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Asked by shobhit to Chris, Dave, David, Fiona, Jack on 22 Jun 2013.
Question: what ENERGY actually is ?
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David Freeborn answered on 22 Jun 2013:
Hi shobit,
This is an interesting question. Today, I think the best answer would be “energy” is “information”. Energy tells us how much information is stored in something. This is why, as we create more complex systems, say atoms are made from smaller subatomic particles, the energy increases. We are creating a more complex structure that encodes more information.
This isn’t an easy thing to grasp. But we can create some simple “thought experiments” that we can use to show there is a deep relationship between information and energy. In particular, the more information we have about any physical system, the more “useful work” or “energy” we can extract from it. This is how heat engines work, for example, and it was by thinking about heat engines that the deep relationship between information and energy was first discovered.
Let’s imagine we have a pump, with two sides, Left and Right (L and R). Each of the sides has one piston attached. We can “do work” or “use up energy” against the system by pushing the pistons in, and compressing the air inside. The system can “do work” or “give us energy” if the air pushes out against the pump.
To keep things simple, let’s suppose there is only one molecule of air in the system, and it can be either on the two sides, L or R. If we don’t know what side the molecule is on, we have no information about the system. Then, it’s impossible to extract any work from the system. On average, if we push either piston in, we might be pushing against the air molecule and doing work. That will be precisely the same as the work the air molecule would do to push the pump back out.
If we know where the air molecule is, we can do work against the system. We can push in the piston on the side which the air molecule is *not* sitting. Because the air molecule isn’t there, we can push the piston in, without costing any energy. Then the air molecule will eventually move, and push back against the piston, and give us energy “for free”.
If we think there is a 2/3 chance of the air molecule being on the Left side, and a 1/3 side of the molecule being on the Right side, then on average we can extract a little bit of work from the system, but not as much as if we had total knowledge about where the molecule is.
It’s easy to generalise this to a more complex system, with many different particles, or many different locations the particles can be. The same principles apply to every possible system.
So the more “information” we have about any system, the more “energy” we can extract. But this is just a special case of an even deeper relationship. Physicists believe pretty energy is just a way of describing the information content of a physical system. In fact, one new theory of gravity is trying to explain the force entirely using information-theory.
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Chris Mansell answered on 22 Jun 2013:
This is how a physicist called Richard Feynman explains what energy is.
“There is a fact, or if you wish, a law, governing all natural phenomena that are known to date. There is no known exception to this law—it is exact so far as we know. The law is called the conservation of energy. It states that there is a certain quantity, which we call energy, that does not change in manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same.”
The following are examples of how we calculate how much energy there is in different situations.
The kinetic energy of a particle (not taking the theory of relativity of particle into account) is its mass multiplied by the square of its speed divided by 2.
The amount of a potential energy that a particle has in a gravitational field (e.g. the Earth’s gravitational field) is its mass multiplied by its height above the ground multiplied by the gravitational constant of the gravitational field.
The amount of electrical potential energy that two charged particles is equal to the product of their charges divided by (4 pi x the distance between the charges x the permittivity of the whatever is in-between them).
The magnetic potential energy a particle is its magnetic moment multiplied by the magnetic flux density.
The electromagnetic energy of a photon is equal to Planck’s constant multiplied by the speed of light divided by the wavelength of the photon.
The amount of energy that would get released in a process where there is less mass at the end than there is at the beginning is the amount of mass that is lost multiplied by the square of the speed of light.
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Jack Miller answered on 23 Jun 2013:
Energy is a weird thing, and I guess it’s useful to explain it in slightly different terms from the answers (both of which are true!) above.
Energy is an example of a conserved quantity — the total energy in a system before something happens has to be the same as afterwards. There are other examples of conserved quantities: electrical charge is a good one, momentum, and (if you temporarily ignore fast things moving close to the speed of light and nuclear reactions) mass is conserved too. Why?
There’s a great theorem by a much-under-appreciated German mathematician, Emmy Noether, which states the following: if the system you’re studying has a symmetry that’s always true, then there are corresponding quantities whose values are conserved properties.
It turns out that you can show that the main properties of conservation of energy and momentum arise just from simple facts about our world. Let’s consider dropping a ball from a height of five metres onto a slab of concrete. It doesn’t matter if I drop the ball in London, Manchester or Osaka: if the ball, concrete and everything else are identical, the ball will bounce in the same way. The laws of physics don’t care where I am — there’s no universal coordinate system that somehow cares where I am. This gives rise to what’s called a symmetry — the laws of physics are invariant under translation. I can drop a ball wherever the hell I want, and the same thing happens. This, believe it or not, is what gives rise to the conservation of momentum.
The same thing is also true for time. I can drop a ball now…or I can wait a week and drop it then. The same thing will happen. The laws of nature don’t care (all other things being identical) what my watch says. This is another symmetry; things are invariant under time translation. This is what really gives rise to energy being a conserved quantity.
When moving things get closer to the speed of light, time and space become mixed (and we have to be careful to use relativistically correct forms of momentum, as momentum and energy also become intertwined), and the above isn’t necessarily true per se. However, there’s another relativistically conserved quantity (called the stress-energy tensor) which actually generates ‘correct’ versions of the above. In the limit of slowly moving things, it’s exactly the same as the above.
Hope the different perspective helped!
All the best,
— Jack
Comments
shobhit commented on :
@david- well you must be familiar with Feynman’s lectures on physics in which he gave an example of a mother and child to tell his view on what energy actually is.. and at the end he said,” i don’t know “like newton said for gravity..but my point is that long ago when joule asked what hotness and coldness actually are then people replied human sensations but physicist’s linked it with science and introduced thermodynamics and also introduced temperature which is just a measure of our sensation but also linked it with k.E of particles. so they basically studied a human sensation and explained its physical sense.My point is that energy is also a human sensation can we do the same with energy too.