Introduction to Thermodynamics

Many people start to feel a bit of apprehension at the thought of delving into physics. Don’t worry! This introduction will just scratch the surface in a few easy steps.

First Law of Thermodynamics

Energy can neither be created nor destroyed.

In physics, there are two major uses of the term energy. First is in the pure physics sense: anything that is moving or has stored potential that can be converted into movement has energy. Second is the common usage: any system that has the ability to do useful work contains energy (sometimes called emergy or exergy in this sense).

The First Law of Thermodynamics is only concerned with energy in the first sense. A moving object has kinetic energy. An object with stored ability to move has potential energy.

If you throw an object, then when that object is moving, it has kinetic energy. However, if you lift an object from the floor and rest it upon a table, it gains potential energy. If you nudge the object off the edge of the table, it will accelerate towards the ground (due to the force of gravity), thus converting potential energy into kinetic energy. Another example of potential energy is a stretched rubber band or spring.

Rules of Thumb

Although there are precise mathematical definitions of energy in its various forms, a person can understand and apply much using just a few “rules of thumb”:

The faster an object moves, the more kinetic energy it has.

For a particular velocity, the more massive an object is, the more kinetic energy it has.

The further an object is lifted or stretched, the more potential energy it has.

Understanding and Applying the First Law

The First Law states that energy can neither be created nor destroyed. Even when one form of energy is converted into another, the total amount of energy remains the same. It can never increase nor decrease for an isolated system. When you lift an object to place it on a table, your body converts chemical energy (food calories) into kinetic energy (the movement of the rising book) that in turn gets converted into potential energy (the height of the book above the floor when can be restored to kinetic energy if the book falls).

Second Law of Thermodynamics

Entropy tends to increase.

Thermodynamics is an important topic in physics concerning the study of converting heat energy into useful work.

As mentioned under the First Law discussion, in physics, there are two major uses of the term energy. First is the purist sense: anything that moves or has a stored potential. Second is the common usage of the term: anything system has the ability to do useful work.

A famous principle of thermodynamics is that the disorder of the universe is always increasing. (Physicists use the more precise term “entropy” instead of disorder). This principle is called the Second Law of Thermodynamics.

 Local decreases of entropy are often at the expense of increasing overall entropy in the universe. In other words, local decreases of entropy result in a net increase in the total entropy of the universe.

The Second Law of Thermodynamics is only concerned with energy in the second sense. If a system is capable of doing work, then it can produce entropy. Another way of viewing this sort of energy is that it is consumed when a system towards equilibrium. While energy in the first sense is conserved, energy in the second sense (i.e. useful energy) is not conserved.

Just as potential energy can transform into kinetic energy, potential entropy can transform into kinetic entropy. However, although kinetic energy can be transformed back into potential energy, kinetic entropy cannot be transformed back into potential entropy. This is why the Second Law is sometimes called the arrow of time; entropy increases in the same direction as time progresses. When only the term “entropy” is used, kinetic entropy is meant. Likewise, common usage of the term “entropy” refers to kinetic entropy.

Kinetic entropy is a measure of how mixed together and averaged-out a system is. For example, a balloon has higher pressure than the surrounding room. If the balloon develops a leak, the pressure in the balloon is released until the balloon reaches room pressure. In the process, entropy is produced.

When the kinetic entropy of a system reaches its maximum possible value, that system is said to be at equilibrium. A system that contains potential entropy is out of equilibrium.

According to the Second Law of Thermodynamics, heat will not spontaneously flow from cooler to warmer objects. The Second Law of Thermodynamics does allow heat to flow from cooler objects to warmer objects, but only if work is applied to achieve this result. A household refrigerator is an example. Moving heat from cooler to warmer objects decreases the combined entropy of those two objects, but producing the work required for this entropy decrease actually produces an even greater entropy increase in the universe. In the case of a household refrigerator, an increased electric bill is evidence of the work required to operate the refrigerator. So the more you decrease entropy locally, the more you increase the total entropy in the universe.

Another consequence is that engines which convert heat energy into mechanical work cannot operate at 100% efficiency regardless of the technology used.

Rules of Thumb

Although there are precise mathematical definitions of energy in its various forms, a person can understand and apply much using just a few “rules of thumb”:

The faster an object moves, the more kinetic energy it has.

The more massive an object is, the more kinetic energy it has.

The higher an object is lifted or stretched, the more potential energy it has.

Heat engines

Engines invert potential entropy into work.

A heat engine operates across a thermodynamic potential, converting that thermodynamic potential into useful work. In doing so, part of the energy used is turned into work while the remainder turns into thermal energy. In fact, useful work can be expressed in the same units as energy.

Such engines are called heat engines, because the classic example of a thermodynamic potential is a temperature difference.

The strength of the potential limits the efficiency of a heat engine.

An important concept is efficiency which is what proportion of the inputted energy is converted to work. An ideal engine is known as a Carnot Engine, which is capable of operating without creating any net entropy. Thus, the highest efficiency at which work for a heat engine can be produced is known as the Carnot Efficiency. Yet, even an ideal engine such as a Carnot Engine cannot operate at 100% efficiency, unless a region of a temperature of absolute zero is available. Such a region is not know to exist.

The efficiency of Carnot Engines is proportional to the strength of the thermodynamic potential being consumed. In classic examples, such strength is represented by temperature differences those engine operate across. For example, a steam engine uses fire for its hot temperature and the much cooler outside air as the cold temperature. Such temperature represents a thermodynamic potential. The hotter the fire, the greater the efficiency of that engine.

Real-life engines are typically much less efficient than Carnot engines. When they operate upon a potential, they create entropy, often lots of it. However, Carnot Efficiency is still a major factor in their efficiency, and much more stright-forward conceptually, so it will be referred to again.

Rules of Thumb

Although there are precise mathematical definitions of energy in its various forms, a person can understand and apply much using just a few “rules of thumb”:

No engine in reality can be 100% efficient.

The greater the temperature difference, the greater the efficiency.

Potential Entropy + Entropy = Constant

Kinetic Energy + Potential Energy + Heat Energy = Constant

More information

For introductory thermodynamic simulations and software, see: Heatsuite.com