ON WORK, ENERGY, AND THE MECHANICAL EQUIVALENT OF HEAT.
QUESTION 28. _For what purpose are all steam engines used?_
_Answer._ They are used to produce _motion_, which is opposed by some
_resistance_. Thus, if an engine is employed to raise grain from a
railroad car to the top of a warehouse, it must produce motion, which
is resisted by the weight of the grain; if it is used to saw wood, it
must give motion to the saw, which is resisted by the fibres of the
wood; a locomotive engine must produce motion of a train of cars, which
is resisted by the air, the friction of the journals and the rolling of
the wheels on the track; if the locomotive is employed on a grade or
incline, besides the frictional resistance referred to it must overcome
that due to its own weight and that of the train, which is gradually
lifted as it ascends the incline. In producing motion opposed by some
resistance an engine is said to be doing “_work_.”
QUESTION 29. _Can this work be accurately measured?_
_Answer._ Yes; but in order to measure anything we must first establish
some accurate standard or unit of measurement. Thus we say a bar of
iron is so many inches long, or a road is so many miles long. In like
manner we speak of so many seconds, or minutes, or hours, or days, or
years, when we speak of time. So it is necessary, in order to estimate
or measure “_work_” in a strictly scientific manner, for us to fix upon
some accurate standard or unit. In this country and in Great Britain
the unit agreed upon for this purpose is the amount of power required
to raise ONE POUND ONE FOOT, and is called a _foot-pound_. If we raise
one pound two feet we do two foot-pounds of work; if three feet, three
foot-pounds, and so on. Again, if we raise a weight of two pounds one
foot high, we likewise do two foot-pounds of work; or if we raise it
two feet high, we do four foot-pounds, and so on. In order to determine
the amount of work done, we must MULTIPLY THE MOTION PRODUCED (_in
feet_) BY THE RESISTANCE (_in pounds_), AND THE RESULT WILL BE THE WORK
DONE IN FOOT-POUNDS.
QUESTION 30. _How many foot-pounds of work are performed in a
pile-driving machine in raising a weight of 1,200 lbs. 24 feet?_
_Answer._ 1,200 × 24 = 28,800 foot-pounds.
QUESTION 31. _When this weight is raised, is the force which was
exerted in raising it annihilated or lost?_
_Answer._ No; because the weight has the capacity of doing an equal
amount of work when it falls, from the momentum[8] it acquires in
falling. This _power of doing work_ which it acquires in falling is
called _energy_. Now, although the weight has no motion-producing power
when it is raised to the top of the machine, yet obviously such action
is then _possible_ which when it rested on the earth was not possible.
It has no energy as it hangs there dead and motionless; but energy is
possible to it, and we might fairly use the term _possible energy_
to express this power of motion which the weight possesses,[9] and
which is therefore called _potential energy_. As soon as the weight is
allowed to fall it acquires a greater velocity the farther it falls,
and its potential energy thus becomes and is called _actual energy_.
[8] Momentum is not a very exact term, but is used here because it
ordinarily conveys the idea we wish to express.
[9] Tyndall’s “Heat Considered as a Mode of Motion.”
QUESTION 32. _How do we explain such phenomena as the heating of a
car-axle while turning under a car, the heating of brake-blocks when
the brakes are applied to car-wheels, the heating of an iron rod by
hammering, and of a turning tool when cutting a piece of metal?_
_Answer._ All of these phenomena are due to the fact that the _actual
energy_ of motion is converted into heat, as has been repeatedly proved
by many able and ingenious investigators and experiments.
QUESTION 33. _When the weight of the pile-driver falls, is its energy
also converted into heat?_
_Answer._ A part is expended in compressing the material into which
the pile is driven and in overcoming the friction of the earth against
the pile, each of which efforts develops heat, and another portion is
converted into heat by the impact or blow of the falling weight on the
head of the pile.
QUESTION 34. _Is all energy convertible into heat and heat into energy?_
_Answer._ Yes. Science has demonstrated very clearly that they are
mutually convertible.
QUESTION 35. _Has it been ascertained how much heat is equivalent to
one foot-pound of work?_
_Answer._ Yes; it has been found, from the most carefully-made
experiments that the amount of heat which is required to raise
the temperature of one pound of liquid water by one degree of
Fahrenheit[10] is equivalent to 772 foot-pounds of work. It must be
remembered that this is the theoretical equivalent of heat, and that
only a very small proportion of this amount of work is ever realized
from the heat developed by the combustion of fuel.
[10] Thermometers are divided into different scales. The one called
the Fahrenheit scale, after its originator, is the one ordinarily
used in this country.
QUESTION 36. _If, then, heat is convertible into work and work into
heat, can the transmutation of the heat of the steam in the cylinder of
an engine into work, and the reverse process, be explained?_
[Illustration: _Fig. 9._
Scale ³⁄₈ in. = 1 foot.]
