Thermodynamics

Thermodynamics I

BY ZACHARY HARTMAN, PH.D., ANTHONY CARPI, PH.D.

Terms you should know

• Kinetic: relating to the motion of objects; the kinetic theory of heat states that heat is a result of the motion of molecules in a material.

• System: a group of related parts that work together to form a whole.

• Work: using force to move an object; a process in which one form of energy is converted into another.

Introduction

Heat is at the core of almost every aspect of our lives. If we stop to think about it, we can see examples everywhere of how heat and other forms of energy drive processes: when you cook eggs in the morning, when you cool a drink with ice cubes, when you burn gasoline to run the engine of your car, even as you sit and read this. In all of these instances, heat and other forms of energy are being interconverted, and you are making use of the laws of thermodynamics. By some measures, thermodynamics is a relatively young field of study. However, research in the field has helped us understand and even shape the world as we know it today. Gaining a fundamental understanding of thermodynamics can help you to appreciate why certain chemical reactions proceed while others never start. Thermodynamics helps explain things as diverse as the thermonuclear reactions that occur in stars and the everyday reactions inside our bodies.

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 how the heat will move and how this can be made into valuable processes, like driving an engine. Therefore, thermodynamics is integral to the study of engineering and physics. However, 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, the heat was thought to be a substance – it could not be touched, seen, or collected like other kinds of substances. However, it flowed from object to object much as 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, temperature changes were caused by the flow of caloric to or from an object. This idea about heat made sense in its day because heat does 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 the 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 the 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 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.

Multiple Choice

Comprehension Checkpoint

Adding heat to ice water

• raises the temperature of the water steadily as the ice melts.

• speeds up the melting of the ice, but the temperature remains the same until the ice is melted.

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. 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 their 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.

Figure 1: Watt's steam engine "Old Bess" preserved at The Science Museum.

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 was struck by French steam engines’ poor state compared to British designs. He was convinced that inadequacies in French engines had contributed to France’s downfall, and he set out to address them.

Figure 2: Sadi Carnot

Carnot and the steam engine revolution (Continued)

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. This was in contrast 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 influential, but it was still based on the flawed notion of the 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. He 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.

Multiple Choice

Comprehension Checkpoint

Energy can be converted from one form into a different form.

• true

• false

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 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.

The First Law of Thermodynamics (Continued)

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 material, and the kinetic theory finally took hold in the field.

Figure 3: Diagram illustrating the First Law of Thermodynamics. On the left, a paddlewheel spins in a beaker of cool water; on the right, the paddle has stopped spinning and the temperature of the water has increased. This is a type of energy conversion where work (the spinning paddle) creates a proportional amount of heat (the warmer water). Throughout the process, though, the amount of energy in the system remains constant.

The First Law of Thermodynamics (Continued)

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.

Multiple Choice

Comprehension Checkpoint

The kinetic theory of heat says that heat comes from

• the motion of molecules in a material.

• the transfer of a heat-containing substance from one material to another.

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. It 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 temperature changes based on the 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 to that of a candle burning (don’t worry, the guinea pigs were fine!). Later, Joule used a crude calorimeter inside the apparatus to equate heat with work. The precise measurement of the heat produced by various processes allowed scientists to better understand the true nature of heat itself.

Multiple Choice

Comprehension Checkpoint

The calorimeter

• was the first device invented to measure temperature.

• allowed scientists to measure changes in temperature inside of a container without outside influences.

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 chemical reactions and equilibrium processes.

Did you know?

Did you know that heat and temperature are not the same things? If you add heat to ice water, the temperature of the water does not change until the ice has melted. This is the first Law of Thermodynamics at work. Anytime heat makes something happen, the laws of thermodynamics apply.

Summary

Without heat flow, nothing can move, no chemical reactions can take place, and no machines can run. This module introduces the concepts of heat and thermodynamics. It explains early ideas about heat and how scientists came to understand that heat and work are two different forms of the same thing. The First Law of Thermodynamics is described (simply put, energy can't be created or destroyed). Other topics include latent heat and the measurement of heat.

Key Concepts

• Thermodynamics is the study of the relationships between heat, mechanical, chemical, and other forms of energy and the effects of these forms of energy on or within a system.

• Heat is a form of energy that moves from areas of high to low, and it can be converted into work energy.

• The First Law of Thermodynamics, a variant of the Law of Conservation of Energy, states that within a closed system, energy may change form but cannot be created or destroyed.

Multiple Choice

What is the definition of thermodynamics?

• The energy available to do work.

• The study of the relationship between heat, work, and energy.

• The amount of heat it takes to move an engine.

• The movement of heat.

Explanation

The word thermodynamics comes from the Greek words for “heat” and “power.” Thermodynamics is the study of heat used as a source of power for work.

Multiple Choice

What was the name of the prevailing theory of heat prior to thermodynamics?

• Heat Equals Work Theory

• Kinetic Theory

• Caloric Theory

Explanation

Many scientists in the late 1700s accepted the “caloric” theory of heat. They believed that heat was a substance that flowed from object to object. The amount of caloric substance in an object gave rise to its temperature.

