Calculating force between atoms

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Discussion Overview

The discussion revolves around calculating the force acting on individual atoms within a crystal structure, specifically manganese, and understanding the relationship between temperature and kinetic energy in the context of atomic interactions and thermodynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks to understand how to calculate the force acting on an atom in a manganese crystal, initially suggesting it is a simple cubic structure.
  • Another participant points out that the total average force on an atom is zero, as it does not accelerate from its position.
  • Participants discuss the relationship between temperature and kinetic energy, noting that temperature is related to average unordered kinetic energy, and that knowing the temperature and mass of an object may not be sufficient to calculate its kinetic energy without additional information about internal degrees of freedom.
  • There is a correction regarding the crystal structure of manganese, with a participant asserting that it is not simple cubic, unlike polonium.
  • Participants explore the concept of calculating components of force acting on atoms, mentioning the use of Coulomb's law and the complexities introduced by electron interactions and lattice structures.
  • Discussion includes the average energy per atom in a crystal, with references to the Boltzmann constant and the implications of having a small number of atoms in a system for thermodynamic approximations.
  • One participant questions whether to consider all electrons or just those from the last orbital for calculations, leading to a discussion on the particles to be included in energy calculations.
  • A later reply suggests that for a larger number of atoms, the average energy can be estimated using the formula ~3kT per atom, but notes that this may be an overestimation and that a better approach could involve the Debye model.

Areas of Agreement / Disagreement

Participants express differing views on the crystal structure of manganese and the appropriate methods for calculating forces and energies in atomic systems. There is no consensus on the best approach to these calculations, and several points remain contested or unresolved.

Contextual Notes

Limitations include the dependence on the assumptions about the crystal structure, the number of atoms considered, and the definitions of temperature and energy in the context of atomic interactions. The discussion highlights the complexity of modeling atomic forces and energies accurately.

casmir
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Hi, can someone please tell me how to calculate force which is acting on one atom. Let's look at the crystal of manganese for example, manganese crystal is simple cube (as far I remember) so how can I calculate force acting on one of 8 atom in that crystal?

And one more question, it's often said that temperature is average kinetic energy of some object. So if It's known to me that some object have temperature of 300K and I know the mass of that object is it possible from this data to calculate kinetic energy of this object?

English isn't my native language so it might be that I'm bit unclear to you, so just warn me and I would try to be more clear.
 
Last edited:
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Force from what, and on which parts?
The total average force is 0, as the atom does not accelerate away from its position.

And one more question, it's often said that temperature is average kinetic energy of some object.
Temperature is related to the average unordered kinetic energy. If you put an object on a moving train, it gains kinetic energy, but not temperature.
So if It's known to me that some object have temperature of 300K and I know the mass of that object is it possible from this data to calculate kinetic energy of this object?
You need to know the internal degrees of freedom of that material.
 
mfb said:
Force from what, and on which parts?
The total average force is 0, as the atom does not accelerate away from its position.
OK, I've set the question wrong, I want to know how to calculate each component of force acting upon each atom.

Temperature is related to the average unordered kinetic energy. If you put an object on a moving train, it gains kinetic energy, but not temperature.
Yeah i know that, but didn't know that term.
 
Oh, and manganese is not simple cubic. The only element that is simple cubic is polonium.
 
Vanadium 50 said:
Oh, and manganese is not simple cubic. The only element that is simple cubic is polonium.
Thanks, but for this example let's take it is :)
 
casmir said:
OK, I've set the question wrong, I want to know how to calculate each component of force acting upon each atom.
Simple coulomb force. As approximation, you could consider both electrons and nuclei at the same position. The distance between atoms depends on the material and lattice structure.

Yeah i know that, but didn't know that term.
It depends on the material. For a simple cubic lattice, you have vibrations in 3 dimensions, with 6 degrees of freedom per atom and therefore an average of 3kT of energy per atom with the Boltzmann constant k. Electrons are more complicated and depend on the band structure - for an isolator, you can neglect them.
 
Simple coulomb force. As approximation, you could consider both electrons and nuclei at the same position. The distance between atoms depends on the material and lattice structure.
So for calculation I'm using just electrons from last orbital or all electrons?

It depends on the material. For a simple cubic lattice, you have vibrations in 3 dimensions, with 6 degrees of freedom per atom and therefore an average of 3kT of energy per atom with the Boltzmann constant k. Electrons are more complicated and depend on the band structure - for an isolator, you can neglect them.

OK, and if we are looking at just one crystal cube with 8 atoms and we know the temperature and mass of this cube how can we calculate energy of this crystal?

Thanks for your answers.
 
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casmir said:
So for calculation I'm using just electrons from last orbital or all electrons?
You are looking for all particles you want to consider.

OK, and if we are looking at just one crystal cube with 8 atoms and we know the temperature and mass of this cube how can we calculate energy of this crystal?
The average energy of the crystal. The whole crystal has just 8 atoms? This is not sufficient to use the usual approximations of thermodynamics (large number of particles), you would need different concepts to analyze this cube.
 
The average energy of the crystal. The whole crystal has just 8 atoms? This is not sufficient to use the usual approximations of thermodynamics (large number of particles), you would need different concepts to analyze this cube.
I thought it would be more simple with just 8 atoms, but OK, let's say that this material has 16000000 atoms and we know the temperature and mass of this material, what is the average energy than?
 
  • #10
~3kT per atom, with k=1.38*10-23 J/K and T=300K this corresponds to 1.2*10-20 J per atom. You can multiply that with the number of atoms to get a rough estimate of the total energy. You can divide it by the mass of a single atom to get the specific heat (J/kg).
Usually, this is an overestimation of the total thermal energy, a better calculation would use the Debye model.
 

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