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### Perfect Crystal

Posted: Sat Jan 27, 2018 1:38 pm
During lecture, we talked about perfect crystals, which I don't really understand. Can someone explain what that is, i.e. what conditions make them a perfect crystal?

### Re: Perfect Crystal

Posted: Sat Jan 27, 2018 1:42 pm
A perfect crystal is just an ideal substance where there is no thermal or positional disorder (as justified by S->0 and T->0), but the main purpose of defining a "perfect crystal" is to create a scale of measuring entropy for realistic substances.

### Re: Perfect Crystal

Posted: Sat Jan 27, 2018 2:02 pm
A perfect crystal is a substance in which all the molecules are lined up perfectly, so there are no planar imperfections. Nothing within the crystal is moving, so the arrangement cannot be altered. This implies that at ground state, a perfect crystal would have a degeneracy of 1, or W=1; subsequently the entropy is zero.

### Re: Perfect Crystal

Posted: Sat Jan 27, 2018 10:53 pm
In friday's lecture, Dr. Lavelle's powerpoint said "if a system can be prepared in only one way, then S=KblnW=0".
What did he mean by prepared in only one way?
I'm not completely sure what he meant by this line and how it relates to the equation.

### Re: Perfect Crystal

Posted: Sat Jan 27, 2018 10:56 pm
Michelle Lee 2E wrote:In friday's lecture, Dr. Lavelle's powerpoint said "if a system can be prepared in only one way, then S=KblnW=0".
What did he mean by prepared in only one way?
I'm not completely sure what he meant by this line and how it relates to the equation.

When it says something can only be prepared in one way, that means the degeneracy, or W in the equation, equals one. When plugging in W=1 into the equation, you get ln1, which equals 0, so therefore S=0.

### Re: Perfect Crystal

Posted: Sun Jan 28, 2018 10:49 pm
Moreover, it means that we can pinpoint exactly where the particles are within an object which we have never been able to do before.

### Re: Perfect Crystal

Posted: Thu Feb 01, 2018 5:31 pm
Can someone explain why entropy of a "perfect crystal" is 0 (S=0)?

### Re: Perfect Crystal

Posted: Sun Feb 04, 2018 2:53 pm
Dang Lam wrote:Can someone explain why entropy of a "perfect crystal" is 0 (S=0)?

At 0K there is no movement of atoms so there is only one possible state for each molecule in a crystal, so it is perfectly ordered and we can know the exact position of everything. Since there is only one possible state, degeneracy is 1, so S = Kb ln(1) = 0

### Re: Perfect Crystal

Posted: Sun Feb 04, 2018 4:04 pm
The perfect crystal is related to the third law of thermodynamics which is about systems in equilibrium at absolute zero (0 Kelvin). It states that the entropy of a perfect crystal at absolute zero is exactly equal to zero.

### Re: Perfect Crystal

Posted: Sun Feb 04, 2018 6:09 pm
basically entropy=0 in a perfect crystal because they must be in perfect order