffix "BARIC" refers to pressure.
This is a system that changes, but keeps a constant pressure. All of
the change is in the volume of gas in the system. Like blowing up a
rubber balloon.

One last "ISO" the suffix now is "THERMAL." Systems that change in
every way but their temperature. You would say that these systems are
in THERMAL EQUILIBRIUM. You can see that the pressure and volume
change. The curve is like the adiabatic process, but there is no
increase in temperature.

The equation of continuity

When trying to solve a problem we are told to look for things that
stay the same. They then can be set equal to each other giving an
equation to solve. Rules like that are easy to state, but examples are
easier to understand.

If we have water flowing through a pipe filled with water, the water
will enter at a certain rate and leave at the same rate. The quantity
pushed in per unit time will push out an equal amount. That seems like
an obvious and reasonable observation. What are we taking for granted
in the argument? We are assuming that water is incompressible, and its
flow is steady it does not speed up or slow down

Volume is expressed as cubic units, cubic feet, cubic centimeters and
so forth. Time can easily be changed from hours to minutes or seconds.
So the rate of flow could be expressed as cubic centimeters per
second. Cubic cenimeters per second could also be expressed as square
centimeters times centimeters per second. Square centimeters and
centimeters per second are measures of Area and velocity,
respectively. So the rate of flow could be expressed as volume per
time equals area times velocity. The velocity would be an average
velocity since we are using the total volume per unit time. The water
in the center of the pipe goes faster than the water at the pipe wall.
However the pipes have cross sectional areas and the water has an
average velocity. So we could use the area of the entry or the exit
opening. If we multiply the cross sectional area of the pipe (cm2)
times the velocity of the water (cm / sec.) we get Cubic centimeters
per second, volume per unit time, a rate of flow. It makes sense both
ways. Does it matter what cross sections we use? If our principle that
the quantity of matter flowing in, is the same as the amount flowing
out then it must also be true everywhere in the pipe.

Therefore we can say that AV = K

Where A is cross sectional area at a point.

V is the average velocity for this point.

And K is a constant 'the rate of flow in the pipe'.

Since the equation is true for any two points in the pipe we can say
that :

A¹V¹ = A²V²

Where A¹V¹ is the area and velocity at one point in the pipe and A²V²
is the area and velocity at another point.

This is called the continuity equation and it shows us that the
velocity of water at a point in a full pipe is inversely proportional
to the cross sectional area of the pipe

at that point. So if a pipe gets bigger the velocity in the pipe will
decrease and vice versa.

Area at X is 10cm² and velocity is 1cm per second

Area at Y is 1cm² and velocity is 10 cm per second

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