# Heat conduction in a solid

avicenna
It is easy to understand heat conduction in a gas as the nucleus of atoms may collide with transfer of kinetic energy. But the space within a solid is vastly empty space and the nucleus of the atoms cannot collide. So if the surface of a solid is in contact with a hot gas, how is kinetic energy transferred across the solid body.

Homework Helper
Imagine the atoms as fixed in position by springs: bump one and others move too.

 re
easy to understand heat conduction in a gas as the nucleus of atoms may collide with transfer of kinetic energy
atomic collisions are not nuclear collisions: there is a negatively charged shroud of electrons around an atom nucleus; the shroud is huge in size compared to the nucleus

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sophiecentaur and davenn
Gold Member
It is easy to understand heat conduction in a gas as the nucleus of atoms may collide with transfer of kinetic energy.
Clearly it's not that easy. Over simplified models can give one a feeling of familiarity with Science but can that be called 'understanding'?

It is easy to understand heat conduction in a gas as the nucleus of atoms may collide ...
That would be more like a nuclear reaction, rather than heat conduction.

avicenna
Imagine the atoms as fixed in position by springs: bump one and others move too.

 re atomic collisions are not nuclear collisions: there is a negatively charged shroud of electrons around an atom nucleus; the shroud is huge in size compared to the nucleus
We don't always go into fine details here; after all we don't have any absolute physics theory. Collision of gas particle are like between billiard balls; in gas, the +ve nucleus would "collide".

I do suspect there are forces holding atoms in the lattice of solids - your spring model. Accepted. So is there a name for the forces holding the atoms of solids together? Van der Wall forces?

We know such transfer of heat - kinetic energy - is through "collisions" in gases. In solid, it would the transfer of kinetic energy through the forces holding the lattice structure. A simple classical model too is helpful; if quantum treatment is better, then we use it.

Mentor
I do suspect there are forces holding atoms in the lattice of solids - your spring model. Accepted. So is there a name for the forces holding the atoms of solids together? Van der Wall forces?
Chemical bonds.

We know such transfer of heat - kinetic energy - is through "collisions" in gases. In solid, it would the transfer of kinetic energy through the forces holding the lattice structure. A simple classical model too is helpful; if quantum treatment is better, then we use it.
Given that you have an interatomic potential, why would it matter if the atoms are freely moving (as in a gas) or moving about an equilibrium position in a crystal lattice?

Note that in metals, the conduction electrons form a Fermi gas which is a very important contributor to heat conduction and the reason why metals are so good at conducting heat.

Homework Helper
in gas, the +ve nucleus would "collide"
No.

Mentor
No.
In a sense, it is correct. The majority contribution to the repulsive part of the interatomic potential is nuclear/nuclear interaction.

BvU
avicenna
No.
No.
Geiger–Marsden experiments where alpha particles are reflected directly by thin gold foil. It is "electrical collision" of +ve nucleus and +ve charged ions.

Mentor
Geiger–Marsden experiments where alpha particles are reflected directly by thin gold foil. It is "electrical collision" of +ve nucleus and +ve charged ions.
There is a difference between ions colliding with atoms and neutral atoms colliding.

BvU
Homework Helper
The nuclei don't come anywhere near another in atomic collisions. The perception of billiard balls in this context is absurd and misleading.

avicenna
The nuclei don't come anywhere near another in atomic collisions. The perception of billiard balls in this context is absurd and misleading.
When an object rests on a table, there is reaction that prevents "merging" of the object to the matter of the table. I only vaguely know it is the electrical force that prevents the the object and the table to merge.

Mentor
When an object rests on a table, there is reaction that prevents "merging" of the object to the matter of the table. I only vaguely know it is the electrical force that prevents the the object and the table to merge.
This is getting off topic.

Given that you have an interatomic potential, why would it matter if the atoms are freely moving (as in a gas) or moving about an equilibrium position in a crystal lattice?
or do you get why solid can conduct heat?

avicenna
This is getting off topic.

or do you get why solid can conduct heat?
My physics background is not yet at a level where I can say I fully understand; one cannot fully understand without a proper background of the physics involved - say interatomic potentials. But I vaguely remember the rudimentary explanations of the kinetic theory of gas where the gas particles collide with the vessel wall. I can't see why this is not somehow related to collision of billiard balls and the electrical forces that separates solid bodies and cause the balls to bounce off each other.

Staff Emeritus
My physics background is not yet at a level where I can say I fully understand; one cannot fully understand without a proper background of the physics involved - say interatomic potentials. But I vaguely remember the rudimentary explanations of the kinetic theory of gas where the gas particles collide with the vessel wall. I can't see why this is not somehow related to collision of billiard balls and the electrical forces that separates solid bodies and cause the balls to bounce off each other.

This is why the field of solid state physics is different than the field of atomic physics. As Phil Anderson has said "More Is Different"!

The fact that it is a solid means that there are strong bonds that hold the atoms and molecules in place. So there is already strong coupling between the atoms that make up the solid. It means that when one part of the solid is agitated (as in the supply of heat), then it is possible (but not always) that this agitation can be transferred.

Metals are good conductors of heat due to two reasons (naive picture): (i) they are usually crystalline solids, so the transfer of the vibrational energy is not "haphazard", unlike amorphous or polycrystaline solids; (ii) the conduction electrons are not only very mobile, and thus, can transfer the heat energy easily, but they also have a rather low specific heat capacity, meaning that they heat up very quickly and easily, and give up heat the same way when they reach cooler regions of the metal.

In solids, vibrational energy (something that is not available in isolated atoms) plays a huge role in their properties. This is what you are missing in your scenario, because you are focusing on free particles that can only transfer energy via collision.

Zz.

BvU and Lord Jestocost
avicenna
This is why the field of solid state physics is different than the field of atomic physics. As Phil Anderson has said "More Is Different"!

The fact that it is a solid means that there are strong bonds that hold the atoms and molecules in place. So there is already strong coupling between the atoms that make up the solid. It means that when one part of the solid is agitated (as in the supply of heat), then it is possible (but not always) that this agitation can be transferred.

Metals are good conductors of heat due to two reasons (naive picture): (i) they are usually crystalline solids, so the transfer of the vibrational energy is not "haphazard", unlike amorphous or polycrystaline solids; (ii) the conduction electrons are not only very mobile, and thus, can transfer the heat energy easily, but they also have a rather low specific heat capacity, meaning that they heat up very quickly and easily, and give up heat the same way when they reach cooler regions of the metal.

In solids, vibrational energy (something that is not available in isolated atoms) plays a huge role in their properties. This is what you are missing in your scenario, because you are focusing on free particles that can only transfer energy via collision.

Zz.
I think this is what I am missing. The answers are in reading more physics as you say - solid state physics,etc.

Thanks to all. I have some things clarified.