This question is actually fairly silly, and probably just reveals that I have a flawed understanding of physics, but here goes:
We all know the old rule that matter can be converted to energy and vice versa; but neither can be created or destroyed. So, I can, for example, fill up my car with gasoline, have the motor convert the gasoline into enegy, and, using that energy, move my car.
My question is, what happens to the energy after it’s spent? I would figure that if energy simply “disappears” after it’s spent, then the universe is, in effect, constantly losing matter. Could there be a day, at some point in the greatly distant future when all the matter has been converted into energy and spent? Or does the spent energy get “recycled” somehow?
You’re on to the First Law of Thermodynamics (energy is conserved). I think the answer to your question lies in the Second Law of Thermodynamics (entropy always increases).
In the end I think it is simply heat that you ‘lose’. That is, we lose it…the universe retains it and it increases the entropy of the universe as a whole.
[sub]I may be off on this but I don’t think burning gasoline counts as converting matter to energy. That is a chemical reaction that simply converts one compound into one or more other compunds and happens to be exothermic. Matter/anti-matter annihilation will get you your matter --> energy conversion. I also think you get some matter/energy conversion in nuclear explosions but IIRC it is only a small percentage of the total amount of mass available. [/sub]
Due to thermodynamics, energy goes from a higher form, stored as chemical bonds in the fuel, to lower forms, heat coming from the radiator, out the tail pipe, through the engine block, and as heat generated by friction in the tire tread and air friction as the car goes down the road (other forms I can’t think off the top of my head right now).
In fact, if the universe dosen’t fall back on itself in a “Big Crunch”, eventually ALL matter and energy will decay into background radiation.
The Law of the Conservation of Energy states that “Energy can neither be created nor destroyed, however it can be changed from one form into another” i.e.
Gasoline (Chemical Energy) >
Heat Energy (ignition)>
Kinetic energy (car moves).
However, there is still the same amount of energy overall after this process. No-one can make energy. What happens to the energy? It is converted into kinetic enrgy. That energy cannot be changed back into chemical energy, but the energy is still there.
There is a multimedia presentation here Law of the Conservation of Energy
So does that mean that there is all this useless kinetic energy running around the planet (basically, the sum of all plant, animal, human and geologic movement since the Earth was formed (to say nothing of the rest of the universe)?
I’m not trying to be obtuse here, this is just something I’ve never really understood and dreading the day that my oldest son will ask me about it.
It’s not that stupid a question, actually. It goes to the heart of the whole branch of physics called thermodynamics.
The short answer is that energy doesn’t disappear, but it turns into heat when it’s used. When you burn gasoline, a lot of the energy goes to heating up the engine (and the air going past the radiator), and some of it goes to pushing the car forward. A lot of the pushing energy goes to moving air out of the way of the car; all this moving air mostly bangs into itself and winds up just heating up the air. Some of the pushing energy stays with the car as kinetic energy, until you stop and turn it all into heat in your brakes.
Heat energy is just kinetic energy, of course, but on a very unorganized scale – when you heat something up all the various molecules are moving faster, but in random directions.
I’ll stop before I fill up pages on how this is just a statistical effect and other points interesting to physics nerds, and say that
the bottom line is that energy doesn’t go away, it just becomes unusable for anything interesting.
Yes even for chemical reactions, the energy released results in loss of mass. However, the decrease in Mass is fairly insignificant. To give you a feeling, here are some typical calculations
The Calorific Value of 1 gal of Gas (E) = 140 KJ
Mass lost to release this heat = 140E3/c^2 = 3.5E-12 kg, or
0.0000000000077 lb, certainly not a difference measureable with 1 gal of gas.
Two things: 1) Generally the “matter” that is the gasoline does not get converted into energy to run your car. If you take the gasoline and the air used in the engine to run your car and weighed them and then you take the products (carbon dioxides and other things spewing out the tailpipe) and weight them, you will find that they weigh the same to great precision. This is the celebrated conservation of mass which is a fine law for nearly every situation you and I will come across.
What does change slightly is the way the atoms are arranged. Chemical bonds are a way to store energy. One way to think of them is as springs that have been compressed by the consituent atoms. Break the bonds and boing, the spring is released. The potential energy stored in these bonds can be converted, in part, into kinetic energy. The “energy” is able to do work on the system (that is move the car forward). However, the no conversion of energy is 100% efficient. This means that some of the energy will get dissipated in the form of heat which is basically an ambient energy background that is impossible to tap because it has no sources or sinks.
