The Laws of Thermodynamics

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Although there is a multitude of solar radiation energy available to us, there are some important limitations to the energy that can be used to fuel all forms of life. The Laws of Thermodynamics explain these limitations. The First Law of Thermodynamics explains that energy can be conserved (by changing from one form of energy to another), but it cannot be created or destroyed. This means that the total amount of energy that is in an isolated system (such as an engine or a generator) remains constant (conserved) over time.

However, the total amount of useful energy output of an isolated system is never equal to the total energy input. This is because some of the input energy will be lost as dispersed heat as it is processed. For example, when energy is being processed through an engine, some of the input energy will be absorbed by the container, causing the molecules that make up the container to vibrate and radiate energy out to its surroundings as dispersed heat. Friction is another example; some energy taken into an engine system will be lost as friction, such as when a piston moves a wheel. There will always be energy lost to the surroundings and energy lost due to friction.

Therefore, using the laws that govern the conservation of energy as our basis, we can summarize the energy balance of an engine in the following equation:

Energy input = energy output + energy lost to friction + energy lost as heat to surroundings

Consequently, it is never possible to obtain as much useful energy output as the total amount of energy input. The efficiency of an engine is therefore defined as the ratio of energy output to energy input, or:

Efficiency = Energy Output/Energy Input

The fact that no real process can ever be completely efficient is a result of the Second Law of Thermodynamics, which explains how the process used by an isolated system to transform energy naturally converts some of its input energy into energy of low quality that cannot be used.

Looking Ahead

In the Energy and Action section ahead, you are invited to do an inventory of incandescent bulbs in your home or school and devise a replacement plan.

Through conversions of energy from one form to another (such as from gasoline to the kinetic motion of a car), useful energy is “lost” as heat. For example, only 15% of the energy in gasoline released to power a car is actually used to move the car forward, while 85% of the energy input is converted into heat that dissipates in the surrounding air. This dissipated energy cannot be captured. Therefore it cannot be used.

Another example is the incandescent light bulb, which only converts 5% of the input energy into light. The remaining 95% of the input energy is lost as dispersed heat. In fact, most energy transformation processes are very inefficient, including photosynthesis, the process by which plants convert solar energy into chemical energy, and which supplies food energy to support a food web.