Second law of thermodynamics examples [Mechanical engineering]

Second law of thermodynamics examples

The second law of thermodynamics

Examples of The Second Law of Thermodynamics
or How Energy Flows from Useful to Not-So Useful

The Unstoppable Tendency of Energy
We've said it often in this website:
Everything that happens is caused by an energy change. Energy changes form, or moves from place to place.

Energy changes are the driving force of the universe. The driving force of all energy change is the unstoppable tendency of energy to flow from high concentrations of energy to lower concentrations of energy.

This is the first thing to understand about the 2nd Law. Energy always and inevitably flows from higher concentrations to lower concentrations.

The next thing to understand is that every time energy changes or moves, some of it, or all of it, becomes less useful. That is the unchangeable result of becoming less concentrated.

One thing leads to another - always, always, always - cause and effect. Energy always, and only, goes from more concentrated to less concentrated. Being less concentrated also means less useful. The cause of everything that happens is a difference in energy concentration. The effect of every energy change is less useful energy.
Get it? Every time energy moves, it moves to a condition in which some of it or all of it becomes less useful.
Less Concentrated = Less Useful


When it comes to doing useful work like running or powering a car, highly concentrated energy is easier to use and more efficient than low concentrations of energy.

Foods like carbohydrates and fats, and liquid fuels like gasoline (petrol for some of you), have highly concentrated potential energy stored in their chemical bonds. They have a lot of energy in a fairly small space and it is efficient for us to convert that concentrated energy into useful energy to keep our bodies and our machines going.

Our Machines Convert Concentrated Energy to Less Useful "Spread-Out" Energy
For example, when a diesel engine turns a generator, the engine's mechanical energy is converted into electricity. The electricity is still pretty concentrated, but not all of the mechanical energy is converted to electricity. Some of the energy "leaks" away through friction and heat. The rotating generator pushes air molecules around. The generator wires are heated up by internal friction as electrons flow through them. The generator cooling fan heats up more air by blowing it over the generator to keep it cool. All of this heat "spreads out" into the air around the generator. The energy is still there, but is no longer useful to us.

A typical generator converts about 90 to 98 percent of the mechanical energy put into it into concentrated electricity. The other 2 to 10 percent leaks away into less useful low grade energy.

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