Energy & Entropy

Energy Basics

Energy is a one-way "street." We cannot recycle energy. Although at the first law of thermodynamics states, energy cannot be destroyed or gained within the context of the universe, it can change forms. In general, the various forms of energy tend to end up dissipating as heat energy.

1st Law of Thermodynamics = energy cannot be gained or lost. The universe has a set amount of energy. This energy can change forms, but cannot be destroyed. When we

2nd Law of Thermodynamics = entropy (or a move towards disorder) increases over time within a particular context or system. If we turn on the heat in our home, then shut off the furnace, the heat energy will slowly dissipate.

3rd Law of Thermodynamics = the entropy of a system refers to a move from order to disorder. Wave or electrical energy are more tightly ordered forms of energy. However, over time this "orderliness" degrades or dissipates as it changes form and leaves a particular system. On Earth, the solar energy that enters into the global system eventually dissipates as heat energy, which leaves the Earth system altogether. However, with increases in carbon dioxide in the atmosphere, increasing amounts of this heat energy is reflected back into the atmosphere (e.g., global warming, "greenhouse" effect).

Energy refers to the ability to move matter or carry-out a chemical reaction. In ecosystems, the most common forms of energy are:

  • chemical energy — such as the energy stored in food;
  • wave (solar) energy — solar energy is converted to chemical energy (food) by chlorophyll in plants; solar energy also heats up land and water masses, which results in water currents and wind patterns; solar energy also provides for evaporation;
  • mechanical energy — wind and moving water erodes land masses and carries materials from one location to another
  • thermal (heat) energy — as mentioned above, heat contributes to air and water currents; heat energy is commonly the final form of energy resulting from the conversions of other forms energy (e.g., if we exercise, we utilize chemical energy from our food stores to make our muscles contract; at the same time, we generate heat energy from these chemical reactions)

Energy as the Monetary System of Ecosystems — When thinking about ecological systems, energy is the ecological equivalent of money in economic systems. We earn money in order to carry out certain functions like buying food, paying our rent or mortgage, etc. However, before we even start spending our money, a certain amount of money is taken from our pay for taxes and benefits. What we receive in our paycheck is our "net income." In an ecosystem, an organism has to expend energy "dollars" to get its food. It expends addition energy dollars to get water, breathe, reproduce, protect itself or build a shelter, and conduct other activities.

Net Energy is the actual amount of energy that is acquired by an organism. A predator utilizes considerable energy in chasing and eating its prey. This predator's net energy is the amount of energy it has taken in after eating its prey minus the amount of energy it expended acquiring its food. Net Energy is often referred to as Energy Return on Investment (EROI). We can use this concept to analyze our own energy consumption. How much energy is used in constructing a particular item, in growing and transporting food, in building a solar power system, etc., and how much energy to we actually get in return. It's a similar idea to thinking about our own economic situations. If we earn $12 an hour, but have to spend $10 an hour on childcare, what is our return on "investment." Or, if we spend $23,000 on purchasing a hybrid car that gets 40 miles per gallon, what is our EROI over keeping our old car that we bought for $15,000, which gets 30 miles per gallon? How much much money do we save over using the old car? However, to get a much more thorough understanding of the situation, we will need to determine the amount of the materials that go into building the new car. How much energy was expended in mining the raw materials, refining and transporting these materials, and building and shipping the vehicle? After figuring out this energy cost, we can figure how much energy is required to operate the new vehicle, including standard maintenance. Then, of course, we have to figure out what the actual lifetime of the vehicle is likely to be… 10, 15, 20 years? Is our new hybrid vehicle likely to save energy over our older vehicle?

Of course, we also need to consider entropy. How much energy is lost in all stages from obtaining materials to using our new car, computer, or even food? When we drive a car, most of the energy derived from burning the fuel is lost as heat energy, which is the entropy of the system. In traditional gasoline engines, only about 25% to 30% of the energy from the chemical reactions (burning the gasoline) is used in moving the vehicle. The rest of the energy dissipates as heat energy. Hybrid vehicles increase the efficiency in terms of gasoline used, but the electrical power is generated somewhere with fairly low efficiency of maybe 33%. Even though, the vehicle operates at lower fuel costs, the total energy expenditure is about the same.


Thermodynamics Review For Ecology

A brief explanation of energy and entropy from the perspective of ecosystems.

Ecology: Laws of Energy



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