Thermodynamics I: _{Caloric theory, latent heat, and the First Law of Thermodynamics}

What is thermodynamics?

The word “thermodynamics” was first used in 1849 to describe the study of heat flow. It comes from two different Greek words, “therme,” which means heat, and “dynamis,” which means power. Taken literally, thermodynamics is the study of heat used as a source of power or work. It uses mathematics to predict the way in which heat will move and how this can be made into useful processes, like driving an engine. Therefore, thermodynamics is integral to the study of engineering and physics, but thermodynamics is also prominent in almost every other branch of science, including astronomy, chemistry, biology, geology, and others.

Understanding heat

Understanding thermodynamics begins with a seemingly simple question: What is heat? Today, we know that heat is the form of energy that affects the temperature of objects by changing the movement of atoms or molecules within those objects. Early scientists did not have such an understanding of heat, though.

Caloric theory

Many scientists in the late 1700s subscribed to the “caloric” idea of heat. In this context, heat was thought to be a substance – it could not be touched, seen, or collected like other kinds of substances – but it flowed from object to object much like water flows from a pitcher. The amount of caloric “substance” in an object was believed to give rise to its temperature: The more it had, the hotter the object was. Therefore, in their view, changes in temperature were caused by the flow of caloric to or from an object.

This idea about heat made sense in its day because heat does, in fact, behave in some ways like a fluid. For example, we know that heat energy flows from areas of high heat to low heat. However, the caloric theory was ultimately flawed and doomed to the archives of science history.

Joseph Black and latent heat

Early evidence contradicting caloric theory dates to the 1760s, when the Scottish chemist Joseph Black performed important work on the different states of water. Black was the first to write about the observation that as ice melted and became water, the temperature of the system remained the same. Adding more heat to the ice water only sped up the melting, but the temperature did not change until the ice had fully melted. If caloric theory was correct, adding more heat to the mixture should raise its temperature. In describing this phenomenon, Joseph Black hypothesized that the ice must store something that is released as it becomes water; he called this “latent heat.”

Today, we know that latent heat is the energy that is released or absorbed as a material changes its state of matter. For example, it tells us exactly how much energy it will take to melt ice or boil water into steam. While the discovery of latent heat is often marked as the beginning of the entire field of thermodynamics, it would take another hundred years before caloric theory disappeared as a description of heat energy and heat flow.

The steam engine drives early thermodynamics research

Many of the great advances in modern society had their roots in the Industrial Revolution of the late 1700s and early 1800s. And a primary driver of the Industrial Revolution was the steam engine, first introduced in industrial capacity by James Watt in 1765 (Figure 1). The steam engine helped drive industry by allowing the placement of factories at locations independent of sources of running water that had traditionally supplied mechanical power. However, early steam engines were not very efficient. In fact, the engine famously invented by Watt and Matthew Boulton converted only about 3% of its fuel into work, a grossly inefficient system. For decades, engine builders tinkered with different parts and fuels to improve its design, but they contributed little to the improvement of the engine’s efficiency. The problem was that none of these early engineers put much thought into the key component that made the engine function: heat. By failing to understand heat and how it moved, engineers failed to make significant progress.

Carnot and the steam engine revolution

In the early 1800s, a French engineer and physicist named Sadi Carnot (Figure 2) began working with the steam engine. Carnot had served in the French military, and coming back from the Napoleonic wars, he had been struck by the poor state of French steam engines compared to British designs. In fact, he was convinced that inadequacies in French engines had contributed to France’s downfall, and he set out to address them.

Carnot approached the problem from a different vantage point than others – he studied the movement of heat and the function of steam in driving the engine as opposed to the mechanical parts of the engine. In 1824, he published his observations in steam engine theory, “Reflections of the Motive Power of Fire.” In this publication, Carnot showed that the work that a steam engine performs is due to the flow of heat from hot parts of the engine to cooler parts, and the efficiency of an engine depends on this temperature difference. Though Carnot’s work was initially well received, it was largely ignored for nearly a decade. Yet this notion of heat flow was the first of its kind, and would eventually lead to major advances in engineering and thermodynamics in the mid-1800s. It marks the true beginning of our formal study of the movement of heat called thermodynamics.

