## Teaching about the nature of energy is supported by 8 key concepts:

1.1 Energy is a quantity that is transferred from system to system. Energy is the ability of a system to do work. A system has done work if it has exerted a force on another system over some distance. When this happens, energy is transferred from one system to another. At least some of the energy is also transformed from one type into another during this process. One can keep track of how much energy transfers into or out of a system.

1.2 The energy of a system or object that results in its temperature is called thermal energy. When there is a net transfer of energy from one system to another, due to a difference in temperature, the energy transferred is called heat. Heat transfer happens in three ways: convection, conduction, and radiation. Like all energy transfer, heat transfer involves forces exerted over a distance at some level as systems interact.

1.3 Energy is neither created nor destroyed. The change in the total amount of energy in a system is always equal to the difference between the amount of energy transferred in and the amount transferred out. The total amount of energy in the universe is finite and constant.

1.4 Energy available to do useful work decreases as it is transferred from system to system. During all transfers of energy between two systems, some energy is lost to the surroundings. In a practical sense, this lost energy has been "used up," even though it is still around somewhere. A more efficient system will lose less energy, up to a theoretical limit.

1.5 Energy comes in different forms and can be divided into categories. Forms of energy include light energy, elastic energy, chemical energy, and more. There are two categories that all energy falls into: kinetic and potential. Kinetic describes types of energy associated with motion. Potential describes energy possessed by an object or system due to its position relative to another object or system and forces between the two. Some forms of energy are part kinetic and part potential energy.

1.6 Chemical and nuclear reactions involve transfer and transformation of energy. The energy associated with nuclear reactions is much larger than that associated with chemical reactions for a given amount of mass. Nuclear reactions take place at the centers of stars, in nuclear bombs, and in both fission- and fusion-based nuclear reactors. Chemical reactions are pervasive in living and non-living Earth systems.

1.7 Many different units are used to quantify energy. As with other physical quantities, many different units are associated with energy. For example, joules, calories, ergs, kilowatt-hours, and BTUs are all units of energy. Given a quantity of energy in one set of units, one can always convert it to another (e.g., 1 calorie = 4.186 joules).

1.8 Power is a measure of energy transfer rate. It is useful to talk about the rate at which energy is transferred from one system to another (energy per time). This rate is called power. One joule of energy transferred in one second is called a Watt (i.e., 1 joule/second = 1 Watt).

## 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 about energy, much of which is based in 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 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 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 changes in temperature.

Radiant energy is the energy from electro-magnetic 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

Companion video by the Department of Energy
View a non-YouTube version of this video

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 frame 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 use to measure energy are Joules. A Joule is the amount of energy required to accelerate a 1 kg object to a speed of 2 m/s, 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 in to 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

How does Work Work? This TED-ed video illustrates the concepts of work and power, which can help us unlock and understand many of the physical laws that govern our universe. In this lesson, Peter Bohacek explores the interplay of each concept when applied to two common objects -- a lightbulb and a grandfather clock.

As is 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), 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.

A chemical battery running a light bulb that illuminates a surface: chemical energy is transformed into electric energy, which is transformed to radiant and (mostly) thermal energy. Examining the labeling on a light bulb allows students to calculate the efficiency by finding the ratio of light output (lumens) to power used (Watts). Higher efficiency light bulbs will produce more lumens of visible light per Watt.

Hand-crank generator/motors and a light bulb show how kinetic energy can be transformed into electrical energy. Connecting two handheld generator/motors together shows how kinetic can be transformed to electric and then back to kinetic.

A Peltier junction (or thermo electric generator) transforms electrical energy into a temperature difference, or a temperature difference into electrical energy.

So-called "happy/sad" balls available from science education supply companies show how the elasticity of a material can affect energy transfer. The "happy" ball is made of a polymer that, when compressed, stores elastic potential energy and releases a similar amount of kinetic energy when it is uncompressed. For example, when the ball is dropped from a height of 1 meter, the gravitational potential energy is converted to kinetic energy as the ball falls. When the ball impacts the floor, the ball compresses and the kinetic energy is converted into mostly elastic potential energy (and some thermal energy). When the ball re-bounds, the elastic potential is converted back to mostly kinetic energy (again, some thermal) causing the ball to re-bound to a significant fraction of the height from which it is released. The ratio of the re-bound height to the release height is the ratio of final energy of the system to the original energy -- an estimate of the energy transfer efficiency. The "sad" ball is made of a polymer that is less elastic. When this ball is compressed, almost all of the mechanical energy is converted to thermal energy and the ball does not re-bound noticeably.

Spring toys and poppers are other examples of similar energy transformations.

A dewar or vacuum flask "thermos" is an excellent example of transfer of thermal energy. Describing how a vacuum flask keeps thermal energy from being transferred either in or out helps students realize that "coldness" is not a quantity, or a form of energy, but rather a lack of thermal energy. Students may be interested in the story of the development of the vacuum flask and how it was commercialized by Thermos who benefited from the fact that Dewar did not patent the idea.

## Teaching materials from the CLEAN collection

Middle school

• The activity, Energy flows, provides a synopsis of energy basics that was written for middle school students.
• 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 systems: atmosphere, hydrosphere, lithosphere and biosphere.

College

• The Solar Water Heater project allows student teams 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.