Food, Energy, Waste

Bullgrit · Mar 16, 2010

OK, I'm going to ask some stuff here that some folks might think ridiculously basic. But I'm gonna ask anyway.

Converting "fuel" to energy -- does the mass of the fuel disappear from the universe? Whether it be rocket fuel converted to thrust, or food converted to body activity.

A rocket has X tons of fuel. When it blasts off, that fuel is converted to thrust energy. Yes? But the exhaust expelled still has the same mass as the original fuel, but is just in a different form. Yes? Instead of being a solid of X tons, it's now a gas of X tons (in a bigger volume). Yes?

We all eat an amount of food, burn some of it as energy for our activity, and "make waste" of some of it. But, (I'm about to make an assumption here), our waste does not equal our food intake. Yes? So the difference in the "intake" and "output" is what was converted to energy for activity. Yes? But if our bodies aren't increasing in weight/mass, that energy . . . "disappeared" the mass we intook?

I understand that the food we eat gets changed down to molecular/elemental levels, but the overall mass of those collected molecules/elements doesn't change, does it?

Bullgrit

Umbran · Mar 16, 2010

Short answer: for just about all intents and purposes, no mass is lost in chemical reactions.

Long answer: In any change in energy, there will be a change in mass. However, for chemical reactions (burning rocket fuel, burning food - it's all pretty much the same reaction - oxidation) the amount of mass lost is so tiny that it is well under our ability to detect the change.

In nuclear fusion and fission, a small but perhaps measurable amount of mass is lost and converted into energy.

Bullgrit · Mar 16, 2010

Umbran said:
However, for chemical reactions (burning rocket fuel, burning food - it's all pretty much the same reaction - oxidation) the amount of mass lost is so tiny that it is well under our ability to detect the change.

So if we eat X amount of food, produce less than X amount of waste, where does the matter/mass converted to energy "go"? If our body doesn't increase in mass, the matter burned up for energy still exists (in the universal sense). Yes?

Like with burning wood: the wood (fuel) is converted to energy, and the resulting ash (waste) is less mass than the starting wood. But the original mass released with the burning still exists (in the universe), right? If we caught all the chemicals/molecules/elements from the burning wood, the total mass would equal the original mass of the unburned wood. Yes?

Thinking about it, myself: is the mass expelled with breathing and sweating? Does body heat carry away mass?

Bullgrit

jaerdaph · Mar 16, 2010

I don't know a lot about thermodynamics, endothermic/exothermic reactions, or energy conversion, but I did stay in a Holiday Inn Express last night! B-)

freyar · Mar 16, 2010

Bullgrit said:
Like with burning wood: the wood (fuel) is converted to energy, and the resulting ash (waste) is less mass than the starting wood. But the original mass released with the burning still exists (in the universe), right? If we caught all the chemicals/molecules/elements from the burning wood, the total mass would equal the original mass of the unburned wood. Yes?

This is the easier example. If you caught up with all the molecules released by burning wood,
then yes, you'd have the same mass as you started with (minus a tiny amount that became heat/kinetic energy, as Umbran mentioned). Chemical reactions are pretty much just about rearranging mass with very little mass turned into (other forms of) energy.

Thinking about it, myself: is the mass expelled with breathing and sweating? Does body heat carry away mass?

Breathing and sweating gets rid of some mass: sweating takes water out of your body, which then evaporates away. Breathing also does the same (though not as effectively). Your loss of body heat also gets rid of mass/energy, but, as noted, it's a completely negligible amount. Here's the deal as best as I can figure: the bulk of what you eat (in terms of mass) passes right through. Some fraction is used just to build/rebuild chemical structures in your body, including storage as fat. Only a very tiny amount (again, negligible as Umbran says) is used to power you (exercise, keeping your body temperature above room temperature, etc).

Theo R Cwithin · Mar 16, 2010

One thing to remember when thinking about burning fuel for energy is that the vast majority of that energy doesn't actually come from converting mass into energy. Rather the energy generated comes from changing the configuration of that mass.

Take a rubber-band and stretch it between two posts. Now it's in a configuration with a lot of tension. If you cut it, it goes flying off and hits someone in the eye-- lots of energy is released, giving the rubberband flight and "oomf" when it smacks into something, but the rubberband itself isn't any lighter. When it was under tension, it had a lot of potential energy; when you cut it, you released that energy stored up in its stretched-out state, yet virtually none of its actual mass was converted into energy.

