The Advent Calendar of Physics: Using Energy

For the sixth day of our advent countdown to Newton’s birthday, we have the first equation that really departs from the usual notation. I’ve gotten to kind of like the way the Matter and Interactions curriculum handles this, though, so we’ll use their notation:


This is what Chabay and Sherwood refer to as the Energy Principle, which is one of the three central principles of mechanics. The term on the left, ΔE represents the change in the total energy of a system, while the two terms on the right represent the work done on that system by its surroundings, and any heat energy flow into or out of the system due to a difference in temperature.

So, why is this important?

This is an equation that is subject to the Indiana Jones and the Last Crusade rule: If you choose wisely, things will go well. If you choose…. poorly, well…

Any time you’re dealing with a bunch of multiple interacting objects, it’s usually possible to choose to define the “system” such that the two terms on the right are zero. In which case, this equation becomes the more familiar statement of the conservation of energy: The total energy of the system at the start of the problem is equal to the total energy of the system at the end of the problem. A wise choice of system thus turns physics into accounting: you have a fixed amount of energy, and can change how you apportion that energy between the subsystems. It’s usually easy to calculate the energy of some of the subsystems, which makes finding the energy of the others a simple matter of arithmetic.

It’s worth writing this out in its full form, though, to emphasize that the total energy of an arbitrarily chosen system is not necessarily zero. If objects outside your system of interest are able to interact with it in ways that do work or add heat, then the total energy can change, and life becomes a little more complicated. That’s an important fact to remember for any case in which your system is smaller than the whole universe.

But does this have any deeper lesson for us? Well, the energy principle is ultimately a result of Emmy Noether’s famous theorem relating symmetry to conservation laws. The underlying reason why the total energy of a closed system (that is, one not interacting significantly with outside objects) is conserved is that the laws of physics are symmetric in time: they will work the same way tomorrow that they did yesterday. According to Noether’s theorem, this means that there must be some quantity that remains constant as you move forward in time, and that quantity is the energy.

(Noether’s theorem also explains momentum conservation: the laws of physics are symmetric in space: they work the same way in Brisbane, Australia as in Niskayuna, New York. According to Noether’s system, this means there must be some quantity that remains constant as you move from one place to another, and that quantity is the momentum.)

So, conservation of energy is much more than turning out the lights when you’re not home. It’s built into the deep structure of the universe, and one of the most fundamental properties of physics.

Come back tomorrow for the next great equation of the season, as we continue counting downt he days to Newton’s birthday.

4 thoughts on “The Advent Calendar of Physics: Using Energy

  1. I recall once reading about somebody (online, of course) arguing against evolution through a conservation-of-energy/entropy trends to a maximum argument: if evolution were true, then there would be increased order/complexity as organisms developed, which violated the entropy principle.

    As the person relating the story to me put it, this particular crank apparently forgot about the big energy-generator 1 AU from the Earth, putting additional energy into the system…

    (Memory is hazy, I may have butchered that completely.)

  2. Will: That’s a really common creationist mistake, actually. It’s one of their standard attacks.

    Part of the problem seems to be two old theological principles, described in this article on Hume (who was attacking them),

    which superficially look like the first and second laws of thermodynamics, though they’re not. They are:

    1. “Nothing can come from nothing” and
    2. “No cause can produce or give rise to perfections or excellences that it does not itself possess”.

    The first is the basis for the cosmological argument for the existence of God, and the second is the basis for believing that evolution can’t happen without divine guidance. If you squint at them, the first one looks sort of like the conservation of mass/energy, and the second looks sort of like the law that entropy always increases in a closed system, if you read entropy as disorder or imperfection. So you can translate old Christian apologetics easily into pseudoscientific language by simple substitution.

    Of course, once you try to be precise about terms and take all the real physical caveats into account, the translation breaks down. There’s no real reason to believe that conservation of mass/energy applies at the moment of the Big Bang, and it even has to be severely modified in general-relativistic cosmology, especially in the presence of dark energy or inflation; and these are precisely the situations that are relevant to the cosmological argument. And the creationist use of the second law is even worse, since entropy is not really the same thing as lack-of-excellence, and the biosphere is nothing like a closed system.

  3. This took me back to my thermo days, though sadly Emmy Noether was never actually mentioned in that class.

  4. Will:

    The law in this post is the 1st law, which doesn’t specify a direction that energy has to flow in. (∆E could be getting bigger or smaller, and Q and W could be positive or negative also.) It’s the 2nd law that says heat has to flow from higher temperature to lower, or not flow at all.

    As for the argument that life can’t exist if the entropy of the whole universe is always increasing, that goes back to where you put the boundaries of your system. If your system is everything inside an imaginary surface that encloses a human body, for instance, the entropy of everything in the system can DECREASE, so long as the entropy of the surroundings INCREASE by the same or a greater amount. This happens through heat transfer through the boundary of the system. When you add up the total entropy of the system plus the surroundings, it’s always bigger, but the system can decrease in entropy.

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