Definition 4.1.1. Energy.
The energy of a system is a physical property measuring its ability to exert forces that can impact the displacement of other systems. Energy is a scalar quantity measured in joules (J).
Section 4.1 What is Energy?
Introductory Activity: Start by writing 2-4 things you already know about energy, each as a single complete sentence.
Like many words used in physics, you probably use the word "energy" in your everyday life. For example, you likely know what it feels like to have "a lot of energy" or you might talk about being "low on energy." Colloquially, energy can be thought of as the capacity to do something. The physics definition of energy will try to take this idea and make it more specific and quantifiable.
Exercises Forms of Energy
Energy comes in many types known as forms. The primary forms you will encounter in physics are kinetic energy (energy associated with motion, including translational, rotational, and thermal kinetic energy) and potential energy (including gravitational, electrical, chemical, and nuclear potential energy). Within a system, energy can transform from one form to another form. You can add together all the different forms of energy in a system to get that system’s total energy, usually symbolized by \(E_{\text{total}}\) or just \(E\text{.}\)
Exercises Energy Transfer
Energy can be transferred from one system to another when those two systems interact with each other in certain ways. When energy is transferred from one system to another, the total energy of one system will increase and the total energy of the other system will decrease by the same amount. The primary means of energy transfer in physics are work (energy transfer associated with a force) and heat (energy transfer associated with a difference in temperature). If a system is not interacting with anything external (or if all external energy transfers sum to zero), you can say that the total energy of the system is conserved, or constant in time. If you were to choose a system consisting of everything in the universe, since there is nothing external to interact with the universe, you can say that the total energy in the universe should be conserved.
Historical Note 4.1.2. Origins of Energy as a Concept.
Historically, Aristotle of Stagira (384–322 BCE) in ancient Greece was the first known writer to use the word energeia (ενεργεια) in his text Nicomachean Ethics. Energeia literally means "in work" or "to contain work". In physics, energy is often defined as the ability to do work.
Exercises Energy Activities
1. Sensemaking: Type of Quantity.
In contrast to the many vector quantities you have studied, energy is a scalar quantity. What is the primary difference between a vector quantity and a scalar quantity?
Answer.
A vector quantity has both a magnitude and a direction, and can be represented by an arrow in space. A scalar quantity is represented only by a single number, and so it does not have a direction, although that number can be positive, negative, or zero.
2. Exploration: Energy Forms for a Skier.
Just before you start skiing down a mountain, your system starts with a large amount of gravitational potential energy. As you ski down the mountain, your system’s gravitational potential energy decreases, transforming into other forms. What other forms of your system’s energy do you think are changing as you ski down the mountain? Are they increasing or decreasing? Do you expect any forms of energy to change in more than one way?
3. Practice: Energy Transfer.
Suppose you have three systems: A, B, and C. Each system begins with a total energy of \(50 \mathrm{~J}\text{.}\) The systems then interact so that system A transfers \(20 \mathrm{~J}\) to system B and system B transfers \(35 \mathrm{~J}\) to system C. Find the total energy of each system after the energy transfers.
4. Sensemaking: Unit Conversion.
Another common unit for energy is the kilocalorie (known colloquially as the Calorie). Look up the number of joules in a kilocalorie. How many joules are in a typical \(2000\) Calorie daily diet?
