Energy Explained

[Farsight]

ENERGY EXPLAINED v2.0

 

The schoolroom textbooks told you that energy is The Capacity to do Work, and work is the transfer of energy. The words go round in circles without getting to the heart of it, and children grow into adults with no real concept of what energy is. So what is it?

 

 

Let’s start by saying that energy is the property of a thing. To illustrate this, I can talk about a red balloon, a red bus, or a red red ruby. All these things have the property that we call red. A thing can be red, but you cannot remove this red and hold it in the palm of your hand. You can remove the paint or the dye, and you can hold that in the palm of your hand, but you’re still holding a thing that is red. You cannot remove the red from the dye to hold the red in the palm of your hand. Even when you imagine red, the image in your mind’s eye is a thing. You always need a thing to be red. There is no such thing as “raw red”.

 

 

You give a thing energy by doing work on it. You “put energy into it”. The parallel between work as in physics and work as in a job are quite striking here. One is to do with energy, the other is to do with money. The money in your bank account is rather like the energy that’s at your disposal. Hence you can spend money like you can expend energy. But the money doesn’t disappear, just as the energy doesn’t disappear. Somebody else now has your money, just as some other thing now has your energy. Think about an old house, nestled in the countryside. It’s picturesque, worth a lot of money, and it’s built out of cob:

 

 

Way back when, some medieval construction crew put some energy into shifting earth and straw to make the walls of this house. They did the same with the wood, which grew out of the earth because the trees put energy into shifting water and CO². The guys made money out of that house. Somebody paid for the energy they put into it, through the work they did moving stuff around. But moving stuff around isn’t what energy is. That’s what it does, not what it is. And it does it to things, things that have mass. So we need to talk about mass and motion to talk about energy.

 

 

Consider a 10 kilogram cannonball, in space, travelling at 1000 metres per second. We talk about how much kinetic energy this cannonball has. We say KE = ½mv² and we do the maths and get five million Joules. But what has the cannonball really got? Its mass seems real enough, I hefted it into my spaceship this morning before I took off. And its motion seems real enough too, because one false move and it’ll be smashing through my viewscreen doing all sorts of damage. To find out more, I take a spacewalk to place a thousand sheets of cardboard in the path of my cannonball. Each sheet of cardboard exerts a small braking force, slowing the cannonball to a halt. This takes two seconds. We know that the cannonball will punch through more cardboard in the first second than in the second second, because it’s slowing down. So we deduce that a cannonball travelling at 1000m/s has more than twice the kinetic energy of one travelling at 500m/s. We can do the arithmetic for each second, then slice the seconds up finer and finer, and we end up realising that the ½v² is the integral of all the velocities between v and 0. But what we don’t realise, is that kinetic energy is a way of describing the stopping distance for a given force applied to a given mass moving at a given velocity. You can flip it around to think about force times distance to get something moving. Or you can think in terms of damage. But basically that cannonball has “got” kinetic energy like it has “got” stopping distance.

 

It’s similar with momentum. That’s a different way of looking at the mass and motion, based on force and time instead of force and distance. We look back to our cannonball and cardboard, and we know by definition that the same amount of time passed in the first second as in the second second. So we realise that a cannonball travelling at 1000m/s has twice the momentum of one travelling at 500m/s. But what we don’t realise, is that momentum is a way of describing the stopping time for a given force applied to a given mass moving at a given velocity. A cannonball has “got” momentum like it has “got” stopping time.

 

But wait a minute. I didn’t fire the cannonball at 1000 metres a second. I dropped it off at a handy spot out near a GPS satellite, then zipped off in my spaceship in a big fat loop.

 

 

It’s me doing 1000m/s, not the cannonball. The cannonball is just sitting there in space. It hasn’t got any kinetic energy at all. I’ve got it. But I don’t feel supercharged with five million Joules of energy coursing though my veins. So where is it? Where’s the kinetic energy gone? It isn’t anywhere really, because all that cannonball has got, is its mass, and its motion. And that motion is relative to me. Kinetic energy is not a thing. It’s just a relative property.

