Q: What is thermodynamics?

A: We might start here by just looking at the etymology of the word. “Thermo” comes from the Greek for “heat”; “dyn” comes from the Greek for “power” (and it has also come to have a meaning pertaining to change).

The word “thermodynamics” came about in English in the 1850’s to describe the emerging technologies (for example, boilers, steam engines, and internal combustion engines) associated with the interconversion of heat and mechanical energy.

We can also think of thermodynamics in terms of the so-called thermodynamic laws:

1. Energy is conserved in an isolated system; and

2. Entropy increases or remains constant with time in an isolated system.

The 0th and 3rd laws deal with temperature and equilibrium. They may be stated as: 

0. If A and B are in equilibrium with C, then A is in equilibrium with B.

3. The entropy of a pure crystalline system at absolute zero temperature is zero.

There are lots of other ways to phrase these laws, and the implications of the laws are vast.

Often, one thinks of thermodynamics as the study of changes of certain properties associated with temperature changes. Indeed, we often study well-defined states, or equilibrium states, defined by thermodynamic properties of materials. However, thermodynamics also has implications for the rates at which changes occur and propagate; this field is often called non-equilibrium thermodynamics, or kinetics, or transport. 

Tags: Entropy, Laws of Thermodynamics

Q: What is the real difference between U (internal energy) and H (enthalpy)?

A: I am going to approach this as an engineer, not a thermodynamicist. In other words, I will address the question “How do we use H and U?”
U is the so-called internal energy. It contains contributions from the thermal kinetic energy of the constituents, and the energy from the arrangement of the constituents (e.g., chemical bonds and intermolecular potentials).

H is the enthalpy. H = U + PV, where P is pressure and V is volume. H, U, and V are usually specific quantities, that is, amount per mass.

Although we can perform energy balances using H or U, the trick is accounting for the work done or used by material flowing into an open system or leaving a closed system.  For this reason, we typically use U for closed systems and H for open systems, like turbines, in which we have material continuously entering and leaving the system. It is a matter of convenience.

Tags: Energy Balance, Internal Energy

Q: We seem to study dimensionless groups in every class, yet I still do not understand the Buckingham Pi theorem. Will I ever understand it? Do I need to?

A: This kind of tool is important when trying to design experiments (either physical or computational) to investigate new or poorly understood phenomena. I recommend working through the examples in the Wikipedia article on the Pi theorem if you want to understand the theorem in action.

Edgar Buckingham was a physicist in the United States in the early 1900’s.  His work on dimensional analysis was published in the time around WWI. His so-called Pi theorem provides a way of combining the parameters that describe a physical problem into dimensionless groups without actually having to appreciate the form of the governing equations; one really only has to know the quantities involved. You are familiar with such dimensionless groups in your study of mechanical engineering: Re, Pr, Nu, Sh, Sc, and so on.  

It is interesting to me that after Buckingham completed his Ph.D., taught at Bryn Mawr for a few years, and wrote a text on thermodynamics, he took a job at a mining camp in Arizona. 

Tags: Buckingham, Dimensional Analysis, Dimensionless Groups

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