Laws of Thermodynamics

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First Law of Thermodynamics

first law of thermo 2The first law of thermodynamics, also called conservation of energy, expresses the principal of the conservation of energy.  This means that the total amount of energy in the universe is constant and that it can neither be created or destroyed.  This law states that for every gain in some type of energy will result in the loss in some other form.  

In general, the conservation of energy can be stated as follows:

Kinetic Energy + Potential Energy = Constant


First Law of Thermodynamics formula

\(\triangle U = Q \;-\; W \) 
Symbol English Metric
\(\triangle U\) = internal energy \(\large{lbf-ft}\) \(\large{J}\)
\(Q\) = heat added to the system \(\large{F}\) \(\large{C}\)
\(W\) = work done by the system \(\large{lbf-ft}\) \(\large{J}\)


Second Law of Thermodynamics

The second law of thermodynamics, also called entropy, abbreviated as S, states that heat cannot pass from a cold body to a hot one by a self-acting process without the help of an external influence.

Entropy is the measure of disorder within a system. The more disorder within the system, energy becomes less available to do work.


Second Law of Thermodynamics formula

\(\large{ \Delta S = \frac {\Delta Q} {T}   }\)
Symbol English Metric
\(\large{ \Delta S }\) = entropy differential \(\large{\frac{Btu}{lbm-R}}\) \(\large{\frac{kJ}{kg-K}}\)
\(\large{ \Delta Q }\) = heat differential (energy) \(\large{\frac{Btu}{lbm}}\) \(\large{\frac{J}{kg}}\)
\(\large{ T }\) = temperature \(\large{R}\) \(\large{K}\)


Third Law of Thermodynamics

The third law is about the ability to create an absolute zero temperature, where the entropy approaches a constant minimum value.  It also states that it is not possible for any system to reach absolute zero.


Zeroth Law of Thermodynamics

When two thermal systems are in equilibrium and they with a third, then all are equal to each other, meaning  \(A=B\)  and  \(B=C\)  then  \(A=C\)

It a system does not transfer heat, it is in thermal equilibrium even though it can transfer heat.


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Tags: Temperature Equations Energy Equations Laws of Physics