20.0 - Introduction
The desire to build more powerful and efficient engines led engineers and scientists
to embark on pioneering research into the relationship of heat, energy and work.
The engines that powered the Industrial Revolution primarily used energy provided
by burning coal.
Much has changed since the first engines were used to drain marshes and pump
water from the coalmines of Great Britain. New sources of energy, from gasoline for
automobiles to nuclear fuel for electric power generators, are used to power
engines. Engines are now used in applications undreamt of by their first designers,
for whom the horseless carriage and nuclear fission would have been science-
fictional fantasies.
Despite the changes, much of the fundamental science developed in the 19th
century is still used to analyze engines. The goal of an engine, to get useful work
out of a heat source, has not changed. Although the technology has advanced and
made engines more efficient, the physics of heat engines as established by 19th
century engineers and scientists applies equally well to the steam locomotives of
their era and to modern electrical power plants powered by nuclear fission.
This chapter focuses on two topics, using the engine as the basis of much of the discussion. One topic is the first law of thermodynamics, the
relationship between the energy supplied to an engine and how much work it does. The other topic is the role of gases in the functioning of an
engine. Many engines use a gas to function; applying some basic principles of how gases behave proves very useful in understanding the
functioning of engines.
The simulation on the right will get you started on your study of engines. Here you see a heat engine, a device that uses heat as its energy
source to do work. There is gas (represented by some bouncing molecules) in the central chamber of the engine, a piston on top, and a “hot
reservoir” on the left from which heat can be allowed to transfer into the gas. In this case, we consider the cylinder, the gas and the piston to be
the system. The reservoirs on the sides allow heat to flow in and out of the system. When heat flows into the engine’s gas, its temperature will
rise (reflecting an increase in its internal energy), and if the piston is free to move, the gas will expand, pushing it up.
In this simulation, you will see what occurs during two distinct engine processes. In the first process, the piston is locked in place while heat is
transferred to the gas. You control the amount of heat that is transferred. The internal energy of the gas is displayed in an output gauge. In the
first step, you should ask yourself: What is the relationship between the amount of heat transferred to the gas and the change in the internal
energy of the gas? Consider the principle of conservation of energy when you ponder your response to this question, and then test your
hypothesis.
In the second process, no heat is allowed to flow in or out of the engine, but when you press GO, the piston will be unlocked so it can move. A
gauge will show you how much work the gas does as it expands or contracts, changing the piston’s position. At the end of this process, note
how much work the gas did and again its change in internal energy. How do the values for the work and change in internal energy relate during
this process?
One last calculation (there are a few here, but you will have taught yourself the first law of thermodynamics when you complete this exercise):
How does the amount of heat you initially transferred to the gas relate to its change in internal energy and the work it does? Write down these
three values and see if you note a fundamental mathematical relationship. You can also apply the physics you learned earlier; consider the
principle of conservation of energy and the work-energy theorem. (The work-energy theorem in its most general form says that the work done
by an external force on a system equals its change in total energy, which includes thermal energy as well as mechanical energy.)
If these are too many questions, read on. The first law of thermodynamics begins this chapter.
20.1 - First law of thermodynamics
First law of thermodynamics: The net heat transferred to a system equals the
change in internal energy of the system plus the work done by the system.
The first law states that the net heat transferred into or out of a system equals the change in the internal energy of the system plus any work
the system does. It is written as an equation in Equation 1, with Qrepresenting the net heat transferred to the system. You saw this law at work
in the simulation in the introduction to this chapter. First, heat was transferred to the gas in the engine, increasing the internal energy of the
system. The increase in the temperature of the gas in the engine reflected the increase in internal energy. When you pressed GO again, the
piston rose. The engine did work, and its internal energy decreased. The amount of work done by the engine equaled the magnitude of its
change in internal energy as the piston rose.
We use the simple engine in Concept 1 to discuss the first law in more depth. Heat flows from the flame into the container, causing the internal
energy of the gas it contains to increase. As the gas’s internal energy increases, its temperature and the average speed of its molecules
increases. As the gas’s temperature increases, its pressure increases and it may expand. The gas does positive work when it expands,
applying a force to the moving lid in the direction of its displacement.