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Thermodynamics: First Law of Thermodynamics

First Law of Thermodynamics

Before reading this section you should have an understanding of forms of energy and energy transfer.

The first law of thermodynamics, also known as the conservation of energy law, will relate work, heat, and total energy together. In general, the first law states that energy cannot be created or destroyed, but instead can only change forms during a thermodynamics process. An example of this would be the even exchange of potential energy to kinetic energy. What I mean by this is when the potential energy of a system decreases then the kinetic energy must increase by the same amount that the potential energy increases, assuming there is no energy loss to friction or drag. This is actually the basic fundamentals of how a rollercoaster works, which is why the first drop is always that highest.

The above example would actually be considered an adiabatic process since the potential energy was transferred directly into kinetic energy. Recall from previous sections an adiabatic process is a process that does not involve any heat gain or loss, so any change in energy has to be due to work. So back to the roller coaster; if the above was to hold true in real life then all roller coasters would be able to consistently drop from the same height, since the kinetic energy should transfer back to potential energy. However, we all know this isn't true, but the first law says energy cannot be created or destroyed. So what happened to that lost energy? Well if energy cannot be created or destroyed then that energy must have gone somewhere, and that would simply mean the energy went into overcoming friction and drag. When friction or drag occurs the object will start to heat up as energy is transferred into the system. This means in real life there is more to a roller coaster then kinetic energy and potential energy, and friction must be considered in the energy transfer process.

Now the first law only focuses on a net change in the total energy of system. So you actually do not need to know what the total energy of the system is, which means to make life simpler you can set an arbitrary zero value for total energy. So this means that your change in total energy, according to the first law of thermodynamics, will be represented by your change in kinetic energy, potential energy, internal energy, and heat if the process isn't adiabatic. This can be seen in equation 1.

Change in Total Energy equation (1)

U = Internal Energy

KE = Kinetic Energy

PE = Potential Energy

Q = Heat Transfer


Forms of Energy Transfer

Energy can be transferred in three forms; two of which you should be familiar with if you read the previous articles. These forms are heat, work, and mass flow. Heat as you should recall is a transfer of energy through conduction, convection, or radiation due to a temperature difference. On the other hand work is an energy transfer that occurs when there is no temperature difference present. Mass flow you may not be as familiar with. Energy transferred from mass flow is due to a fluid passing through the system. The fluid passing through the system could potentially remove energy, or add energy to the system depending on the temperature difference of the fluid moving through the system, or any viscous effects that might be present between the fluid and the system. Mass transfer cannot occur if the system is a closed system.

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