The First Law of Thermodynamics

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Thermodynamics Gateway Page
In this module:
Kinetic and Potential Energy
The First Law of Thermodynamics
Systems and Sign Conventions

Do you remember the circus act where an elephant would stomp on one end of a plank, sending a performer shooting up into the air until he or she landed on the shoulders of a fellow performer? Let's look at this act from the perspective of kinetic and potential energy.

Click on the step numbers below to see how the circus act can be explained using kinetic and potential energy. Click on the mouse icon at left to clear the images and text.

Both the elephant and the performer are on the ground. Let's call this zero potential energy. (Because potential energy is always measured relative to some position, you can always define some convenient position as zero potential energy. It is often convenient to define ground level as zero potential energy, as in this example.)
The elephant uses some energy (potential energy) from the hay it ate that morning to raise up its front legs. Now the front part of its body has a higher potential energy.
The elephant lets its legs drop, converting the potential energy into kinetic energy.
When the elephant lands on the plank, the kinetic energy is transferred to the performer, who shoots into the air.
As the performer gains height, he slows down until he lands on the shoulders of his fellow performer. When he does, all his kinetic energy has been converted to potential energy.

Do you see all the different forms of energy there are in this example? This example illustrates an important General Rule in chemistry, the First Law of Thermodynamics.

Energy can be neither created nor destroyed. It can be transferred from object to object and changed into different forms, but the total energy before and after any process is always the same.

Think of some event or process that has happened to you today. It could be driving to the store, eating breakfast, doing your homework, or anything else. Try to write down as many transfers and interconversions of energy that happened during that process as you can.

The First Law of Thermodynamics