PHYSICS

Thermal expansion: the extreme heat
of a July day in Asbury Park,
New Jersey, USA, caused these
railroad tracks to buckle
(Wide World Photos)
the initial and final equilibrium states (that is, for various processes),
one can find that Q ­ A is the same for all paths connecting
the initial and final states. We conclude that the quantity
Q - A is determined completely by the initial and final states of
the system, and we call the quantity change in (Q -
A) the
internal energy of the system, which is independent of the path.
The internal energy function is U and changes in the internal
energy can be expressed as:
t1U = U,­ U; = Q - A (12.4)
where all quantities must have the same energy units. This
equation is known as the first law of thermodynamics.
12.4. THERMODYNAMIC SYSTEMS
A thermodynamic system is that part of the universe that is
under consideration. A real or imaginary boundary separates the
system from the rest of the universe, which is referred to as the
surroundings.
Thermodynamic systems are classified into three categories:
• Isolated systems do not exchange energy or matter with the
exterior;
• Closed systems exchange energy with the exterior but not
matter;
• Open systems exchange both energy and matter with the
exterior.
When a system is at equilibrium under a given set of conditions,
it is said to be in a definite thermodynamic state.
12.5. THE FIRST LAW OF THERMODYNAMICS
The first law of thermodynamics is a generalization of the law
of conservation of energy and includes possible changes in
internal energy.
Suppose a thermodynamic system undergoes a change from an
initial state to a final state in which Q units of heat are absorbed
(or removed) and A is the work done by (or on) the system. If
the quantity (Q - A) is measured for various paths connecting
90
. 12.6. THE SECOND LAW OF THERMODYNAMICS
The second law of thermodynamics establishes which processes
in nature mayor may not occur. The field of thermodynamics
evolved from a study of heat engine - a device that converts
thermal energy into other useful forms of energy, such as
mechanical and electrical energy. It is useful to represent a heat
engine schematically as in fig. 12.2. The engine (represented by the
circle at the center of the diagram) absorbs a quantity of heat
(Q,,) from the high-temperature reservoir. It does work (A) and
gives up heat (Q) to a lower-temperature heat reservoir. The net
work (A) done by the engine equals the net heat flowing into the
engine:
A = Q -
(12.5)
h
The thermal efficiency (17) of a heat engine is defined as the
.... , ---~I ratio of the net work done to the heat absorbed
during one cycle:
Heat engine
Q c
1] =~=Q,,-Qc =l-~
Q" Q" Q"
(12.6)
Fig. 12.2. Schematic representation of a heat engine
that receives heat Q] from a hot reservoir, expels
heat Q 2
to the cold reservoir, and does work A
91