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PY105 Notes - Applying energy concepts
Includes good example on accounting for air resistance.

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Special points of emphasis

Work, Energy, and Power


As physical systems undergo change, one question always on the short-list is: What is happening with energy?

As the system changes configuration, what happens to the energy within the system and between the system and its surroundings?

Energy dissipation represents an important set of phenomena. In the context of Thermodynamics, we call it heat flow from the system.

Almost any real world transformation of potential energy into kinetic energy coincides with an exchange of energy between a system and its environment, usually as sound energy or heat flow. The friction on a block sliding down an inclined plane is a typical problem, leading to energy loss to the surroundings through heat flow.

Work, Energy, and Power


Atomic Theory

DC Current

Energy may transform from one form to another within a system or it may be exchanged between the system and its surroundings. Always watch for the movement of energy between a system and its surroundings.

Photon energy is often the means by which an electro-dynamic system transfers energy to its surroundings. The quintessential example is the emission of light from an excited atom as it returns to the ground state.

Another common situation involves heat loss from a resistor, usually initiated by collisions between current electrons and conductor atoms at the molecular level which leads to a temperature difference between the resistor and the environment leading to heat flow.

Work, Energy, and Power

The Second Law of Thermodynamics and Heat Engines

The relationship of a transformation with regard to a particular kind of statistical view of order will be a crucial point of view we will learn later to apply to judge the transformations of energy. When you are applying this point of view, you are judging a transformation in terms of the total change in entropy of the system plus its surroundings. This is an incredibly important framework for viewing physical change because it helps you to understand why some directions of change are spontaneous and some are not.

We will have a great deal more to say about this as this MCAT course progresses, but here, in the context of energy, let it suffice to understand that some kinds of energy represent a more or less disordered state than others. Spontaneous change leads to an increase in the disorder (entropy) of the universe. This is the Second Law of Thermodynamics, one of our topics in Module 6.

Most real world transformations are spontaneous, i.e. irreversible. In other words, it is much more likely for the sliding block to produce sound than for sound to make the block slide back up the plane.

The understanding that the entropy of the universe is always increaseing is a very important concept governing the transformations of physical systems within their surroundings. When you move further into the physical sciences, you want to add to the first question: What is happening with energy? A second question: What is happening with entropy?

This is a preview of a very important set of concepts which we will approach with quite a bit of depth later in this course. We will have a great deal to say about entropy later in the Interdisciplinary Discussions. For now, keep in mind that not all directions of change are equal. Real world energy transformations are not reversible.

The WikiPremed MCAT Course is a free comprehensive course in the undergraduate level general sciences. Undergraduate level physics, chemistry, organic chemistry and biology are presented by this course as a unified whole within a spiraling curriculum.

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