9.5 Heat Transfer Mechanisms
331
By transferring energy in the form of heat, open systems can lose energy to
another system without doing any work on the second system. This means we can
identify the first term in Eq. (9.3) as the heat input, δQ:
δQ =
i
dn i i .
(9.6)
We see that heat transfer causes a redistribution of the occupation numbers of
the possible energy levels in the system, without changing the energies of the
microstates. Later, we will associate such redistributions as a change in the disorder
of the system.
9.5 Heat Transfer Mechanisms
We can identify two distinct methods for the transfer of energy by heat: conduction
and radiation. Some authors also include convection; others add phase change.
But convection is macroscopic transport of material carrying energy that does not
necessarily come from heat transfer.
Phase change also may involve heat absorption or release, but the energy
released or absorbed does not have to come from heat transfer. Material undergoing
phase changes, such as water going from a solid to a liquid, or a liquid to a
gas, may produce or absorb heat energy while spatially reconfiguring their atoms
or molecules, without any chemical change. A phase transformation alters the
microscopic energy states i and may change the occupation numbers n i of the
atoms or molecules, but not necessarily so. If several phases are mixed together, and
each involves heat energy transfer to change, then under ordinary circumstances
the temperature of the mixed system is preserved until conversion to one phase is
complete. A glass of ice water accepting heat demonstrates this behavior. 11
The distinction between heat conduction and radiation is one of spatial range.
Conduction occurs when short-range forces, usually electric, act between incoherently moving particles, causing a net energy transfer when one set of particles carries
more energy than a second set. Heat transfer by radiation is caused by long-range
interaction between two sets of particles, that interaction being the exchange of
electromagnetic waves. The waves are generated by the incoherent jiggling of the
particles in both sets, causing charges with the particles to accelerate and radiate.
11 Under more extraordinary circumstances, substances can become ‘supercooled’ or ‘superheated’, i.e. not change phase even when cooled or heated past the ordinary phase-change
temperature. This condition may occur when ‘nucleation’ sites are missing. For example, in the
case of supercooled gases, there would be no locations where adhesion of molecules to a surface
brings those molecules together over a sufficient time for weak bonding to form between them, and
then quasi-stable clusters of molecules can form.
331
By transferring energy in the form of heat, open systems can lose energy to
another system without doing any work on the second system. This means we can
identify the first term in Eq. (9.3) as the heat input, δQ:
δQ =
i
dn i i .
(9.6)
We see that heat transfer causes a redistribution of the occupation numbers of
the possible energy levels in the system, without changing the energies of the
microstates. Later, we will associate such redistributions as a change in the disorder
of the system.
9.5 Heat Transfer Mechanisms
We can identify two distinct methods for the transfer of energy by heat: conduction
and radiation. Some authors also include convection; others add phase change.
But convection is macroscopic transport of material carrying energy that does not
necessarily come from heat transfer.
Phase change also may involve heat absorption or release, but the energy
released or absorbed does not have to come from heat transfer. Material undergoing
phase changes, such as water going from a solid to a liquid, or a liquid to a
gas, may produce or absorb heat energy while spatially reconfiguring their atoms
or molecules, without any chemical change. A phase transformation alters the
microscopic energy states i and may change the occupation numbers n i of the
atoms or molecules, but not necessarily so. If several phases are mixed together, and
each involves heat energy transfer to change, then under ordinary circumstances
the temperature of the mixed system is preserved until conversion to one phase is
complete. A glass of ice water accepting heat demonstrates this behavior. 11
The distinction between heat conduction and radiation is one of spatial range.
Conduction occurs when short-range forces, usually electric, act between incoherently moving particles, causing a net energy transfer when one set of particles carries
more energy than a second set. Heat transfer by radiation is caused by long-range
interaction between two sets of particles, that interaction being the exchange of
electromagnetic waves. The waves are generated by the incoherent jiggling of the
particles in both sets, causing charges with the particles to accelerate and radiate.
11 Under more extraordinary circumstances, substances can become ‘supercooled’ or ‘superheated’, i.e. not change phase even when cooled or heated past the ordinary phase-change
temperature. This condition may occur when ‘nucleation’ sites are missing. For example, in the
case of supercooled gases, there would be no locations where adhesion of molecules to a surface
brings those molecules together over a sufficient time for weak bonding to form between them, and
then quasi-stable clusters of molecules can form.