Burning gasoline or digesting food is similar, except instead of cutting rubberbands, these processes are cutting chemical bonds in the molecules. Big molecules get broken down into several smaller ones, and the energy that was tied up in holding them together is released-- and that's the energy we're getting access to by tanking up a car or eating cheetos, not the energy from mass-to-energy conversion. If you could track down every atom involved in the reactions, you'd find that they're all accounted for, before and after!

Umbran · Mar 17, 2010

the_orc_within said:
One thing to remember when thinking about burning fuel for energy is that the vast majority of that energy doesn't actually come from converting mass into energy. Rather the energy generated comes from changing the configuration of that mass.

Technically, while the chemical bond is there, the energy it represents is mass. A minuscule amount of mass, undetectable even in aggregate, but present.

Fundamentally, chemical bonds are analogous to the bonds within an atomic nucleus. Different particles, but the same basic idea. In nuclear (fission and fusion) reactions, a fraction of a percent of the mass of the atom is released as energy. In chemical reactions, the fraction of mass converted to energy is much, much smaller.

Umbran · Mar 17, 2010

Bullgrit said:
So if we eat X amount of food, produce less than X amount of waste, where does the matter/mass converted to energy "go"? If our body doesn't increase in mass, the matter burned up for energy still exists (in the universal sense). Yes?

Yes. The general posit is, "Matter and energy can not be created nor destroyed, but only converted."

So, there was some energy stored in the chemical bonds ("chemical potential energy"). When released it most usually becomes another form of energy. If you use food energy to lift a heavy box onto a high shelf, the chemical potential energy gets converted into potential energy of position of the box. If you throw a baseball, it gets turned into kinetic energy of the ball.

Most of the energy released eventually finds its way into heat - random motion of molecules and atoms. The thrown baseball eventually hits the ground, and dissipates it's kinetic energy through friction with the ground, and heats the ground and itself ever so slightly.

If we caught all the chemicals/molecules/elements from the burning wood, the total mass would equal the original mass of the unburned wood. Yes?

For most human intents and purposes, yes, the mass of the original wood would equal the mass of ash, gases released and oxygen used in burning.

As a minor technicality - do note that our measuring techniques have limits. There are mass differences so small that we cannot measure them. My home scale only displays differences down to about 1/8th of an ounce, for example. If less than 1/8th of an ounce of mass were missing, I'd not notice it. Even the best-made scientific scales have limits on their precision.

Thornir Alekeg · Mar 23, 2010

In terms of the human body, one thing to keep in mind is that the typical person's body weight is more than half water. You lose liquids through waste excretion, sweating and respiration. On the solid side there is waste excretion and then smaller amounts through shedding of skin and hair (only a couple of pounds per year).

When you are trying to reconcile the amount of food you eat with the amount of waste you produce, do not forget that most foods have a significant amount of water weight as well. Much of that food-borne water will removed from the solids in the large intestine and eventually will be excreted in the urine.

Jack7 · Apr 1, 2010

It depends a great deal upon whether you believe, and more to the point, whether the universe actually is, a closed system, or an open system? If it is an open system then where does "lost" matter, mass, and/or energy go?

If the universe is truly a closed system and the laws of thermodynamics are true, then it is hard to imagine anything being truly lost due to any reaction. However as Umbran implied much depends on how one seeks to measure loss.

Does matter converted to energy loss mass in relation to the total mass of the universe (the question I suspect you are actually asking) or does it merely lose mass in relation to the matter converted as dependant upon the physical confines (operational area) of the reaction. (That is to say does the mass bleed into the ambient environment to such a diffuse degree that it becomes in effect un-measurable as a practical matter (despite neither adding to or subtracting form the total mass of the universe).

Then again you have to consider this proposition when considering how you measure mass. An object in motion has a greater kinetic mass (relative to how fast it is moving, an object at light speed has infinite mass, depending upon the action of gravity and how it interacts with other objects) than an object at rest (which unless sit is large enough to generate a measurable gravitational field consists primarily of potential mass). Energy causes motion in most cases. Therefore the mass lost through conversion into propulsion (as an example of mass converted to forward momentum work force), while seemingly lessening the "motionless mass" through burn expulsion of converted fuel (I am not speaking of weight effect in a gravitational field, though movement creates a relative gravitational field) creates a relative effect of kinetic mass through velocity which tends to compensate for static loss.

Of course my examples are of physics, not biochemistry.

But the same general principles apply, except as regards living organisms.
All living organisms are open systems so loss is always occurring in truly open systems.
Living organisms have to be open or they would not require an environment.