 

So, let’s examine this property. How do you make something move? Easy. Hit it with something else that moves. And how did you make that something else move? Where did it all start? I pitch you a baseball, you whack it with a bat, and it flies away at twenty metres per second. You made that baseball move. Now, where did the energy come from to make it move? From your muscles: “The release of ADP and inorganic phosphate causes the myosin head to turn, causing a ratchet movement. Myosin is now bound to actin in the strong binding state. This will pull the Z-bands towards each other. It also shortens the sarcomere...”.

 

 

It all gets a little complicated, but in the end it’s all down to bond angles. Bonds within molecules change, and the change releases energy. Sometimes it’s a simple single change of bond angle, something like a leaf spring letting go and giving something a flip. Sometimes there’s more than one bond angle change, in a molecule that resembles an elasticated deckchair surging from one configuration to another. And sometimes the molecule takes a rather different shape. A familiar shape:

 

 

Collagen looks like a spring because it is a spring. That’s why it’s elastic. It’s in your skin. It makes your skin elastic. You see the same shape repeated in a great many organic molecules. It’s in your muscles too.

 

 

The molecules look like springs because they are springs. That’s the size of it. The energy to move your muscles is stored in tiny compressed springs. Yes, they’re electromagnetic springs rather than solid springs, but let’s face it, that’s what all solid springs are. They are electromagnetic in nature. That’s how the muscular energy is stored. It’s the same for chemical energy, and I quote:

 

In the early 1980's it was pointed out that cubane's very high density and high heat of formation would make it an especially good explosive, especially if each carbon could have a nitro group attached. The resulting molecule would decompose to eight molecules of carbon dioxide, and four molecules of nitrogen, and release a lot of heat in the process. A cubane with a nitro group on each carbon is called octanitrocubane. Several factors are important in making a good explosive. The decomposition must be energetic. In cubane derivitives, the strain energy ensures a very energetic decomposition.

 

Did you catch that? It’s the strain energy. There’s compressed “springs” in there. It’s the same with nuclear energy, only the springs are stronger. The sun gets its energy from nuclear fusion. Squeeze hydrogen atoms together and you make helium. But when you do, twang, something lets go, and things spring out between your fingers, things like photons. Yes, there’s more to it than that because actually it’s a three-stage process and the recipe goes like this: 4 1H + 2 e --> 4He + 2 neutrinos + 6 photons.

 

 

But the gist of it is simple. And it’s even simpler for matter/antimatter annihilation. Take one electron. Add one positron: bang. Two springs let go so totally that they’re just not there any more, and photons come bounding out at gamma-wave energies.

 

 

A photon is an interesting thing. Particle physics comes with mental baggage that says it’s a speck, a point, a particle. But we have long-wave radio which reminds us that photons can be 1500m long. A photon isn’t a speck. It’s more like a slink in a slinky spring. No, not a slink, because a photon isn’t a longitudinal wave like a sound wave. A photon is a transverse wave. It’s more like a shimmy.

 

 

The slinky spring here is space, that vacuum void with its permeability and permittivity. A photon is like a ripple on an electromagnetic ocean between the stars. A boat on this ocean can ride the ripple and the ripple just passes on by. But tie that boat to the sea bed with a rubber rope, and you can capture the energy of the ripple, and you can save it in starch or oil.

 

 

Space isn’t really an ocean. It doesn’t have a surface, it isn’t a liquid, and there’s no substance to it. But mathematically speaking, space can be likened to a block of ghostly transparent rubber. And a photon is a stress travelling in it, a transverse wave of vacuum stress. A tree can capture a photon via photosynthesis and use it to make leaves and branches, and then you can use the wood to build your house. It all comes down to springs, elasticity. The energy is in the compression, the pressure, the stress. And in the end, this stress is somehow in space itself.

 

 

It’s not obvious. You don’t always realise the energy is there. But then you get a “phase change”, and something happens. Then you realise it was there all along. Like you realise that the cool night air was moist when the water vapour condenses out and it starts raining warm rain. Sometimes it’s more dramatic. Sometimes it’s called “symmetry breaking”. It’s rather like a big hidden spring letting go. The Big Bang was something like that.

 

 

All these springs and elastic are just analogy of course. Analogies are based on the tangible things we experience with our senses, and these are not the things of the subatomic world. So analogies can be dangerous, like too much butter. But the Universe will wind down to the sameness that we call entropy. The stresses in space will eventually even out. Because in the end that’s what energy really is. Stress in space. It’s in the space between the stars, and in the spaces in the atoms. It’s everywhere.

