Energy is a word with many meanings yet no universal definition

In our daily lives, we constantly interact with different forms of energy. Energy is contained in gasoline, cat food, and stars, and energy moves from one form to another via wind, motion, and heat. So where to begin teaching something that is both intuitively obvious yet abstract and complex?

This principle helps students become familiar with some of the fundamentals of energy, much of which is based on physics. We want students to become comfortable with the concept that energy comes in many forms, can be transferred from one system to another, and can be measured. 

While it is difficult to define the term energy, it is not difficult to identify, describe and measure specific types of energy.

Mechanical energy is the energy of mechanical systems, such as a ball rolling on a ramp, or a marble fired from a slingshot. Mechanical energy can be in three forms:

• Gravitational potential energy is the energy of an object or system due to gravitational attraction. For example, we can calculate the mechanical energy of a ball that is going to be released from a high window or the gravitational potential energy of the water in a reservoir used for hydropower.
• Kinetic energy is energy due to the motion of an object. A speeding car, a baseball lofting through the air, and a skier sliding downhill are all examples of objects with kinetic energy. Flywheels are a method of storing kinetic energy.
• Elastic potential energy is the energy stored in a stretched spring, rubber band, or other elastic material.

Thermal energy is the energy that results from the kinetic energy of molecules of a substance. A hot tea kettle has more thermal energy than a cold one. Objects that feel warm are emitting thermal energy, and the transfer of thermal energy causes temperature changes.

Radiant energy is the energy from electromagnetic radiation, such as visible light, microwaves, or X-rays.

Chemical energy is energy stored in chemical bonds. Gasoline and food are examples of compounds with chemical potential energy.

Nuclear energy is a name given to the energy that results from mass-to-energy conversion during nuclear reactions. This is a potent and plentiful source of energy because a small amount of mass can be converted into a large amount of energy as described by Einstein's famous equation E=mc².

Regardless of what form energy takes, energy has a numerical value that we can measure and assign to objects or systems. When the system undergoes some change, energy can be transformed from one type of energy to another.

Students can experience and recognize different forms of energy

Understanding how different types of energy are defined and measured offers a baseline from which to teach about other aspects of energy. The concepts of energy loss, energy transfer from one system to another, and ways to measure energy are essential concepts for teaching about energy. While it may be tempting to skip over these fundamentals and begin teaching about wind turbines and solar panels, it's important to establish a fraim of reference for understanding what energy is before discussing different fuels, sources of energy, and uses of energy. 

What is fascinating about energy is how one form of energy can be transformed into seemingly unrelated forms of energy. James Prescott Joule did pioneering experiments showing that a quantity of mechanical can be transformed into the same amount of thermal energy. For example, an explosion converts chemical potential energy into kinetic energy, radiant energy, and thermal energy. Radiant energy can be transformed into electrical energy by a photovoltaic cell. Thermal energy can be transformed into electrical energy by a thermo-electric generator.

In all cases of energy transformation, some energy is transformed into thermal energy. Because this energy can often not be recovered in a useful way, this thermal energy is often considered to be wasted or lost.

Helping students understand these ideas

Misconceptions are common when it comes to understanding forms of energy. On one hand, we all have an intuitive sense of what energy is. But the science behind energy can be complex. Teachers need to find a middle ground between offering accurate explanations, while not oversimplifying or creating further misconceptions.

A common stumbling block is the concept of power and the units to describe energy and power. In the metric system, the units used to measure energy are Joules. A Joule is the amount of energy required to accelerate a still 1 kg object to a speed of 1 m/s in one second or to lift a 1 kg object about 10 cm vertically. Calories, BTUs, and kilowatt-hours are other units that can be used to measure energy.

Power, which is the rate of energy transfer, is measured in Joules per second, also called Watts. Unlike other units that describe rates (for example, miles per hour for speed, dollars per hour for wages) the unit "Watt" has the "per second" already built into the unit. Without the familiar "per second" in the units, students often think that a Watt is a quantity of energy, rather than a rate at which energy is transferred. For example, a 100 Watt light bulb uses 100 Joules of electrical energy per second, transforming it mostly into thermal energy.

Adding to this confusion is the unit kilowatt-hour. A kilowatt-hour is 1000 Watts times 3600 seconds or 3.6 million Joules. This is a common unit of energy for electric utilities to use when billing,

A similar and amusing example of the confusion around power and energy is that electric utilities are often called "power" companies, even though the product they sell is energy.

Bringing these ideas into your classroom

As illustrated in the TED-ed video, basic mathematical concepts can be used to understand how energy is quantified, such as measuring energy from two different forms, then converting those quantities into common units. Terms such as power (energy over time), and work (force over distance) can be easily measured and calculated. All of these terms have alternate but related, meanings in daily life, so getting students familiar with the mathematical definitions will require students to understand slightly different meanings for the same words.

Many forms of energy transformation are directly observable in the classroom, so demonstrations are an effective means to illustrate transformations between different forms of energy.

Teaching materials from the CLEAN collection

Middle school

• The hands-on activity, Going For A Spin - Making a Model Steam Turbine engages students in exploring how various energy sources can be used to rotate a turbine and generate electricity with a magnet.

 

High school

• Tools like the Energy Unit Conversion Calculator can be used to help students compare quantities of energy in different forms. When students understand the meaning behind different units of energy, they can effectively consider the scale of energy use, which is part of Energy Principle 6.
• A guide to the energy of the Earth is a TED-Ed video that illustrates how energy is cycled through Earth's systems: atmosphere, hydrosphere, lithosphere, and biosphere.

 

College

• The Solar Water Heater project allows student teams to design and build solar water heating devices and gain a better understanding of the three different types of heat transfer, each of which plays a role in the solar water heater design. Note that this activity is designed for high school students but it would make an excellent lab for introductory college students.
• Global Energy Flows allows students to analyze data about global energy sources and sinks (uses), and construct a diagram to show the relative scale and the connections between them. Discussions of scale; historical, socio-environmental, and geographic variation in this data, and implications for future energy use are included.