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Energy: The Quantity of Motion (2012)

Page history last edited by Joe Redish 11 years, 2 months ago

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You've probably heard the term "energy" for most of your lives. When you were a toddler your parents might have complained, "he/she has so much energy I just can't keep up with him/her." The term has lots of everyday meanings.

 

But in your science classes you certainly will have heard the term in its more technical sense -- these molecules have a particular binding energy; ATP is the energy currency of the cell, and so on.  But what is energy really?  For a physicist, in the end everything is energy -- because we have learned that mass is a form of energy.*  But that doesn't help very much.  Here are a few key ideas to get us started.

 

  • At beginning, the best starting point for building up the concept of energy is motion. The place where historically the idea of energy started and the place where we will start is with the sense that energy is something we will associate with moving objects.   Energy is not simply another quantity we can associate with a moving object, in addition to things we already learned such as velocity, acceleration, momentum.  As you will see thinking about the energy of a moving object will allow us to tackle and quantify motion for a lot more situations.
  • At the end, energy becomes the universal currency of physics -- it's stuff that can, in principle, be converted into motion or that has come from motion. The key here is that it is possible to think of energy as something that is conserved.  While energy can be transformed from one form to another (and may be more or less useful in different forms), the total amount of energy will be the same. This is a powerful idea and will give us many useful tools for understanding complex processes.

 

The way this works is that we follow a process that has turned out to be immensely useful in organizing our thinking about physics and indeed about much of science.

 

We look at our usual "simplest possible cases" and decide how we might quantify the concept of motion. We find two ways of doing this -- momentum and energy. 

 

One of them, momentum helps us think about collisions between two objects - conservation of momentum helps us tremendously to think about collisions in a simple way - even all collisions of all atoms in a gas will conserve momentum so a gas does not spontaneously start to move. 

 

However, momentum is not very helpful when tackling for example the following problem:  "How fast will a skateboard be at the bottom of a hill, 100 feet below the starting line"  The skateboard interacts with the earth, so conservation of momentum tells us that the momentum of the skateboard and earth combined is conserved, but that's not helpful since we cant measure the momentum of the earth easily...  In this situation, and many others, we will be able to use a concept of energy conservation in the following sense:  The ability to generate motion at the top of the hill will be turned into actual motion at the bottom of the hill.  The key is that we will be able to measure transformations of the energy of motion into other forms.

Every time we find a situation that looks like the sum of the energies we have defined is not conserved, we try to discover where the energy is hidden.  In every case this has led to the discovery of lots of new kinds of energy and the restoration of the idea that energy is conserved.

 

This process sounds circular. What good is it if every time our law fails we introduce a term to save it? Isn't this sloppy science?

 

Well, no. We might decide this was a useless process if we had to introduce a new kind of energy for essentially every new experiment or phenomenon we looked at. But we start with the energy of motion (kinetic energy) and quickly add the energy of relative position (potential energy). We soon discover that something being hotter can be considered a kind of energy (thermal energy) and corresponds to an increase of an internal hidden motion of an object's atoms and molecules. We then discover that there is energy stored in the structures of atomic bonding to form molecules (chemical energy) and this, in the context of quantum mechanics, can be interpreted as kinetic and potential energies distributed probabilistically. We know that light can carry energy, and finally, Einstein suggests that mass can be considered a form of energy. And that's where it now stands.

 

So over a period of 400 years we have invented 6 kinds of energy.** And they have served us to describe millions of experiments and situations. This is very much a part of our modeling of the world and science's ways of knowing. We create ways of thinking about phenomena that are appropriate for those phenomena (choosing a channel on cat television) and then try to stitch our understandings together (building coherence). With energy, we have developed one of the most powerful tools in the scientific arsenal -- and it all begins with thinking about motion.

 

Follow-ons:

* The energy content of mass is given by Einstein's famous equation, E = mc2 where c is the speed of light in vacuum, a universal constant = 3 x 108 m/s. This implies that 1 kg of mass has an energy content of almost 1017 Joules -- a huge amount. The conversion of a tiny fraction of an atom's mass into energy is responsible both for the energy we extract from nuclear and chemical reactions.

 

** As of this writing, the scientific community has learned that the galaxies of our universe seem to be moving away from each other at increasing speeds -- accelerating rather than slowing down as would be expected from the fact that they attract each other gravitationally. Some source is providing huge new energies of motion to objects of galactic scale. If, as we expect, there is still a conserved total energy, this implies that there is a new kind of energy that is being transformed into kinetic energies of galaxies. We don't know what this is, but for know we are calling it "dark energy". Stay tuned!

 

 

Joe Redish 7/29/11

Wolfgang Losert 11/14/12

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