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The Laws of Thermodynamics



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The laws of thermodynamics stem from the study of work, energy, energy transfer, and the relationship between them. Initial work with these theories began in the late 18th century but gained prominence in the 19th century as people sought more efficient machines and engines. Ultimately, the goal was to have a perpetual motion device that would never run out of energy but when this was proven impossible, the laws of thermodynamics were soon developed. There are three basic laws of thermodynamics and then a fourth law which was discovered after the initial three.

First law

The first law of thermodynamics basically states that any heat added to a system will either cause a change in the system’s internal energy or cause the system to perform work. There may also be a combination of the two happening. Similar to the conservation of energy, heat cannot also be created or destroyed while going into a system. It can only be transformed into another type of energy.

This law can be worked into a mathematical representation which is shown below.

Ef – Ei = delta E = Q – W

Following from left to right, the final energy (Ef) of the closed system minus the initial energy (Ei) of the closed system equals the change in internal energy of the closed system (delta E), which in turn also equals the amount of energy transferred (Q) minus the work performed (W). The Q is positive if energy is transferred into the system and negative if energy is transferred out of the system. Also, the W is positive if work is being performed by the system or negative if work is being performed on the system.

Second law

The second law of thermodynamics deals more with the restrictions placed on the system and what is possible or what can be done. There is an initial energy and if no energy enters or leaves the system, then the final energy will always be less than that initial state. This is true for all energy transfers and is referred to as entropy. A closed system will either have constant entropy or the entropy will increase because energy always moves away from its source. Energy transfer will happen in a single direction and this will not be reversible. A system will also never be 100 percent efficient because energy will be lost or dispersed as heat.

An example of the second law is a person walking on a frozen lake and they fall through the ice. That person had an initial energy state when they fell into the lake and they can’t get it back unless energy is provided to them. The closed system is the lake and the person. As the person struggles to get out of the icy water, their energy is expended performing work on the system to get out. Also, heat energy is dissipated into the cold water as the person’s body temperature lowers. Because of entropy, the energy in the person’s body moves into the cold water as heat loss. That person cannot get back any heat from the cold water.

With the example, the terms Q and W in the first law equation cannot both be positive which is true for all systems. In this case, heat will dissipate away from its source (into the system) and work will be performed on the system. Unless that person performed enough work to get them out of the water, then they won’t survive. That person doesn’t have perpetual energy or motion; the energy in the body will eventually all be converted to work or dissipated into the cold water as heat energy. Unless energy is added into or out of the system, then there will be no more energy and the person’s energy will never be higher then their initial state when they first fell into the water.

Third law

The third law of thermodynamics has to do with absolute zero which is the lowest possible temperature; zero in the Kelvin scale and -273 degrees in the Celscius scale. At this temperature, there is no motion as molecules stop moving. The internal energy of a system is therefore zero and the entropy is also zero. People have yet to reach any state of absolute zero and the third law also states that it is impossible for a system to reach this state in a finite number of steps or operations. Therefore, as the temperatures approaches absolute zero, entropy approaches a constant (zero) but is never able to reach this state.

Zeroth law

The final law was discovered last but because of how fundamental and important it is, the law is typically referred to as the zeroth law because it should come before the others. This law states that two systems at equilibrium with a third system are therefore at equilibrium with each other. So basically if A=C and B=C, then similar to the transitive property in math, A must equal B.

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  • InfoBoxCallToAction ActionArrowhttp://www.physicsplanet.com/articles/three-laws-of-thermodynamics
  • InfoBoxCallToAction ActionArrowhttp://www.grc.nasa.gov/WWW/k-12/airplane/thermo1.html
  • InfoBoxCallToAction ActionArrowhttp://www.physics4kids.com/files/thermo_laws.html
  • InfoBoxCallToAction ActionArrowhttp://www.physicalgeography.net/fundamentals/6e.html