[hamdani yusuf (OP)]

You still would not be talking about exchange of heat, so it still wouldn't be anything to do with the title of the thread.

What does it take to say that there is heat exchange in a process?

[alancalverd]

For there to be an exchange of heat, the mean internal kinetic energy of one object must decrease and that of the other object must increase. We measure mean kinetic energy as temperature. If two objects are at the same temperature, their temperatures will not change by putting them in contact with one another. That's how temperature is derived from the zeroth law.

[Eternal Student]

Hi.

What does it take to say that there is heat exchange in a process?

   What level of answer did you want?

General principles:
Heat isn't a substance or a property that some matter can have.   A system can have a temperature or some internal energy but it does not have anything you can call "heat".   It should be possible to identify two systems if there is a heat flow, energy flows out of one system and into the other system.  While the energy is in transfer, that is "heat".  This exchange of energy is commonly called a heat flow or heat exchange.

Medium level answer:
   Heat  or "heat flow"  describes the natural and automatic transfer of energy from a body at a higher temperature to one of lower temperature when they come into thermal contact.
   Two bodies are in thermal contact if there is atleast one path along which heat can flow.

You will then focus in on a precise definition of heat by either:  excluding certain energy transfers  OR you'll take a constructive approach - start with an empty set of permitted energy transfers and directly specify the mechanisms of energy transfer that will be included in the definition of heat flow.

The Exclusion method to define heat exchange:
   Certain forms of energy transfer are specifically excluded.   Heat should not be thermodynamic work done on a system.   Also there should not be a direct transfer of matter (which might have some internal energy) from one system to the other.

The constructive method (start from nothing and inlcude things):
    The mechanisms of energy transfer that define heat include conduction, through direct contact of immobile bodies, or through a wall or barrier that is impermeable to matter; or radiation between separated bodies; or friction due to isochoric mechanical or electrical or magnetic or gravitational work done by the surroundings on the system of interest....
     ... (Convection might be included but Wikipedia choose to exclude it and argue that although it's an intermediate process, the ultimate flow of energy is achieved by conduction or radiation once the fluid has has been carried to a particular location).

[Both these approaches are based on the Wikipedia entry for heat and some text is a direct quotation].

Best Wishes.

[hamdani yusuf (OP)]

Certain forms of energy transfer are specifically excluded.   Heat should not be thermodynamic work done on a system.   Also there should not be a direct transfer of matter (which might have some internal energy) from one system to the other.

Can you give some examples?

[hamdani yusuf (OP)]

The mechanisms of energy transfer that define heat include conduction, through direct contact of immobile bodies, or through a wall or barrier that is impermeable to matter; or radiation between separated bodies; or friction due to isochoric mechanical or electrical or magnetic or gravitational work done by the surroundings on the system of interest....
     ... (Convection might be included but Wikipedia choose to exclude it and argue that although it's an intermediate process, the ultimate flow of energy is achieved by conduction or radiation once the fluid has has been carried to a particular location).

Heat conduction can be seen as near field radiation. It's extremely rare case for two ordinary objects to collide at atomic level.

[Eternal Student]

Hi again.

Can you give some examples?

    There's not much point.   There are text books and other sources of information if you're really interested.   We've also already spent some time discussing work done by (or on) a system which shows a volume change.

    What seems more relevant is just to jump straight to a higher level answer:
"Temperature",  "thermal contact" and "heat"  are all very difficult things to define.  At least, they are difficult to define or explain at any microscopic scale (where you're trying to take a reductionist approach and break things down to the smallest indivisible units like particles with well defined properties).   However, they can be easily defined (or really just decalred to exist) on a macroscopic scale.  This is the usual development of the subject matter that is called "thermodynamics".

    For example, if you try to take a microscopic definition for "heat" then it is not supposed to include a transfer of matter from one system to another  BUT  a transfer of photons such as infra-red radiation is something we would very much like to include in the definition of heat.   What makes a photon different to some other particle with energy?   If an electron and positron cross a barrier then that would seem to be a transfer of matter across the barrier and would not be considered as a heat transfer.  However, if they annhilate on one side of the barrier, only some photons cross the barrier and then the photons interact with some nucleus on the other side of the barrier to reform particles and anti-particles  -  that would seem to be just fine.

Best Wishes.

[alancalverd]

Heat conduction can be seen as near field radiation. It's extremely rare case for two ordinary objects to collide at atomic level.

