Section 3.3 Energy
Can you define energy? Although everyone has a general idea of the meaning of energy, a good definition is hard to find. Most commonly, perhaps, energy is defined as the “ability of a system to do work.” This is not a very complete definition. Energy can produce other effects which cannot possibly be considered “work.” For example, heat can flow from one object to another without doing work; yet heat is a form of energy, and the process of heat transfer is a process that produces an effect.
A better definition of energy, therefore, states that “energy is the capacity for producing an effect.” Energy exists in many forms. For convenience, we usually classify energy according to the size and nature of the bodies or particles with which it is associated. Thus we say that Mechanical energy is the energy associated with large bodies or objects, usually, things that are big enough to see. Thermal energy is energy associated with molecules. Chemical energy is energy that arises from the forces which bind the atoms together in a molecule, Chemical energy is demonstrated whenever combustion or any other chemical reaction takes place. Electro-magnetic energy is energy that is associated with the electron and charged particles. Light, x-rays and radio waves are examples of electro-magnetic energy. Nuclear energy is associated with the particles in the nucleus, and is governed by Einstein’s famous equation \(E = mc^2\)nEach of these types of energy (mechanical energy, thermal energy, etc.) must also be classified as being either stored energy, or energy in transition. Stored energy can be thought of as energy that is actually “contained in” or “stored in” a substance or system. There are two kinds of stored energy: potential energy, and kinetic energy. When energy is stored in a substance or system because of the relative positions of two or more objects or particles, we call it potential energy. When energy is stored in a substance or system because of the relative velocities of two or more objects or particles, we call it kinetic energy.
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Mechanical Energy – energy associated with relatively large bodies or objects.
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Stored forms of mechanical energy
- Mechanical potential energy
- Mechanical kinetic energy
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Mechanical energy in transition
- Work
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Thermal Energy – energy associated with the very small particles called molecules.
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Stored forms of thermal energy
- Internal potential energy
- Internal kinetic energy
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Thermal energy in transition
- Heat
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If you do not completely understand this classification, come back to it from time to time as you read the following sections on mechanical energy and thermal energy. The examples and discussion given in the following sections will probably help you to understand this classification.
