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# Energy - neither created nor destroyed?

saratdear
According to the Law of Conservation of Energy, energy can neither be created nor destroyed, right?

Assuming that ALL of the energy forms we have on earth arrived here in some or the other form, then how does this Law hold when we get new energy from the sun every day?

What I am trying to say is, today we get some amount of energy from the sun. It is photosynthesis-ed by plants, and then converted to other forms, etc.etc. But the next day, we get a whole lot more of energy. This energy wasn't here yesterday, and it wasn't sent to the sun from the earth in any other form, either. So, effectively, we get 'new' created energy, right?

Can anybody clear this up for me?
Voodoocat
Actually, energy is neither created nor destroyed in a closed system. Since the sun is inside our system (not solar system but energy system) there is no violation.
Bikerman
OK. The way to think of this is as what we call a classic 'blackbody' problem.
The earth absorbs and emits radiation. Radiation is absorbed from the Sun (mostly in the form of visible light) and emitted into cold space (mostly in the form of infra-red radiation). The amount emitted is roughly equal to the amount absorbed. This is obvious (otherwise, if it were less then we could continue to heat up until we reached the same temperature as the sun, and if it were more then we would cool down to the temperature of space). Now, a small part of the energy from the sun is used by plants, bacteria and algae to photosynthesise - they absorb high frequency 'visible' light and emit lower frequency (infra-red) heat.

So, on one side we have a hot Sun at about 6000 degrees C. On the other side we have empty space at about -270 degrees C. We therefore reach a 'thermal equilibrium' somewhere between the two. If we had no atmosphere this equilibrium temperature would be about -19C. Because the atmosphere 'traps' some of the infra-red emitted by the earth the actual equilibrium temperature is much higher - about 15 C.
Bikerman
PS - as for the source of the energy (the sun), energy comes from the fusion of hydrogen into helium. Thus matter is being converted into energy (according to the famous e=mc^2) and the overall energy is conserved (since mass and energy can be regarded as interchangeable).
Bikerman
Quite right - my mistake again...I really must stop 'taking shortcuts' in explaining things
Arnie
Well last time it was my mistake
infinisa
Hello Bikerman & Arnie

Let me see if I get this.

I guess from Arnie's link, what you're saying is that when the sun emits energy through the fusion of hydrogen atoms, the energy that's released is nuclear binding energy (that's energy from the strong nuclear force field, right?).

On the other hand, some kinds of energy show up as mass: nuclear binding energy and kinetic energy, for instance. So does that mean that E=mc^2 refers not to the conversion of mass into energy, but that the mass varies depending on certain types of energy (binding, kinetic?) If so are there other kinds of energy that manifest themselves as mass? Why do some and not others (if that's the case)?

One consequence of this logic is that if you were to think of mass as a kind of energy, you would end counting some kinds of energy twice, right?

Grateful for any explanation
infinisa
Hi Bikerman

Now I see this can't be right either, because as you say here:

 Quote: The only method I can think of that would convert mass directly to energy would be matter-antimatter anihilation, although we could argue that relativistic mass is another example....

...And the reverse process could also be an example - e.g in Hawking radiation, where energy is spontaneously converted into a matter-antimatter pair at the boundary of a black hole, and one of the two just escapes while the other stays inside, thus making what would normally be a very temporary phenomenon into a permanent one.

So does that mean we do have to consider mass as a kind of energy after all (for energy to be conserved)? Then what about the double counting I just referred to in my previous post?
Bikerman
It's a deep question and I'm not going to launch into an explanation yet - I need to check my facts, with a couple of people much more qualified than me, first. Watch this space and I promise to come back to this question.
saratdear
OK - Energy can neither be created nor destroyed in a closed system - and the sun is inside our system. But is it possible to think of a system including just the earth?
leontius
The law applies to the whole universe, not just to earth. So if earth gets more energy then there will be less energy in the sun. Also I do not think that the law is really applicable now that we can create energy from matter (or vice versa). The more approriate law would be the Law of conservation of energy+mass.
gtoroap
Great explanation, Voodoocat. I think that everyone has a correct answer to the main question...
Bikerman
Infinisa
to be going on with let me offer this;

All mass/energy is 'quantizable' (ie reducible to fundamental 'bits'). For example, in the case of em then we quantise to the photon.
Now, in the case of fermions (matter) then we quantise to various fundamental particles, including the quark and electron. In the case of the fundamental forces (em, strong, weak, [gravity?]) we quantise to appropriate bosons.
Now, each of the fundamental 'quantised' entities - let's call them particles - has an associated energy. Therefore we can regard mass and energy as interchangeable to the extent that all 'stuff' is made up of fundamental quantised 'bits' which have their own associated energy...

Now, as to the main question of mass/energy interchangeability:
Yes, all energy can be represented as mass (and vica-versa).
Thus if an entity has potential energy (consider, for example, a compressed spring) then it has more mass than the same entity without potential energy (the uncompressed spring). Potential energy is just 'book-keeping' really. Physicists will not normally talk about 'mass' at all when considering these issues - they prefer to talk totally in terms of energy. They used to talk about 'relativistic mass' - ie the extra mass that an entity has by virtue of its motion (kinetic energy), or its potential energy (compression, heating etc), but the term has largely gone out of favour nowadays..

That's my own take - I will, as promised, consult with wiser heads before coming back to this....
infinisa
Hi Bikerman

Thanks for this info.

You say:
 Quote: Yes, all energy can be represented as mass (and vica-versa).

That's interesting - I had no idea that this applied to potential energy - that a compressed spring has more mass than an uncompressed one!

So does that mean that to count all energy just once, you can measure either energy OR matter but not both?

And what then is meant by "the conservation of mass-energy"? Is it also based on the view that mass & energy are interchangeable rather than complementary?

Thanks
chatrack
i agree with Statement of energy Conservation.. You should not forget that ENTALPY & ENTROPY of our earth is increasing day by day!
Bikerman
infinisa wrote:
Hi Bikerman

Thanks for this info.

You say:
 Quote: Yes, all energy can be represented as mass (and vica-versa).

That's interesting - I had no idea that this applied to potential energy - that a compressed spring has more mass than an uncompressed one!

So does that mean that to count all energy just once, you can measure either energy OR matter but not both?

And what then is meant by "the conservation of mass-energy"? Is it also based on the view that mass & energy are interchangeable rather than complementary?

Thanks

Yes - you have got it. You can either count total mass or total energy and the laws of conservation were therefore combined after Einstein to say that mass-energy is conserved rather than either in particular. For most purposes we consider mass and the conservation of mass. It works well enough for non-nuclear processes. Chemical energy is tiny so recombining atoms in different molecules doesn't really change the mass appreciably. Likewise heating or compressing objects adds such a tiny amount of mass (relativistic mass) that we can ignore it. Even kinetic energy doesn't add appreciable mass until you start travelling very fast indeed (a good proportion of the speed of light).

So, there are two cases when physicists WOULD measure in 'energy' terms rather than 'mass' terms.
1) When dealing with extremely small 'events' - like colliding particles in the LHC
2) When dealing with extremely fast 'objects' - like photons from distant galaxies, for example.

(In other words - in quantum events or in relativistic events).
Arnie
Chemical energy is tiny - well, that's a matter of "relativity".
Bikerman
 Arnie wrote: Chemical energy is tiny - well, that's a matter of "relativity".

LOL...you know what I meant...
infinisa
Hi Bikerman

Thanks for your explanation. It was simple and clear.