Chapter 3: Energy and Entropy
Thermodynamics is the branch of physics that studies the nature of heat and its relation to other forms of energy. This chapter will introduce you to the first and second laws of thermodynamics—and some of their surprising implications, which will be important in later chapters. We’ll begin with a few simple concepts.
Work is the exertion of a force that moves something for a distance, and is equal to the magnitude of the force times the distance the object moves in the direction that the force is exerted:
work = force × distance
Earth’s gravitational force moves a falling apple downward (the same direction that the force vector is pointing), so work is done on the apple.
However, the earth’s gravitational pull doesn’t do any work on the moon, because the moon’s direction of motion is perpendicular to the direction of the force. Work is done only when an object moves in the direction that the force is exerted. The force of Earth’s gravity pulls on the moon, but the moon isn’t getting any closer to us. In other words, the moon isn’t moved by Earth’s gravity. The force of gravity merely changes the direction of the moon’s velocity as it orbits, so no work is done.
The joule is also the standard unit of energy, as we’ll see in a moment.
The standard unit of work is a joule (symbolized with an uppercase “J”), which is the amount of work done by 1 newton of force exerted for a distance of 1 meter:
1 J = 1 N × 1 m
1 J = 1 N m
1 J = 1 kg m2/s2
How much work is done when lifting a 5 kg rock 3 meters upward? First, we need to figure out how much force (in newtons) is required to lift a 5kg rock against the force of gravity. Near the earth’s surface, a 5 kg rock weighs 49 N. (See this note in chapter 2
for an explanation of how to calculate the weight of an object near the earth’s surface.) It takes 49 N of force to lift a 5 kg rock (plus a negligible amount more than that to accelerate the rock upward), so the amount of work done when lifting a 5 kg rock 3 meters is 49 N × 3 m = 147 J.
Both work and energy are scalar
quantities, not vector quantities. (For an explanation of the distinction between scalar and vector quantities, see this section of chapter 2
Energy is the ability to do work—that is, the capacity or potential to exert a force that moves something. There are numerous forms of energy: kinetic energy, potential energy, and thermal energy, to name a few. (These will be introduced in the pages that follow.)
A system is any part of the physical universe (i.e., any physical thing or collection of physical things). Everything outside the system is called its environment. A system is open if it exchanges matter and/or energy with its environment, and closed (or isolated) otherwise. The universe as a whole is perhaps the only perfectly closed system, but many other systems exchange so little matter and energy with their environments that for most practical purposes they can be regarded as isolated. For example, a sealed thermos is almost a closed system: hardly any energy can enter or escape through the insulated body of the thermos, and little if any matter passes through the sealed lid.
A system has energy if it has the ability to exert a force that moves something. (The thing that is moved may be a part of the system itself, or it may be a part of the environment.) The amount of energy a system has is the same as the amount of work it is able to do; hence the standard unit of energy is the same as the standard unit of work: the joule.