Thermodynamics is the branch of physics that focuses on the relationship between heat and other forms of energy. Specifically, it describes how thermal energy is converted to and from other forms of energy and how it affects matter.
Force, Work, and Energy
A force is a push or pull acting upon an object that results from its interaction with another object. There are a variety of force types categorized by whether or not they result from the contact the interacting objects or happen over a distance. Frictional force, tension force, normal force, air resistance force, applied force, and spring force are all contact forces, as they only occur between objects when the objects touch each other. Gravitational force, electrical force, and magnetic force are forces that occur over a distance when the interacting objects are not touching. Contact forces cease if the interacting objects stop touching, but distance forces remain even when the objects touch. For example, when a skydiver jumps out of a plane, gravitational force pulls him toward the ground. There are many other forces at play here, but for the sake of this example, let's ignore them. Earth exerts gravitational force on the skydiver over a distance, which pulls him to the ground. Once the skydiver lands, gravitational force continues to hold him in contact with the ground, and will do so until a stronger force is exerted in the opposite direction.
When a force causes an object to move, such as the gravitational force pulling the skydiver to the ground, work has been done. For work to occur, some force must cause a displacement, or movement, of the object it is working upon. Work can be expressed mathematically as the product of a force and the displacement of the object under force in the direction in which the force is applied. The equation is W=F*d*cosΘ, where W is the work, F is the force, d is the distance displaced, and the cosine of angle theta (Θ) is the direction in which the force displaced the object.
Energy is a property of matter, as it is an object's ability to and capacity for doing work by applying force to another object. Energy is the “currency”, if you will, for doing work. For any given amount of work to be done, an equal amount of energy must be expended. Some common forms of energy are:
- Kinetic Energy: This is the energy an object in motion possesses due to its motion.
- Potential Energy: This is the energy of a resting object due to its relative position to other objects, stresses within itself, electric charge, or any other factor that might result in a force acting upon it. It is the resting object's potential to move.
- Elastic Energy: Elastic energy is the potential mechanical energy of an object that has been either compressed or stretched, as with a spring. It is equal to the work done to stretch or compress the object, which depends on the spring constant (k), as well as the distance of the deformation.
- Chemical Energy: This is the energy stored in the chemical bonds between atoms and molecules. It is released in chemical reactions, and often produces heat as a by-product of the reaction. Batteries, fossil fuels, and food are all examples of stored chemical energy.
- Radiant Energy: Radiant energy travels by wave. Examples include light, both natural and artificial, sound, x-rays, and microwaves.
- Electrical Energy: Electrical energy is the energy generated by electrons moving through an electrical conductor.
- Thermal Energy: Thermal energy is heat, which is generated by the movement of tiny particles within an object. The faster these particles move, the more heat, or thermal energy, is generated.
Law of Conservation of Energy and the First Law of Thermodynamics
In 1840, James Prescott Joule discovered during an experiment that the heat generated in a coil of wire is proportional to the square of the current running through it. Over the following decade, Joule sought to unify the study of electrical, chemical, and thermal energy by conducting dozens of experiments he hoped would demonstrate their inter-convertibility and quantitative equivalence.
Perhaps the most famous and successful experiment was his gravity driven water wheel. He used a falling weight to drive a large paddle wheel that was sealed inside a container of water. He calculated the potential energy of the weight, which is the energy it had due to the gravitational force the Earth was exerting on it,and its potential to fall the distance it was suspended above the Earth. He reasoned that when the weight fell, it would transfer nearly all of its potential energy into kinetic energy for the paddle wheel. As the weight fell, the wheel turned and the water churned. This warmed the water by a small, but noticeable degree. Joule measured the amount of energy the water gained and found that it matched exactly the amount of energy that the weight had lost.
Joule's work resulted in the law of conservation of energy and the international scientific unit of measuring energy (the joule) being named for him. The law of conservation of energy states that energy can neither be created nor destroyed. It can, however, change forms and flow from one place to another.
In thermodynamics, a system is the part of the universe being studied, and its surroundings are the rest of the universe that surrounds and interacts with it. An open system exchanges both energy and matter freely with its surroundings. A closed system exchanges energy, but not matter. An isolated system exchanges neither energy nor matter with its surroundings.
The first law of thermodynamics is an adaptation of the law of conservation. It states that the total increase in the energy of a system is equal to the increase in thermal energy plus the work done on the system. In other words, heat is a form of energy, and therefore subject to the principle of conservation. Since energy can neither be created nor destroyed, the net heat supplied in any system's thermodynamic cycle must equal the net work done on the system.
Entropy and the Second Law of Thermodynamics
While energy can not be destroyed, it can move from one place to another. The second Law of Thermodynamics states that thermal energy, or heat, cannot be transferred from a body at a lower temperature to a body at a higher temperature without the addition of energy. In other words, heat will always move from hotter areas to colder areas. Since this movement always occurs, energy can dissipate, or be “lost” as wasted heat energy that is unavailable to do work.
