Ask a physicist anything. (5)

[TheReasoner]

 

[mzungu]

YouTube - Hahaha

 

[Wiccan_Child]

Hey! Where is the physicist. Is he on a Tea break

I was actually out celebrating the royal wedding at a barbecue while we all wore our poshest suits and dresses. Fun times!

 

[mzungu]

I was actually out celebrating the royal wedding at a barbecue while we all wore our poshest suits and dresses. Fun times!

I wish them a happy marriage and healthy kids. Hey yanks eat your hearts out. You should have remained a colony!

 

[Gracchus]

I wish them a happy marriage and healthy kids.

If I read my history aright, that is unlikely.

Hey yanks eat your hearts out. You should have remained a colony!

No thanks! I would just as soon not have my tax dollars supporting such amateur show business talents. Royalty is like English cooking: almost always unappetizing, hard on the digestion, and leaving a bad smell.

Who are they but the overly inbred and unremarkable descendents of murderous pirates?

 

[mzungu]

Royalty is like English cooking: almost always unappetizing, hard on the digestion, and leaving a bad smell.

WOT You have obviously not tasted a good steak and kidney pie, nor a hearty English breakfast with bangers (yummy) and Marmite on buttered toast.

 

[Wiccan_Child]

WOT You have obviously not tasted a good steak and kidney pie, nor a hearty English breakfast with bangers (yummy) and Marmite on buttered toast.

Marmite? MARMITE?! It's the Devil's detritus, that's what it is! Now a Full English, that's a decent meal:

 

[mzungu]

Marmite? MARMITE?! It's the Devil's detritus, that's what it is! Now a Full English, that's a decent meal:

I prefer my tomatoes fried; But what the heck YUMMY anyway!!!!!!!!!!!!

 

[TerranceL]

Marmite? MARMITE?! It's the Devil's detritus, that's what it is! Now a Full English, that's a decent meal:

Is that mushroom grilled? And is that black circle supposed to be sausage?!

 

[TheReasoner]

Is that mushroom grilled? And is that black circle supposed to be sausage?!

It's black pudding. Yummy

 

[mzungu]

By the way folks; I make my own British banger (Sage, black pepper, touch of nutmeg). Also to top up the hearty English breakfast I always eat a bowl or two of RICE KRISPIES (to our down under friends: RICE BUBBLES).

When it comes to breakfast,; No one beats the Brits. Oh! by the way; Texas beef steaks and french fries do not constitute a breakfast so you Texans can go chase a tornado or fly a kite.

 

[chris4243]

Is the null hypothesis disproven yet?

 

[Wiccan_Child]

Is the null hypothesis disproven yet?

By definition... yes and no. Welcome to quantum logic

 

[TheReasoner]

This time I have a serious question I was hoping you could help me with. It's simple enough, but I can't seem to get the logic right, or my professor has made a mistake. Here goes:

You're presented with those three systems. The task: What ensemble would you use to find the equilibrium in each system?
System a: Isolated system. I'd maximize entropy.
System b: Piston, variable volume. I'd use Helmholtz free energy F(T,V,N). Equilibrium is reached when the free energy is minimized
System c: Constant volume. I'd use Gibbs free energy G(T,p,N). Equilibrium is reached when the free energy is minimized.

However, the solution is the opposite for systems b and c. Gibbs for b and Helholtz for c.


Darn it. I drew this up in LaTeX, typed it up and I got it while typing this. Oh well. Could you answer anyway, just to get your take on the problem? (not going to let that drawing and typing go to waste)

 

[Wiccan_Child]

This time I have a serious question I was hoping you could help me with. It's simple enough, but I can't seem to get the logic right, or my professor has made a mistake. Here goes:

You're presented with those three systems. The task: What ensemble would you use to find the equilibrium in each system?
System a: Isolated system. I'd maximize entropy.
System b: Piston, variable volume. I'd use Helmholtz free energy F(T,V,N). Equilibrium is reached when the free energy is minimized
System c: Constant volume. I'd use Gibbs free energy G(T,p,N). Equilibrium is reached when the free energy is minimized.

However, the solution is the opposite for systems b and c. Gibbs for b and Helholtz for c.


Darn it. I drew this up in LaTeX, typed it up and I got it while typing this. Oh well. Could you answer anyway, just to get your take on the problem? (not going to let that drawing and typing go to waste)

My take is that since the Gibbs free energy is a function of pressure and temperature (G = U + pV - TS), we'd use that on a system of variable pressure. The Helmholtz free energy is a function of just entropy (A = U - TS), so it is used when temperature and volume are held constant. In other words, A is a measure of the energy you have to put into a system to achieve a final state once you've accounted for spontaneous heat transfer from the environment, while G is a measure of the energy you have to put into a system to achieve a final state once you've accounted for heat transfer and volume changes.

So, I'd agree with your answers: use G for (b) and A for (c). For a system under constant temperature and pressure, equilibrium is reached when A is minimised. In (b), you have to account for a changing volume, which is where G comes in.

 

[mzungu]

My take is that since the Gibbs free energy is a function of pressure and temperature (G = U + pV - TS), we'd use that on a system of variable pressure. The Helmholtz free energy is a function of just entropy (A = U - TS), so it is used when temperature and volume are held constant. In other words, A is a measure of the energy you have to put into a system to achieve a final state once you've accounted for spontaneous heat transfer from the environment, while G is a measure of the energy you have to put into a system to achieve a final state once you've accounted for heat transfer and volume changes.

So, I'd agree with your answers: use G for (b) and A for (c). For a system under constant temperature and pressure, equilibrium is reached when A is minimised. In (b), you have to account for a changing volume, which is where G comes in.

Pardon my ignorance but; How is it possible to achieve a steady state when according to Dirac's equation; Matter and antimatter is constantly appearing only to return back to energy. Of course with time some particles of mass remain and thus increasing entropy.

So would this not mean that a steady state is impossible?

 

[chris4243]

A steady state is steady, not static. Equilibrium is a steady state. (otherwise there would be no steady states)

 

[mzungu]

A steady state is steady, not static. Equilibrium is a steady state. (otherwise there would be no steady states)

I never meant static. I mean steady state and it seems that such a state is possibly impossible if we take into account Dirac's equation.

 

[chris4243]

It seems to me you want to apply "steady state" to things that are not state variables.

 

[Wiccan_Child]

Pardon my ignorance but; How is it possible to achieve a steady state when according to Dirac's equation; Matter and antimatter is constantly appearing only to return back to energy. Of course with time some particles of mass remain and thus increasing entropy.

So would this not mean that a steady state is impossible?

I don't believe so.

Thermodynamically, a steady state is one which is largely consistent over time: its pressure, volume, temperature, chemical potential, etc, are constant. But this doesn't mean that it can't be dynamic.

As you point out, particle-antiparticle pairs are constantly popping into and out of existence all the time. But a system can still be in a steady state because, for every particle popping into existence there is a particle popping out of existence. For every positive charge, there is a negative charge.

So, though there are peculiar processes like these going on, the state variables of the universe (such that they are) are, or at least can be, constant. There's no reason why the quantum foam would upset this steady state, since the chemical potential of the universe is still constant.