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# Energy types

• Some energies are stored and “waiting” to be released, such as chemical energy (fuel in a car) and elastic energy (in a taut crossbow spring). They will push on their surroundings – if released; they have the potential to cause motion. Let’s invent the term potential energy for all such stored energies.1
• Other energies are already released and “on their way”, such as kinetic energy (“motion energy” enabling the thrown ball to knock stuff over).2

Saying ‘chemical energy’ and ‘elastic energy’ or ‘chemical potential energy’ and ‘elastic potential energy’ are then synonymous – you might see these terms used interchangeably in different contexts and literature.

Looking around in our world, there are many types of energy. A quick brain storm gives us a list like in the table below (more can be found in Resource: Energies overview).

Energies
Gravitational potential energy... when things are high upBook on a shelf, Earth-moon attraction
Elastic potential energy... is "spring-back" energy Spring, trampoline
Electric potential energy... when charges want to flowCircuits, lightning
Chemical potential energy...... in chemical bondsFuel, food/nutrition, gun powder
Kinetic energy... when things moveThrown ball, moving car
Thermal energy... when things are hotCharcoal, stove

With such many different types of potential energies, let’s choose a common symbol to mean ‘potential energy’, such as the letter $U$.3

If we want to, we can easily categories these energies in different ways. Typical examples are

• internal energy (thermal, often-times chemical),
• mechanical energy (gravitational potential, kinetic, elastic potential),
• electromagnetic energy (electric potential, magnetic potential),

We might do so now and then when convenient.4

References:

1. Kinetic Energy, Discussion’ (web page), Glenn Elert, The Physics Hypertextbook, www.physics.info/energy-kinetic (accessed May 23th, 2020)
2. On the General Law of the Transformation of Energy’ (article), William J. M. Rankine, The Philosophical Magazine and Journal of Science, vol. 5, 4th series, issue 30, 1853, books.google.dk/books?id=3Ov22-gFMnEC&pg=PA106 (accessed Jun. 12th, 2020), chapter 18, page 106
3. The World of Physics’ (book), John H. Avison, Nelson, 2nd ed., 1989, books.google.dk/books?id=DojwZzKAvN8C&pg=PA148 (accessed Jun. 12th, 2020), ISBN 0-17-438733-4, chapter 8.4, page 148
4. Potential and Kinetic Energy’ (web page), Math Is Fun, www.mathsisfun.com/physics/energy-potential-kinetic.html (accessed Jun. 12th, 2020)
5. Potential Energy Definition and Formula’ (web page), Anne Marie Helmenstine, ThoughtCo., 2019, www.thoughtco.com/definition-of-potential-energy-604611 (accessed Jun. 12th, 2020)
6. Potential energy’ (web page), Dr. Jeff Cruzan, xaktly, 2016, www.xaktly.com/PotentialEnergy.html (accessed Jun. 12th, 2020)
7. Sears and Zemansky’s Univesity Physics with Modern Physics’ (book), Hugh D. Young & Roger A. Freedman, Pearson Education, 13th ed., 2012