Question about energy

[cold14]

Well recenlty i had the most random tought, is energy a constant? It seemed stupid at first then i realised that it makes a bit more sense that it does at first. is energy a constant? I mean our energy comes from food which comes from plants which comes from the sun. so basically energy is just transferred to different types of energy all the time. But this is where im stuck: what is kinetic energy transfrerred to? i first thought it would be heat but the amount of heat is too small.

am i wrong or what? and what is movement energy tranferred to exactly :blink:

EDIT: OMG lololol i wrote this post and i totally leftout the bit that i was supposed to be askin about, which is is energy a constant. post fixed, read again makes a bit more sense now LOL

but you guys have seemed to have answered my question so thanks

Edited January 2, 2006 by cold14

[kernatch]

chemical energy, heat, sound... lots of stuff.

haha, i can't really say.

[JoDaddy]

Laws of Thermodynamics: Energy is neither created or destroyed - only converted.

Also with every conversion, some energy is lost forever - hence the world is slowly going towards entropy.

[kernatch]

that's interesting. lost forever - so what exactly happens to if the first law is true?

[JoDaddy]

It's still there - it's just we can't get it anymore. There's another law that says something to the effect of you need more input than your output. It's the reason a self-propelling machine isn't possible.

[JoDaddy]

Wow - my laws are all messed up, in case you wanted to know. The Law of Conservation of Energy states that total input must equal total output - energy cannot be created or destroyed. The Laws of Thermodynamics explain that energy is lost while being converted, and that all systems tend toward entropy.

Hope you weren't needing this for a science test. :-)

EDIT - more on Kinetic energy conversions

[Blackima]

As said before and as everyone knows energy cannot be created or destroyed (at least that's whatwe comfortably believe at the mo')

And yah, you'll find that most energy just ends up being converted to heat energy (which is actually just the kinetic energy of particles, but nvm)

Kinetic energy also usually just ends up being transfered to heat energy

When JoDaddy says energy is lost, he doesn't mean destroyed, he just means it's no longer usable by us (too disorded). It's still there, jsut in the form of heat energy (usually) though. I don't really like saying energy is lost though, sounds too much like it's being destroyed and that's ather confusing if you believe energy cannot be cretaed / destroyed. Though energy can be lost from a system, ie the total energy within the system remains the same but energy can enter or leave that system.

And in answer to your last question "and what is movement energy tranferred to exactly". Snappyfool's basically answered it for yah. It can be transfered to basically anything. Though most likely heat cause that's often the most entropically favourable path

[EDIT] Oh oh. While we're on the subject of energy. Here's a nice little tidbit. The theoretical physicists out there reckon they've figured out what the TOTAL energy in our visible universe is. I have to admit I agree with them purely cause it's such a beautiful answer and makes life a lot easier. They reckon the total energy of the universe is...

joules.

Freaking beautiful isn't it? [/EDIT]

Edited January 2, 2006 by Blackima

[Post-It Note]

Then what does negative energy mean? If the total energy was 0J, then there must be a lot of negative energy somewhere.

[Blackima]

Then what does negative energy mean? If the total energy was 0J, then there must be a lot of negative energy somewhere.

Heh heh, Bingo! :yes:

I didn't even realise this until a while back, and the whole concept threw me when I realised the way I perceived energy was wrong :p

Basically whenever you see objects in attraction, whenever you've seen people refer to "potential energy", that's all been negative energies. You've probs wondered why the formula for gravitational potential energy is U=-GMm/r rather than just U=GMm/r. And you might have been confused by why people talk about a higher object having a greater g.p.e than a lower object when you know a higher value of "r" should result in a lower g.p.e

Well at least, I've always wondered that :p, and it turns out that all "potential energys" are actually negative. And to be fair there's a LOT of potential energy out there. Just the earth being within the sun's gravitational "field" means it has a potential energy of -5*10^27 megajoules. Every bond has a negative energy associated with it, that's why bond enthalpies are given as a negative number.

Anyways, there's a lot of negative energy out there, a hell of a lot, some may think there's even more negative than positive energy, though at the moment the "in" thing is to consider that there are equal ammounts of positive and negative energy to make the total energy of the universe = 0J

[edit] actually if we're being pedantics electric potential energy isn't always negative. when there are two oppositely charged objects they'll have negative potential energy, when there are two similarly charged objects they'll have positive potential energy. I guess a better rule of thumb is when you see attractive forces, you're likely to find negative energies

Edited January 3, 2006 by Blackima

[Rob2687]

and you can't go explaining g.p.e with out that little graph they like to include in all the textbooks. :p

Edit:

[_sphinx_]

^exactly as drawn above, the best example of -ve work[energy] is the variation of the gravitional potential -GM/R with the displacement from the earth R. As we move towards the earth, we're doing work WITH gravity, but the definition of the word gravitional potential is the work done AGAINST gravity when a one kg mass is brought from infinity to a point. Because of that, WD (energy) MUST be of increasing negative values as we move towards the earth while at infinty, it's zero. The graph is, as drawn above, a -ve logarithmic curve.

Nother example:

We defined dW to be the change in the WD BY the gas on a piston.

If in a certain situation, you compressed that piston then work is done ON the gas so dW must be -ve.

You'll get the hang of it. When you define work in a certain direction aided by a force, WD is +ve and then you go in the opposite direction of this force, then the work done will be -ve. Work done is tehe energy btw. and WD: work done