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How the First Law of Thermodynamics Keeps You Alive

Updated on November 7, 2016
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Katie graduated with both a BA in Chemistry from BYU and a BA in Spanish from UVU in 2016. She graduated from medical school in 2020.

Thermodynamics Is Important to the Basic Sciences

Thermodynamics Is Important to the Basic Sciences
Thermodynamics Is Important to the Basic Sciences | Source

The First Law of Thermodynamics

The first law of thermodynamics states that the change in internal energy, ΔU, of a system is equal to the work done on the system (w) plus the heat transferred to the system (q). This is stated mathematically in the following equation:

First Law of Thermodynamics: ΔU=q+w

U is internal energy, q is heat, w is work

Defining Internal Energy

The internal energy of a system, U, is the sum of all the potential and kinetic energy contributions of the atoms, ions, molecules and compounds of the system and their interactions.

At a given temperature, the total energy of a system is equal to the sum of the kinetic energy, the potential energy and the internal energy of the system, as shown in the following equation:


KE is kinetic energy, PE is potential Energy, and U is Internal Energy

The First Law of Thermodynamics and the Law of Conservation of Energy

The first law of Thermodynamics is a restatement of the law of conservation of energy which says that energy can be neither created nor destroyed, but can be transformed from one form of energy to another.

The first law of Thermodynamics also establishes work and heat as the two forms of energy flow. Work is the application of a force across a distance or the net movement of matter. Heat, as described above, is the flow of energy by random collisions of molecules and flows down the thermal energy gradient from hotter to colder objects.

How the First Law of Dynamics Keeps you Alive

An important example of the first law of thermodynamics is the exchange of energy throughout the biosphere. Not only do organisms exchange energy, but energy is transformed into various forms to meet the organisms’ needs.

Plants have specialized organelles, called chloroplasts, which convert carbon dioxide, water and light energy from the sun into chemical energy in the form of glucose and oxygen. This process is called photosynthesis and is outlined in the equation below. The plant then converts the chemical energy stored in glucose into various forms of chemical and mechanical energy as necessary for growth.

When humans and animals eat plants and other animals they extract glucose for their own energy needs. These creatures also obtain oxygen through respiration. The oxygen and glucose can then be used by that organism for chemical, thermal, electrical and mechanical energy as needed.

In the process of utilizing the glucose and oxygen, animals give off carbon dioxide. Although it’s waste for humans and animals, plants need this carbon dioxide to complete photosynthesis. Thus, respiration and photosynthesis form a cyclic symbiotic relationship. As you can see, no new energy is created and no energy is destroyed; energy simply changes forms, just as the first law of thermodynamics predicts.

Photosynthesis: 6CO2+ 6H2 O+light energy→6O2+6C6H12O6

CO2 is Carbon Dioxide, H2 O is water, O2 is Oxygen, C6H12O6 is glucose

How the First Law of Thermodynamics and Hess' Law Relate to Your Body's Proteins

Hess’ law is closely related to the first law of thermodynamics. It states that the heat absorbed or evolved in a process must be the same regardless of the manner in which the process takes place.

Hess’ law also states that the sum of the changes in energy that accompany each individual step in a process is the total change in energy brought about by that process.

This means that not all steps must be thermodynamically favorable for a process to occur. Steps that aren’t thermodynamically favorable can occur when they are coupled with thermodynamically favorable steps.

This concept can be seen in the biological world in the folding of proteins. As long as the final shape of the protein is the most energetically favorable conformation, it’s okay if some of the interactions between amino acids are energetically unfavorable. These unfavorable relationships just have to be outweighed by the energetically favorable relationships around them.

For Information on the Zeroth, Second and Third Laws of Thermodynamics

You can view information on the Zeroth Law of Thermodynamics here and information on the Second and Third Laws of Thermodynamics here.

The First Law of Thermodynamics on Khan Academy


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