Multiple Choice

Which of the following provides evidence against the caloric theory of heat?

• Heat doesn't flow from hot objects to cold objects as predicted by caloric theory.

• Adding more heat to ice water does not raise the temperature as predicted by caloric theory.

Explanation

If the caloric theory were correct, adding more heat to the mixture should raise its temperature since the caloric substance would move into the water. Joseph Black found that adding more heat to ice water sped up melting, but the temperature did not change until the ice had fully melted. This showed that heat is a form of energy that can be put to work to melt the ice; that is, heat and work are two different forms of the same thing: energy.

Multiple Choice

What breakthrough is said to mark the true beginning of the field of thermodynamics?

• The discovery of a substance called “caloric.”

• The invention of the thermometer.

• The discovery of latent heat.

Explanation

Scottish chemist Joseph Black experimented with different states of water. He observed that adding heat to ice water did not change the temperature of the water. Rather, it went into changing the water from a solid to a liquid. He called the energy associated with changing one state of matter to another “latent heat.” The discovery of latent heat is often marked as the beginning of the field of thermodynamics.

Multiple Choice

What is latent heat?

• Heat that doesn’t move from high temperature to low temperature, also known as “lazy heat.”

• The heat required to move an object from its “latent” position.

• The heat required to convert the state of matter of a substance without changing its temperature.

Explanation

Adding heat to ice water makes the ice melt faster, but the water temperature doesn't rise until the ice fully melts. This is because of latent heat, the energy that is released or absorbed as one state of matter changes to another.

Multiple Select

Which statement is true about the First Law of Thermodynamics?

• It states that energy can't be created.

• It states that energy & studies can be destroyed.

• It states that energy can't be destroyed.

• It is another name for the Atomic Theory.

• It is another name for the Law of Conservation of Energy.

Explanation

The Law of Conservation of Energy, also known as the First Law of Thermodynamics, states that in a system that is closed to its surroundings, energy can change forms but can't be created or destroyed.

Multiple Choice

Why was the invention of the calorimeter important?

• It was the first device invented to measure temperature.

• It allowed scientists to measure changes in temperature inside of a container without outside influences.

Explanation

The invention of the calorimeter was a crucial development in the study of heat and energy flow. This device allows for the accurate observation of changes in temperature based on processes inside of a container free from outside influences. The precise measurement of the heat produced by various processes gave scientists a better understanding of the true nature of heat.

Multiple Choice

Which is a true statement about energy conversion?

• One form of energy cannot be converted into another form of energy.

• Heat can be converted other forms of energy only if caloric is present.

• There is no relationship between heat and work.

• Heat can be converted into other forms of energy.

Explanation

In a closed system, energy can change forms but cannot be created or destroyed. Heat, one form of energy, can be converted into other forms of energy. Thermodynamics is all about how heat and other forms of energy can be interconverted to make work happen.

Multiple Choice

Which is true according to Rudolf Clausius’s 1850 treatise on work and heat?

• The more work that is done, the less heat is used up.

• The more work that is done, the more heat is consumed.

Explanation

According to Clausius, when heat causes work to be done, the quantity of heat consumed is proportional to the work done. Therefore, the more work that is done, the more heat is used up.

Fill in the Blank

What type of storage system uses mechanical energy to store energy?

Hint: 1 word, 8 letters.

Explanation

Flywheels store energy as kinetic-mechanical energy. A flywheel's spinning wheel is levitated by a magnet inside a vacuum, which reduces almost all friction. The concentric wheel is made of high-tensile-strength fibers embedded in epoxy resins. These wheels spin at 200,000 - 100,000 rpm. Speed is everything: the faster the flywheel can go, the more power it can store. If scientists could make even stronger material, then someday flywheels could hold many terajoules of energy. A terajoule is equal to a thousand billion joules. It takes about 30,000 joules to make a cup of tea. Only a few terajoules were used to power astronauts to the moon and back. Flywheels can charge in 15 minutes, and do not need maintenance for twenty years. They are lightweight, reliable, and have a ninety-five percent power efficiency. Flywheels are not affected by the weather like chemical batteries and do not hurt the environment. In the 1950s, flywheel-powered buses were used in Yverdon, Switzerland. Flywheels were also used in steam engines of the 1970s. The main disadvantage of flywheels is that they can explode at any second because the wheel inside is spinning so fast.

Multiple Select

What 2 factors affect air resistance?

• size

• mass

• shape

• height

• weight

Explanation

Size and shape are the 2 factors that affect air resistance. Air resistance works with surface area, so the more surface area, the more air resistance. Think about when you drop 2 pieces of paper: one crumpled and one flat. The crumpled one falls faster because there is less air resistance acting on the paper.

Poll

How much do you agree or disagree with this statement? I found this lesson very interesting.

• Strongly Agree

• Agree

• Neither agree nor disagree

• Disagree

• Strongly Disagree