Consider two macroscopic systems A
and A′ that can interact and exchange energy. Together they form a joint
system A*.
- The
joint system A* is isolated.
- Therefore,
its total energy remains conserved.
- Interaction
between A and A′ can occur mainly in two ways:
- Thermal
interaction
- Mechanical
interaction
A general interaction may involve both
heat and work.
1. Thermal Interaction
If the external parameters
remain constant, the energy levels of the system remain unchanged. Any
energy exchange is then called thermal interaction.
- Energy
is exchanged in the form of heat.
- If
the system absorbs energy:
- If
the system loses energy:
Since the combined system is
isolated:
Therefore,
Thus, heat absorbed by one
system equals the heat released by the other system.
Key idea
In thermal interaction, the energy
levels remain fixed, but the distribution/probability of energy among the
fixed levels changes, causing a change in the average energy.
2. Thermal Insulation / Thermal
Isolation
Two systems are thermally
isolated when there is no exchange of heat between them.
Characteristics
- Heat
exchange is prevented.
- External
parameters remain constant.
- If
the system is initially in equilibrium, it remains in equilibrium as long
as the external parameters remain unchanged.
- Materials
such as asbestos and fiberglass can be used for thermal insulation.
All processes occurring under
thermal insulation are called adiabatic processes.
Thermal isolation: No heat exchange
3. Adiabatic Interaction
Suppose A and A′ are thermally
insulated from each other. If their external parameters are changed,
energy can be exchanged between them through mechanical work.
This is called adiabatic
interaction.
Characteristics
- Systems
are thermally insulated.
- Heat
exchange does not occur:
- External
parameters can change.
- Energy
levels of the system can change.
- Energy
is transferred through macroscopic work.
If average energy increases:
This represents work done on the
system.
If average energy decreases:
This represents work done by the
system.
Since the combined system is
isolated:
Therefore,
Hence, work done on one system
is equal in magnitude and opposite in sign to the work done by the other system.
Two reasons for change in average
energy
During adiabatic interaction,
average energy can change because:
- Energy
of individual states changes
due to changing external parameters.
- Probability
of occupation of the states changes.
Therefore, adiabatic interaction is
also called mechanical interaction.
4. General Interaction
When two systems interact such
that:
- external
parameters are allowed to change, and
- heat
can also be exchanged,
the interaction is called general
interaction.
Thus, both thermal and
mechanical interactions occur simultaneously.
The total change in average energy
is
where:
= energy change due to thermal interaction
= energy change due to change in external
parameters, i.e. macroscopic work