## 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.

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What type of energy is pictured here?The Thunder Dolphin rollercoaster in Tokyo Japan reaches speeds of 81 mph. Photo by Ben Garney.

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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

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.

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## References

What is Energy? from EIA Energy Kids, this unit covers energy basics, types of energy, energy units and energy calculators.

PhET simulations for teaching Energy, Work and Power. Interactive simulations that allow students to "experiment" with changing variables in different energy systems.

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The Physics Classroom is an online, free-to-use physics website developed primarily for high school physics students and teachers. For example, the animation of Energy Transformations for Downhill Skiing illustrates the relationship between work and energy.