298
THE
RELATIVITY
PRINCIPLE
E
=
h
i
-
€cl
[80]
G
=
q
3
E(i
1
~
F
7
/2
A
part of
these values is
to be
allotted
to
the
electromagnetic
field,
and
the
rest to
the massless
body
that
is
subjected to
forces
due to
its
charge.1
§15. The
entropy and temperature of
moving
systems
Of
the
variables
that determine the
state
of
a
system,
we
have
thus far
only
used
pressure,
volume,
energy,
velocity,
and
momentum,
but
have not yet
dealt with thermal
quantities.
The
reason
for this
was
that
for
a
system's
motion
it is irrelevant
what
kind of
energy
is
supplied,
so
that
we
had
no
reason
to distinguish between
heat
and
mechanical
work.
However,
we now
want
to
introduce thermal
quantities
as
well.
Let
the
state
of
a moving
system
be
completely
determined
by
the
quantities
q,
V,
and
E. Obviously,
in the
case
of
such
a
system
we
have to
consider
as
the heat
supplied
dQ
the total
energy
increase
minus the
work
produced
by
the
pressure and
that
spent
on
increasing
the
momentum,
so
that
we
have
dQ
=
dE
+ pdV
-
qdQ
.
(23)
After
having
so
defined the heat
supplied
for
a moving
system,
we can
intro-
duce
the absolute
temperature
T
and
the
entropy
n
of
the
moving
system
by
considering
reversible
cyclic
processes
in the
same
way
it is
done
in
text-
books
of
thermodynamics.
For
reversible
processes
the
equation
[81]
1Cf.
A.
Einstein,
Ann. d.
Phys.
23
(1907): 373-379.
[82]