# Do QM and Thermodynamics interact?

1. Nov 22, 2012

### AndySA

I'm actually quite scared of asking this due to my very basic knowledge regarding Quantum Mechanics. Actually, this is my first hiccup: is there a difference between Quantum Physics and Quantum Mechanics?
I always thought one was the theoretical investigation and the other the application but after reading a bit I am not so sure any longer. A quick definition clarifying the conflicting info I am getting would help.

My actual question: is Quantum Mechanics bound to the 1st law of thermodynamics?

Or any classical thermodynamics restriction for that matter. I've been thinking about this and just when I find I can say: no, I think of a way that can maybe be a: yes. I've tried the Google approach and have given up. I've tried reading here and am now more confused than before. So I hope someone can sort this out explaining it in simple terms - and that there actually is an answer.
Thanks

2. Nov 22, 2012

### Staff: Mentor

I think "Quantum Physics" and "Quantum Mechanics" are the same.

Depends on your favorite interpretation - in general, it is.

Modern thermodynamics can be derived from quantum mechanics.

3. Nov 22, 2012

### ZapperZ

Staff Emeritus
I am not exactly sure what the issue is here.

Thermodynamics is statistical mechanics. And in statistical mechanics, you can have quantum statistics. That is when a fermion or a boson matters. And Thus, quantum mechanics is applicable to thermo.

For example, you can write the partition function for a quantum system with discrete energy states, and you can derive all the same themo state variables and functions from that.

Zz.

4. Nov 22, 2012

### AndySA

Thanks for the explanations.

I understand that quantum mechanics can explain thermodynamics.
What about the other way around? My problem is that I don't know whether one can state that the conservation of energy even applies to quantum mechanics.

Is this a valid question: does energy conservation work if a particle traverses the Higgs field and picks up mass?

5. Nov 22, 2012

### Bill_K

Energy is exactly conserved in quantum mechanics. And it is locally conserved, meaning that it is conserved at every point in space, at every moment in time.
The Higgs field does not "give mass" to particles. It permits particles to have mass in a way which is consistent with electroweak symmetry. The origin of mass is not understood, but the Higgs field is not the cause. Most certainly the Higgs field does not cause the mass of a particle to change with time.

6. Nov 22, 2012

### AndySA

That makes it 2 very clear statements. Thank you.

It shows that I was wrong on the first part and misunderstood the function of the Higgs field. I suppose typical layman misconceptions.

7. Jan 1, 2013

You asked two questions: 1. The difference between Quantum Physics and Quantum mechanics and 2. The relation between Quantum mechanics and the 1st law of thermodynamics (your actual question).

1. 'Mechanics' is only a part of 'physics'. (you have many other parts such as - statics, heat, Sound, Light, electricity and magnetism, to mention a few). In mechanics we use Newton's laws to describe motion of bodies. When we try to describe the (motion) propagation of light, we find Newton's laws do not serve the purpose adequately. In place of Newton's laws we use Laws of Quantum mechanics (QM) OR Wave mechanics (WM). These are much more complicated compared to Newton's laws. That is why people are afraid of QM and WM, just as you were 'quite scared' even to pose your question! Mechanics deals with the study of motion of macroscopic bodies and is simple (relatively speaking) while QM &WM deal with study of motion of light and other particles at atomic level and is complicated.

2. If you had asked the relation between QM and TD it would have been more interesting. you stopped at the fist law of TD! Anyway, in mechanics (dynamics) we have the law of conservation of energy, but that does not include 'heat' as a form of energy. TD includes heat also as a form of energy and must be taken into account while the book-keeping or accountancy is done for energy.

TD deals with conversions of energy in general and heat and other forms of energy in particular.

Statistical mechanics (Which is also mechanics) is a boarder case, relating Newton's mechanics, TD and QM.

8. Jan 1, 2013

### dextercioby

Radhakrishnam, 'wave mechanics' is a term coined to describe SchrÃ¶dinger's work of 1926 and in modern usage it is a subtheory of Quantum Mechanics in particular or Quantum Physics in general.

Quantum Mechanics is the subtheory of Quantum Physics describing systems of point particles with finite classical (Hamiltonian) DOF (where available).