Joule's experiments expand Carnot's work

Carnot’s work in steam engine theory was monumentally important, but it was still based on the flawed notion of caloric theory. Other scientists who followed in Carnot’s footsteps would sound the death knell of caloric. One of the first scientists to expand on Carnot’s work was James Prescott Joule, who began to study the steam engine that his family used to power their local brewery to determine if it could be replaced by the newly invented electric motor.

Before he became manager of the family brewery, Joule was trained in physics by none other than the English atomist John Dalton. Joule studied the amount of work (or what he called “duty”) that each motor could produce, and found that despite its inefficiencies, the steam engine fared significantly better than the electric motor. Joule continued to study the nature of work and energy, and in the 1840s he conducted one of his best-known experiments. Joule used the energy from a falling weight (mechanical energy) to drive a paddle wheel through water, generating heat from the friction; this observation proved that energy can be converted between forms. (Learn more about this research in our Energy: An Introduction module.)

Joule’s experiments contributed to our modern understanding of energy: Heat and work are two different forms of the same thing: energy. This represented a direct challenge to caloric theory. How could the material that caused things to be hot be the same as that which resulted in work? Many scientists of the time were skeptical of Joule’s interpretations and sought flaws in his work. As such, it was slow to be accepted.

The First Law of Thermodynamics

The caloric theory of heat began having problems explaining all of the observations of scientists as early as 1760. However, it met its biggest challenge in the mid-1800s when Rudolf Clausius, a German physicist and mathematician, published a series of papers that laid the groundwork for the so-called “kinetic” theory of heat. Clausius appreciated the ideas published by Carnot, but he had a problem with the caloric theory. If heat is a material, like all matter, then it cannot be destroyed, as Carnot said in his work. But Clausius said that the opposite must also be true, that if heat cannot be destroyed, then it cannot be created. This is obviously incorrect though – the simple act of rubbing two objects together, like your hands on a cold day, can increase their temperature, and thus create heat.

Referring to the work of Joule, Clausius formalized the relationship between work and heat in his treatise On the mechanical theory of heat, published in 1850. In it, he provided the first statement of the kinetic theory of heat, in which heat is derived by the motion of molecules in a material. “In all cases in which work is produced by the agency of heat, a quantity of heat is consumed which is proportional to the work done,” he stated. This one sentence is credited with two landmarks: It fractured caloric theory, and it is one of the earliest formal statements of the Law of Conservation of Energy, which states that in a system that is closed to its surroundings, energy can change forms but cannot be created or destroyed. The Law of Conservation of Energy has come to be known as the First Law of Thermodynamics (see Figure 3). Clausius was one of the first scientists to note that heat and work were proportional to one another, essentially representing two different forms of energy. This proved that caloric was not a material, and the kinetic theory finally took hold in the field.

Clausius is also largely credited with formalizing the Second Law of Thermodynamics. As he put it, “Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.” Put simply, cold objects cannot warm up hotter objects; this requires some kind of external influence. A full understanding of this phenomenon requires an understanding of a concept called entropy.

Capturing heat

The study of heat in the early 1800s was hindered by a more practical problem: measurement. Heat could not be captured in a jar like a gas. It could only be detected by changes in temperature. Accurate thermometers and temperature scales had been around since the early 1700s, so measurement of temperature was not a problem. However, scientists using early thermometers did not know how heat and temperature were related. Joseph Black’s research on latent heat had challenged the conventional wisdom that heat and temperature were interchangeable, and raised questions about the nature of heat as a material, but it did not yet provide answers.

A crucial development in the study of heat and energy flow was the invention of a device called the calorimeter. This is an insulated container that allows for the accurate observation of changes in temperature based on process inside of the container free from outside influences. The French chemist Antoine Lavoisier used an early calorimeter to show that the gas and heat exchange that occurs when guinea pigs breathe is similar in nature to that of a candle burning (don’t worry, the guinea pigs were fine!). And Joule later used a crude calorimeter in his experiments, allowing him to equate work with heat. The precise measurement of the heat produced by various processes allowed scientists to better understand the true nature of heat itself.

Looking ahead

Thermodynamics is a discipline firmly anchored in the study of energy and its transitions. From our desire to improve the steam engine, we came to understand the interplay between different forms of energy like heat and work. However, the story becomes much more complex. The concepts of enthalpy, entropy, and free energy come into prominence as we shift our attention to processes such as chemical reactions and equilibrium.