Entropy is this loss of heat energy, and is used as a measure of the degree of disorder in a system. As energy is transferred from one form to another, some is lost as heat. As the energy in a system decreases, the disorder in the system, and thus the entropy in a system, increases.
During any naturally occurring process, molecules generate heat through friction, collisions, and possibly chemical reactions. While this heat energy is conserved rather than destroyed, as mandated by the first law, it is made unavailable through dissipation. When this happens, the second law states that it cannot be reversed. Therefore, all naturally occurring processes are irreversible.
Even in the case of a theoretically reversible process, the net change in entropy would be zero. Since entropy can only stay the same or increase, the second law of thermodynamics tells us that the entropy of any given system will increase over time, and that the entropy of the universe will increase over time. Entropy can not decrease.
The Third Law of Thermodynamics
As previously explained, entropy is sometimes referred to as waste energy, or energy that is no longer available to do work because it has left in the form of dissipated heat energy. Since there is no heat whatsoever at absolute zero (0 degrees Kelvin), there can be no waste energy given off as heat at this temperature.
Entropy is also a measure of the disorder of a system. A perfect crystal is by definition perfectly ordered, but any positive value of temperature means that there is molecular motion within the crystal, which causes disorder. The third law of thermodynamics states that the entropy of a pure crystal at absolute zero is zero.
No movement means no disorder. Since absolute zero is the absolute absence of heat, which means there is no molecular movement, there can be no negative value for temperature in degrees Kelvin. Since entropy is zero at absolute zero, and there is no possible negative value for temperature below absolute zero, there can be no possible negative value for entropy. For these reasons, there can be no physical system that ends with lower entropy than it began, so entropy always has a positive value and is always increasing, even if slowly.
The Zeroth Law of Thermodynamics
The zeroth law of thermodynamics states that if two systems are both in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other. This establishes temperature as a fundamental and measurable property of matter.
The body of knowledge that makes up the science of thermodynamics was developed over hundreds of years and has many contributors. The ideas of conservation and entropy and the first three laws have been around for a long time. Even where not fully understood, the names are almost universally recognized.
When a fourth law of thermodynamics was proposed, this became an issue. It makes logical sense to be placed before laws 1-3 because it is the supposition upon which all three are based. However, the three laws were too well known to be restructured as four with the new law being #1. The solution was to name it the zeroth law and list it before the first law. While it appears last in this article for the sake of explaining its existence more easily, the logical order of the laws of thermodynamics is actually zeroth law → first law → second law → third law.
Thermodynamics is the science of heat energy and its interaction with other forms of energy and matter. Energy is the ability to do work, and is a property of matter. Work occurs when energy is expended by one object as it exerts a force on another object that results in a displacement, or movement. Force is a push or pull exerted on one object by another, such as the gravitational pull between Earth and a skydiver who has just jumped out of a plane.
The zeroth law of thermodynamics states that if two bodies are in thermal equilibrium with a third body, then they are in thermal equilibrium with each other. This establishes temperature as a fundamental and measurable property of matter. It has to be true for the other three laws to follow, so it belongs at the top of the list rather than being the fourth law. However, because it was formalized long after the first three were accepted and commonly taught, renaming the other three laws would be very confusing. Calling it the zeroth law is the perfect solution. It's the scientific equivalent of following up a fiction series with a book that takes place before the original series and calling it a prequel!
The law of conservation of energy states that energy can not be created or destroyed, but it can chance form and move from one place to another. As heat is a form of energy, it is subject to conservation. Therefore, by the first law of thermodynamics, the total increase in the energy of any isolated system has to be equal to the increase in thermal energy, or heat, plus the work done on the system.
The second law of thermodynamics states that heat energy can not be transferred from a body at a lower temperature to a body of a higher temperature. Therefore any heat energy that leaves one area is irreversibly lost to that area. Heat energy will always spontaneously flow from hotter areas to cooler ones, so the loss of heat energy will always occur in natural processes. Since entropy, or the dissipation of heat energy as a form of waste, will almost always occur and cannot be recovered, the entropy of all systems and the universe itself tends to increase over time to approach equilibrium.
The third law of thermodynamics states that the entropy of a pure crystal of a pure substance at absolute zero is zero. Since there is no molecular movement at absolute zero, and no heat, no heat can be lost. This law further explains why entropy will always have a positive value. If there is no possible negative temperature value below 0 degrees Kelvin, then there can no possible entropy value below 0. This reinforces the idea that all thermodynamic systems will move toward equilibrium and then stop. Since the only possible values for entropy are zero and above, the entire universe will, eventually, reach thermal equilibrium and stay there.