 

In physics, stress is the same thing as pressure, which is the same as negative tension. To quantify energy and get the correct dimensionality, we also have to know that stress is force per unit area, and energy is force multiplied by distance. We have to multiply a stress by an area and then by a distance to get energy. So we multiply stress by volume. We have to multiply the degree of stress by the volume of space that is stressed. Then we can calculate how much energy is there. And then we’ve got a new definition of what energy really is:

 

Energy is the capacity to do work, and is in barest essence a volume of stressed space.

 

That’s what it is. Energy is a volume of stressed space. That’s why you can’t hold it in the palm of your hand. You know you can’t hold stress in the palm of your hand, and a volume of it doesn’t make it something you can get hold of either. That’s why you can’t hold pure energy in the palm of your hand. Just as you can’t hold a photon in the palm of your hand. A photon is a travelling stress, it’s travelling in space, and it has to travel. It isn’t pure energy, because it travels. Nothing is “pure energy”, just as there’s no such thing as pure pressure. Because energy is the property of a thing, even when it’s the very last property that makes a thing the thing that it is.

 

But oddly enough, you can hold energy in your hand. It’s a subtle difference, but it’s very simple. Just squeeze a fist. Use your right hand. Squeeze it tight. Now touch your left thumb to your right thumb. Feel that blood pressure. Now look at the volume of your fist. Stress is the same thing as pressure, and there’s a volume of it in that fist. Your fist has energy. And if you swing that fist, it has even more.

 

As to how, it’s all to do with pushing little circles into spirals and making little springs. But to explain that, I’ll have to explain mass.

 

[thedarkshade]

Huge Huge Huge, but awesome. Here's another "drop" compared to what farsight wrote!

[timo]

The schoolroom textbooks told you that energy is The Capacity to do Work, and work is the transfer of energy.

Are you sure of that (meaning: Did you actually check any schoolbooks to verify that claim)? Only schoolbook I have on my shelf is Resnick, Halliday, Krane "Physics", which sais

Consider a particle acted on by a constant force F and assume the simplest case in which the motion takes place in a straight line in the direction of the force. In such a situation we define the work W done by the force on the particle[/i'] as the product of the magnitude of the force F and the magnitude of the displacement s through which the force acts. We write this as W=Fs.

(emphasizing copied from the book, not done by me).

[thedarkshade]

Are you sure of that (meaning: Did you actually check any schoolbooks to verify that claim)?

I think he is right!

When we talked about energy on my physics classes the definition they gave to us was: "The ability to perform an action!"!, and that's it!

I went into further details to learn more but at school they only said that and moved into formulas!

[timo]

When we talked about energy on my physics classes the definition they gave to us was: "The ability to perform an action!"!, and that's it!

That has nothing to do with what I said for two reasons:

1) I was talking about schoolbooks, not what you recall from your classes. If Farsight had said "as I recall/think/imagine ..." rather than "schoolroom textbooks tell you ..." I'd have had no problem with the statement.

2) I was saying that a sample schoolbook does not define work via energy. This then breaks up the circular reasoning Farsight sees. Whether energy is then defined via work (I assume by "perform an action" you meant "do work" - action also exists as a physical term but has a different meaning) or not does not matter anymore.

[thedarkshade]

That has nothing to do with what I said for two reasons:

1) I was talking about schoolbooks, not what you recall from your classes. If Farsight had said "as I recall/think/imagine ..." rather than "schoolroom textbooks tell you ..." I'd have had no problem with the statement.

Well in school they teach you according to the books implemented by the government, so if they've said "Energy is the ability to perform an action", that is because that was written in the books that we were supposed to finish that year!

 

edit: and I just read one of Richard Muller's writings and the first line into Definitions chapter is: Energy is the ability to do work!

[swansont]

I suppose one could ask Farsight to define all of his terms without any circular/self-reference. Just for the irony.

[thedarkshade]

I found this while browsing into "Physics for Future Presidents" and I thought it could be interesting (at least I found it so:doh:) to this subject.

 

Just check under Energy - Energy and Explosions - Amount of Energy, and you will find a table that contains some interesting data!