Sunday, January 22, 2012

Food is not energy

A response to a response to my last post--NASA, food is not energy.

Language matters, especially to young children trying to make sense of the world. I remember being utterly confused as a child thinking that Karl and Groucho were the same guy--how dangerous could the Russians be if they were led by a man with a fake mustache who made silly movies?

As adults, with reasonable frames of reference, we laugh at obvious holes in our schema. The best comedians make a living at pointing out the oblivious obvious.

Children, however, will try to weave the inconsistencies into their worldview that already exists. They don't get jokes because they're so busy trying to make sense out of everything.

And they do, internally if not correctly.

As we get older, we learn that people will laugh at us if we do not share a common schema, so we learn to laugh at jokes we do not get, then wrestle quietly with the punchline, stuck in our brain like a piece of corn caught between molars.

We live in a Newtonian universe. Einstein was a smart guy, and his work led the way to all kinds of remarkable things, but we'll not be transforming matter into energy in our classroom, nor energy into matter.

 Matter is matter, and energy is energy.

But what about food? And plants? And sunlight?
I'll get to those in a moment.

is usually defined in public schools as something that has mass and occupies space, and we toss this at kids as though they have some special understanding of mass. 

I certainly don't, so I use a different definition my students can grasp--matter is "stuff." I can tell them it has inertia, or I can tell them that if I throw it at them fast enough they will feel it. (Yes, I know, some particles fly through us since we're mostly empty space...another story for another day.)
is usually defined in public schools as the ability to do work, and we toss this at kids as though they have some special understanding of a physicist's concept of work.

I certainly don't, so I use a different definition. Energy is some quality that can cause a change in stuff. That is, of course, a lousy definition, hardly covers all its various forms. I might say that if stuff has changed, then energy was involved, also a limited definition. 

I'd rather use crippled definitions with their defects discussed than the "real" definitions in textbooks that tell us nothing new. (Starting science units with "vocabulary" just adds to the fun.)

My students are told, upfront, that I have problems grasping the concepts of matter and energy. These are hugely difficult concepts. If you truly grasp them, you own the universe, and no one owns the universe. No one.

Stuff is stuff, and energy is energy, and in Newton's world, "never the twain shall meet."

NASA tells teachers that "food is energy," and it's simply not so.

I put a pie in a slingshot and fire it your way, you will feel it. It's stuff. It moved, made a sound, broke into several pieces, warmed up my face, all evidence of "energy," but the stuff is still the same stuff.

If I burn propane by mixing it with oxygen, I mix the stuff around a bit, but I will end up with exactly the same amount of stuff (defined as the measurement of force exerted by that stuff on a scale placed between it and the Earth) in the form of water and carbon dioxide. Exactly the same. For all the light and heat and noise released, the amount of stuff remains exactly the same.

(You can easily demonstrate that water comes out of this reaction--grab a propane torch and flash the flame over cool metal--use a desk leg or a faucet.)

This is a big deal. 

If kids get through their first 8 years of public school knowing nothing else besides the conservation of mass and energy, we'll take it from there.

So where is this thing called energy? Bad question--it is no "thing."

How is energy stored in food? Better question, but still almost impossible to answer if you do not have a reasonable grasp of chemistry, so let's leave food for a minute and go to a 5 pound rock. 

If I drop a 5 pound rock on your head, how much damage does it cause? Well, that depends on how high the rock was (relative to your head) before it was dropped. The higher the drop, the more damage done, the more energy released. We call this potential energy, a deceptively difficult concept.

If I pick up a rock, it is the exact same rock it was when it as still on the floor. It is now in a less stable position by virtue of having been lifted from the floor, but it's still the exact same rock. It can make more change now when I drop it--louder sound, more damage--but it's still the same rock before and after I drop it.

The potential energy is not "in" the rock, it's in the rock's relative position to the floor. The less stable the rock's position, the more energy it "has."

The rock got less stable because I invested kinetic energy using my muscles. My kinetic energy came from, the potential energy created by the unstable complex organic molecules we call "food"--when I exercise, I convert unstable food molecules into more stable water and carbon dioxide molecules. I need oxygen to help strip the electrons off the food molecules.

The mass of a molecule of glucose and the oxygen molecules needed to break it down need to break them down is exactly the same as the mass of the carbon dioxide and water molecules left when the energy has been released..

The potential energy "in" food came from a plant's ability to combine carbon dioxide and pieces of water together into a larger, less stable compound, using the energy of sunlight.

You cannot weigh sunlight because it's not stuff, it's energy.

Plants do not "eat" sunlight. Stuff is stuff, energy is energy. Food is not energy. It is stuff.

Plants recycle the stuff, but they cannot recycle energy. Energy goes from useful to less useful to even less useful.

And where does sunlight come from? Here's where Einstein joins the party--hydrogen atoms are fused into helium, a tiny bit of mass converted to tremendous amounts of energy.

That's fascinating and deserves study but not until later, when a child knows what food is.

Newton and the Marxes lifted from PD sources.
Potential energy diagram from McGraw-Hill here.


Sue VanHattum said...

Thank you.

I'm pretty sure I used to think there was some mass to energy conversion going on in our bodies, and was puzzled by how huge an amount of energy is supposed to come from little amounts of mass (E=mc^2, but shouldn't there be a conversion constant in there? Are we measuring energy by kg*km^2/hr^2?).

Jenny said...

There is a lot for me to mull over here, but my quick, initial take-away is that I might have to become a high school science teacher if you can really just "grab a propane torch" at any time. I'm jealous.

doyle said...

Dear Sue,

I believe just under a a gram of matter was converted to energy in the bomb used at Nagasaki, though I need to check that.

A gram.

A slice of the brand of bread I eat weighs 36 grams, a HUGE amount of pure energy if it could be converted into such.

Thankfully, though, the carbohydrates are just broken down into smaller pieces of stuff, but still stuff.

I'm going to dodge the math question except to say that for an energy/mass equivalence, c^2 has to somehow be 1, and is, I guess, the conversion constant you're looking for.

Or I may be utterly confused (again)--I'm sure David Rudel will be happy to straighten me out. =)

Dear Jenny,

Pish posh! Go get one and use it wisely. The kids will be amazed.

(To be fair, I'm not sure grabbing a propane torch for class is kosher in my district, either.)

David Rudel said...

One of the most direct take-aways from the points you bring up here is that we must be more careful about how we define roles like "producer."

Often "producer" is defined as something that "makes its own food" or something that "gets its energy from the sun."

Neither of these is either accurate or healthy to inculcate in young minds. A producer can build carbon dioxide into more complicated substances. That is the "production" that is going on.

doyle said...

Dear David,

Criminy, I still have a review or two to do.

I've been pushing the books. I'm not sure folks like being told much how confused they might be.

Or maybe we're all so busy keeping up with nonsense that what matters slips away.

Keep hammering.

David Rudel said...

Thanks, Mr. Doyle.

When I finish my current writing projects (don't get too excited, it's a couple chess books), I hope to have time to start a science myths blog.