What is Energy? Introduction

Units:

What is Energy?

Section A. Introduction

What is always present but never visible? ENERGY! Energy is a difficult concept to understand because it is not a concrete object that you can see or touch. To understand what energy is you must first understand what it does. That is, although energy isn't visible you can detect evidence of energy. Jumping, moving a wheelchair, eating, and singing all require energy. Nonliving things also use energy - a clock, vacuum cleaner, and mechanical toys all require energy to move. Work is involved whenever anything moves a distance, and energy is needed to do work. Therefore, energy is defined as the ability to do work.

Much like mass or volume, energy is a property of an object. It's just that energy is more abstract than some other properties. Although energy itself isn't visible, you can detect evidence of energy.

Movement, sound, heat, and light provide evidence that energy is present and being used. Sound is produced when we strike something. But does sound do work? Yes, sound can move things. Sound waves move the tiny bones in your ear and shake windows when a loud truck passes by. Sound waves are also evident in the vibrations from playing a radio.

Our body is working even when it appears to be still. Breathing, blinking, and digesting food all require energy. For us to do these activities, our bodies burn the energy in food. We know this is happening because we feel warm (burning generates thermal energy [heat]). Therefore, heat is evidence that energy is being used.

If the energy is the ability to do work, how does thermal energy fit into this definition? Thermal energy can melt an ice cube or make water boil. Therefore, the definition of energy can be amended to energy is the ability to do work or to organize or change matter.

Light is another observable form of energy. Light can change things. When light shines on your arm it makes it feel warm. When light shines on a green plant, the plant can make food (Excerpt from KEEP Activity Guide "Evidence of Energy").

Since energy is a property of matter, scientists have discovered ways to measure and quantify energy. Measuring energy helps to understand how it is used, how it changes forms, and how to increase energy efficiency.

In this unit, you will learn more about energy forms, measurements, and resources.

Next reading: What is Energy? - Section B. Two Main Forms of Energy.

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