Themal energy and temperature

Discussion in 'Multiple Subject Tests' started by jello, May 30, 2005.

1. jelloRookie

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May 30, 2005

me again!!

the difference between thermal energy and temperature is an interesting topic

temperature is when the molecules move, the speed.
heat measures how fast the molecules are moving, measures the energy
thermal enegry is the quantity of molecules that make up an object

is it fair to say that (bigger is better) - the bigger the object, the more thermal energy it has? A big object has many molecules and even if they aren't moving as fast as a heated cup of water it still has more molecules so more thermal energy...

why do we even care about thermal energy?

3. TeacherGroupieModerator

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May 30, 2005

Jello, you've very nearly got it.

Thermal energy is indeed molecules running around. (This makes thermal energy a kind of kinetic energy.) Temperature is a way of measuring thermal energy (molecules running around faster or slower) in the particular place where the thermometer is. Think of roasting a turkey (I hope you're not vegetarian, or if you are, bear with me): it matters where the thermometer goes, because both bone and air conduct thermal energy faster than muscle tissue does. How fast the turkey cooks depends on how hot the oven is (how much thermal energy there is for the turkey to absorb) and how big the turkey is (how much turkey there is to absorb the thermal energy - the bigger the turkey, the longer it takes for thermal energy to penetrate deeply; and now we know the science behind the recommendation that stuffing (or dressing, or whatever you want to call it) be cooked OUTSIDE the turkey. And even if it's boiling (100 degrees C, 212 degrees F), a cup of water simply doesn't pack enough thermal energy to cook a turkey thigh, let alone a whole turkey.

Thanks for asking this question! I looked up THERMAL ENERGY on Answers.com and learned something about the two laws of thermodynamics. The first law tells us that one kind of energy can be converted to another without loss - chemical energy in a nerve cell, for example, converts to electrical energy as the nerve impulse jumps across the synapse to the next nerve cell, and then it converts back to chemical energy to move along the next nerve cell, and so on. The second law, though, tells us that thermal energy - heat - is special: you can convert other types of energy TO heat without loss, but you can't convert back FROM heat to other kinds of energy without loss. So thermal energy is a lower quality form of energy.

Great question!

4. jelloRookie

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May 30, 2005

LOL - love the turkey and stuffing explanation !!

5. TeacherGroupieModerator

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May 30, 2005

You're inspirational. I'd never quite made that connection till today.

Oh, and please insert a right hand parenthesis ) after "OUTSIDE the turkey."

6. Eki75Rookie

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May 30, 2005

Would it also depend on how BIG the oven is? Now I am confused. lol. If a bathtub of water has more thermal energy than a cup of coffee (both liquids at the same temp), do MORE molecules running around have more thermal energy than a lesser amount of molecules running around at the same rate? If that is so, there has to be an terminal limit to thermal energy, no? If a 4 sq ft. oven at 300 degrees cooks a 10 lb turkey in 5 hours, would a 6 sq ft. oven at 300 degrees cook it faster? Would a 6000 sq ft. oven at 300 degrees cook the turkey in seconds (theoretically)? There has to be something I am missing. This IS a really good question. Thanks for bringing it up, Jello!

7. TeacherGroupieModerator

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May 30, 2005

Wow, great questions! And much deeper than anything you're likely to see on CSET. But since you asked...

The turkey in the 6-cubic-foot oven (I assume you meant to type cu., not sq.) might very well cook a little faster, simply because of the way cooking works - the turkey (or so the Second Law of Thermodynamics tells us) is absorbing heat from the surrounding warm air, which means the air right next to it is continually losing heat that needs to be replenished (so to speak) from the air a little farther away, and since there's more thermal energy in the larger oven, presumably that heat exchange is going to happen a little faster. This is presumably the principle on which convection ovens work - that is, instead of depending on a larger space, the convection oven blows the hotter air around.

I don't think, though, that the 6000-cu.-ft. oven would make that much difference, because the turkey itself is going to be a factor - how fast it can absorb heat AND transfer that heat inward from the surface. Similarly, a poached egg isn't going to cook significantly faster in 2 gallons of water than it is in 2 cups.

8. Eki75Rookie

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May 30, 2005

OH! I get it. I was missing the part about the surface area of the thing being cooked and its contact with the thermal energy. If the oven was 100 times larger, it would still cook in the same amount of time because the turkey's surface area is the same and has a limit to the amount of heat/time it can absorb. Makes perfect sense. Thanks!

9. TeacherGroupieModerator

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May 30, 2005

Thanks tremendously for asking. I learned from this too - had been sort of skating over the top of a couple of these concepts before, and now I know this better.

I don't know which is more fun: helping someone else Get It, or Getting It myself. Whee! Can we play some more, pleeeeease?