Once the work is done of the system, the energy has been either fettered away into heat or stored in some other way (perhaps by getting your car to get up a hill, thus storing gravitational potential energy).
Einstein showed in certain treatments of special relativity that one can measure a quantity that looks like mass which is not conserved. In effect, the mass could be converted away if you looked carefully enough, into a form of energy. Mass-energy idea was born. The theory then said that all forms of energy had something called relativistic mass. However, that mass was derived by dividing by a ridiculously large conversion factor (c^2) which makes any mass due to the normal amounts of energy we deal with on a day to day basis pretty small. Still, theoretically, if you measure the reactants of the air and the gasoline and then the products of the heat, carbon dioxide, etc. you should find that the reactants and the products have the same relativistic mass as before. In practice, it’s very hard to measure the difference in mass due to the energy that is stored in the system because of the super-large conversion factor, and since it’s such a negligible factor for most of the things we deal with, we ignore it and say that mass and energy are conserved, but energy can be converted into a form where it can no longer be used to do meaningful work.
Wow, four other posts and a reply while I was composing my magnum opus.
Zev – you’re right on with your third post. Though the earth isn’t a closed system (to use physics lingo). What happens is that the earth radiates heat into space, so all the excess kinetic energy is being pushed out into space as infrared radiation, which will eventually hit something and heat it up. So all the kinetic energy (heat) from dinosaurs running around isn’t just on earth, it also heated up the rest of the universe. It might be worth noting also, that a lot more heat is created on earth from radioactive decay in the earths center, and sunlight hitting the earth, so this isn’t a big effect temperature wise.
As Enola said, eventually, all the energy in the whole universe will be heat, and everything will be at the same temperature, and there won’t be any energy left to do anything interesting with. Excluding any funky cosmological expansions, contractions, or other things we don’t understand yet, of course.
Oh, and Zev, you’re right on with your second post, too. If you very very, very accurately weigh some gasoline and oxygen, then react them (i.e. burn the gasoline), and cool off and weigh the resulting CO2 and H20, you’ll find a tiny, tiny, tiny (tiny, tiny) bit less mass afterwards.
Energy by itself is useless. What it useful is the difference in energy levels.
Richard Feynman had an analogy using water and towels. Towels have a certain amount of dryness. Let’s say you are sopping wet, and you dry yourself off with the towel. At some point, you and the towel are equally wet. At this point, even though the towel still has some dryness left, it is useless to you, because it is no more dry than you are. If you want to get dryer, you need to either apply more energy to the towel (leaving it out to dry, putting it in a clothes dryer), or find a new towel that is starting out fresh and dry.
Some matter gets converted to energy in the example you had. It is still the equivalent amount as ever. But it is no longer in a very useful form. Heat is only useful as an energy source working against a colder place. Doing that work will even out the heat levels. Eventually you get to a point where everything in the universe is at the same heat level. There is no more or less energy than ever, but the ability for that energy to actually do anything is gone, since every place is at the same energy level.
That heat level is extremely low by the way. Very very low. At that point, it is essentially the same as the background radiation.
At that point, the matter has indeed all been spent. Protons have decayed, there’s nothing left but ubiquitous low level energy that can’t actually do anything.
Is this an actual loss of matter or just a loss of energy? (Give me chance to explain here)
Ordinarily we don’t think of energy as having mass yet, since energy and mass are interchangeable, we can see the same effects as mass with just energy. For instance, say I place the earth in a box (ignore the box except as a means to contain the earth). If I magically converted the entire earth to energy it would still gravitationally affect those outside the box just the same as it did before conversion.
So, back to our chemical reaction. Is this ever so tiny loss in mass an actual conversion of matter into energy or am I merely measuring the loss of energy stored in the system that is now lost to heat?
Whack-a-Mole… heat has “mass” in the formulation talked about by andy_fl. Those infrared photons that have radiated away or excited some vibrational mode of some molecule in the atmosphere added to the energy of the products of our combustion will ultimately have the same amount of energy as before.
As to whether it’s an actual “conversion” of matter into energy, it depends on what you define as “matter”. Generally speaking, physicists refer to matter as anything with a non-zero restmass. If you actually measure the restmasses of these particles in the center of momentum frame you will find no loss of rest-mass.