_Answer._ Yes. Take a cylinder, fig. 9, and, in order to make the
conditions of the experiment as simple as possible, imagine it to be
placed in a vacuum. Now let saturated steam be admitted under the
piston so as to fill the cylinder half full at an absolute pressure of
100 lbs. If we will allow this steam to expand to double its volume
and raise the piston _without doing any work_, and then repeat the
experiment with a load of 50 pounds on the piston, whose area is
one square inch, it will be found that the temperature of the steam
is sensibly less, after lifting the weight, than in the previous
experiment, in which it expanded without doing work, showing that part
of the heat was abstracted from the steam by doing work, or, in other
words, was converted into work. If then, after the steam has expanded
and lifted the weight, we press the piston down so that the steam under
the piston is compressed to its original volume, we shall find that
its temperature is the same as before, as the work done in compressing
it is converted into heat. In these experiments it is assumed that
there is no friction of the piston, nor loss of heat from radiation or
conduction. The same phenomena can be observed in machines used for
compressing air, which is heated to so high a temperature when it is
compressed that it is necessary to cool the cylinders of such machines
by circulating a current of cold water around them.
QUESTION 37. _What practical relation is there between the
convertibility of heat into work, and the conducting and radiating
properties of different substances explained in answer to Question 27?_
_Answer._ The fact that heat is only another form of energy, or
“the power of doing work,” indicates that its loss by conduction or
radiation lessens that power just as much as or more than the loss
or waste of coal would, and therefore every effort should be made to
protect the different parts of engines from loss of heat by covering
them with substances which conduct or radiate very little heat. Care
should also be taken to exclude cold air from circulating in contact
with these parts, and excepting for supporting combustion, the nature
of which will be explained hereafter, it should be excluded from the
heating surface of boilers.
QUESTION 38. _What is meant by the term_ LATENT HEAT OF EVAPORATION_?_
_Answer._ By _latent heat_ is meant that heat which _apparently_
disappears when water or other liquids are vaporized. Thus, it
is found that if any quantity of water is converted into steam at
any pressure, it is necessary not only to heat it to a temperature
equivalent to that of the steam, or to the boiling-point, but after
it has reached that temperature an additional amount of heat must be
added in order to keep up the process of boiling. Notwithstanding this
addition of heat to the water, the temperature of the steam produced
will not be higher than that of the boiling water, thus showing that a
considerable quantity of heat is absorbed, the only effect of which is
to change the water into a gas or steam. This apparent disappearance
of heat can be shown if we take a pound of boiling water whose
temperature is 212 degrees and mix it with a pound of ice-cold water
at 32 degrees. The result will be a mixture of two pounds of water of
a mean temperature of 122 degrees. If now we convert a pound of water
into steam at atmospheric pressure, the steam will heat 6.37 lbs. of
ice-cold water up to 122 degrees, showing that a pound of steam at
atmospheric pressure contains over six times as much heat as a pound
of water of the same temperature as indicated by a thermometer. A
similar apparent disappearance of heat occurs when other liquids are
evaporated, and when ice or any other solid is converted into a liquid.
QUESTION 39. _What is the explanation of these phenomena?_
_Answer._ The exact reasons which will explain them fully are probably
not yet clearly understood, but it is at least extremely probable that
when any substance is changed from a solid to a liquid, or from a
liquid to a gaseous condition, “a large portion of the heat is spent
_in doing work_ against the force of cohesion.”[11] The particles of
solid bodies, as we know, are so united that it requires more or less
force, according to the nature of the substance, to tear them apart.
Now we can conceive that the heat is changed into a form of energy,
and in that condition resists this attraction of the particles to each
other, and that being thus transformed it has lost the capacity of
expanding the mercury in the thermometer. A similar effect takes place
when a liquid is converted into a gas. In the former condition the
particles move freely about each other and have little or no attraction
for each other, but when it becomes a gas they have a _repulsion from_
each other. The heat is thus converted into the energy of repulsion,
and therefore is in reality no longer in the condition of heat and
consequently does not affect the thermometer. We can illustrate this by
supposing that by using steam heat is converted into work by raising
the weight, or drop as it is called, of a pile-driving machine. When
the weight is raised to the top of the guides from which it falls,
although, as already explained, the heat is converted into _potential
energy_, yet if we attached a thermometer to the drop we would not
find that it was any warmer than before the drop was raised. If it
were possible to make an instrument sufficiently sensitive to indicate
an instantaneous change of temperature in the weight while falling,
we would not find any increase of its temperature at the instant it
had acquired its greatest momentum and just before it struck the
object under it, although its potential energy would at that instant
be converted into _actual energy_ of motion. If, however, the weight
should strike an unyielding object, its actual energy would at once
be reconverted into heat, which our thermometer would indicate. The
phenomenon of what is called latent heat of evaporation seems to be
very similar to that described--the heat when the water is changed from
a liquid to a gaseous condition is transformed into energy, which, as
already stated, has no effect upon the mercury of the thermometer.
[11] Balfour Stewart on the Conservation of Energy.
QUESTION 40. _What is meant by the_ TOTAL HEAT _of steam?_
_Answer._ The “total heat of steam” is a phrase used to denote the
sum of the heat required to raise the temperature of water from some
given point up to the boiling-point due to a given pressure, and of the
heat which disappears in evaporating one pound of water under a given
pressure (or _latent heat of evaporation_.) Thus the latent heat of
one pound of steam at atmospheric pressure (14.7 lbs.) is 966.1 units;
and 212 units of heat are necessary to raise water from zero to the
boiling-point; therefore the total heat counted from zero of steam of
atmospheric pressure is 1,178.1 units. At 100 pounds absolute pressure
the latent heat is 885.5 and the sensible heat 327.9 degrees; therefore
the total heat measured from zero is 1,213.4 units.