No. "Radiation" excludes energy transfer by collision conduction or convection. Words have very precise meanings in physics, which is why some of us despise priests, politicians and philosophers. And sloppy journalists.

[hamdani yusuf (OP)]

Heat conduction can be seen as near field radiation. It's extremely rare case for two ordinary objects to collide at atomic level.

No. "Radiation" excludes energy transfer by collision conduction or convection. Words have very precise meanings in physics, which is why some of us despise priests, politicians and philosophers. And sloppy journalists.

Water and ice have very precise meanings in physics. It doesn't mean that they can't have the same substance. What I wrote there is comparable to saying that ice is just frozen water.

Radio wave, microwave, visible light, and X ray are also manifestations of the same phenomenon.

[Bored chemist]

Water and ice have very precise meanings in physics.

Not if you are talking about a single molecule.

That's part of the problem.

[Bored chemist]

It's extremely rare case for two ordinary objects to collide at atomic level.

In the condensed phase (liquid, gas or dense SCF) they are already in contact.
Even in the gas phase, collision frequencies are very high- GHz for ordinary air.

[hamdani yusuf (OP)]

Water and ice have very precise meanings in physics.

Not if you are talking about a single molecule.

That's part of the problem.

Is conduction have a  precise meaning for a single molecule?

[hamdani yusuf (OP)]

It's extremely rare case for two ordinary objects to collide at atomic level.

In the condensed phase (liquid, gas or dense SCF) they are already in contact.
Even in the gas phase, collision frequencies are very high- GHz for ordinary air.

What's the maximum distance between two molecules where they are said to be still in contact?
I was referring to collisions between atoms from two adjacent macroscopic solid objects.

[Bored chemist]

Water and ice have very precise meanings in physics.

Not if you are talking about a single molecule.

That's part of the problem.

Is conduction have a  precise meaning for a single molecule?

No.

But energy transfer from one molecule to a second by collision is different from energy transfer by radiation.

[Bored chemist]

What's the maximum distance between two molecules where they are said to be still in contact?

It's hard to say, but if one mechanically  transfers energy to the other they can't be far apart.

[Eternal Student]

It might be worth stating that a lot of the discussion has been continued on your other thread:
"What is temperature"
https://www.thenakedscientists.com/forum/index.php?topic=80604.80

[alancalverd]

What I wrote there is comparable to saying that ice is just frozen water.

No, it's very different. Ice is indeed one (actually several) macroscopic phase of an ensemble of H2O molecules, but radiation and conduction are entirely different phenomena.

The rate of heat exchange between two bodies by conduction depends on the temperature difference  T_h - T_l. The rate of heat exchange by radiation depends on T_h⁴ - T_l⁴. 

[hamdani yusuf (OP)]

What I wrote there is comparable to saying that ice is just frozen water.

No, it's very different. Ice is indeed one (actually several) macroscopic phase of an ensemble of H2O molecules, but radiation and conduction are entirely different phenomena.

The rate of heat exchange between two bodies by conduction depends on the temperature difference  T_h - T_l. The rate of heat exchange by radiation depends on T_h⁴ - T_l⁴. 

Imagine two parallel square metal plates, 1 m² each. First plate has temperature of 200K, while second plate is 400K. Initial distance is 10 meters. Measure heat exchange rate, and record as P0.
Reduce the distance to 1 meter, or 10% of previous distance. Measure and record the heat exchange as P1. Repeat the process until those plates are considered touching.
Will we find abrupt change or discontinuity in the record? At what point?

[Bored chemist]

Will we find abrupt change or discontinuity in the record?

Yes

At what point?

When they touch.

There will be a very sharp increase just before they actually touch- at the distance when electron tunnelling becomes significant.

[hamdani yusuf (OP)]

Will we find abrupt change or discontinuity in the record?

Yes

At what point?

When they touch.

There will be a very sharp increase just before they actually touch- at the distance when electron tunnelling becomes significant.

Is there any experimental evidence for that?

[alancalverd]

If the separation s exceeds twice the diameter d, we are in "far field" where the rate of exchange  by radiation tends towards a 1/s² law.

If 0 < s < d we approach a "near field" radiative exchange that is almost independent of s.

If s = 0 the exchange is conductive   

It's all pretty standard stuff in radiation physics and thermal engineering.