ELECTRICAL AND MECHANICAL ENERGY
The production of electricity is simply a transformation of energy from
one form into another, usually mechanical energy is changed into
electrical energy and a dynamo is simply a device for effecting the
transformation.
Prof. Fessenden truly remarks there are two independent
properties of matter--gravity and inertia--and these give two
ways of defining force and energy.
It should always be remembered that electricity is something real,
although not easily defined. And then, too, while it is not matter and not
energy, yet under proper conditions (it having the power of doing work) it
is convenient to speak of its performances as electric energy. The
following questions and answers, although few in number, may present the
subject with clearness.
=Ques. What is energy?=
Ans. Energy is the capacity for doing work.
Steam under pressure is an example, a spring bent ready to be
released is another form, again, water stored in an elevated
tank has capacity for doing work. These examples illustrate
_potential energy_, as distinguished from _kinetic energy_.
Potential energy may be defined as _energy due to position_, and
kinetic energy, as _energy due to momentum_.
=Ques. What is matter?=
Ans. Matter is anything occupying space, and which prevents other matter
occupying the same space at the same time.
=Ques. What name is given the smallest quantity of matter which can
exist?=
Ans. The atom.
An atom means that which cannot be cut, scratched, or changed
in form and that cannot be affected by heat or cold or any known
force; although inconceivably small, atoms possess a definite
size and mass.
=Ques. What is a molecule?=
Ans. A molecule is composed of two or more atoms.
=Ques. What is the behaviour of these minute bodies?=
Ans. They are perpetually in motion, vibrating with incredible velocities.
=Ques. Why at this point are definitions of energy and of matter most
useful?=
Ans. Because, as stated, all electric action is an exhibition of energy,
and energy must act through matter as its medium.
=Ques. What is the difference between electricity and magnetism?=[8]
Ans. The ultimate nature of neither is known. There are, however, some
differences. To sustain a current of electricity requires energy. To
sustain magnetism requires no energy. A current of electricity is always
accompanied by a magnetic field of peculiar form. Magnetism alone cannot
produce electricity. Electricity can do work; but magnetism cannot in the
same sense--and alike with electricity, neither can it exist without
contact with matter.
=Ques. How is energy transmitted from one part of a material substance to
another?=
Ans. Gradually and successively. It requires a medium and also time.
=Ques. What is the principal use or function in mechanics of electricity?=
Ans. It is purely that of transmission. It corresponds to ropes, shafts
and fluids as a medium of conveying and translating power or work.
=Ques. What is work?=
Ans. Work is the overcoming of resistance through a certain distance.
As a quantity of water moving from a higher to a lower level
will do work, so also will a quantity of electricity falling
through a difference of potential.
=Ques. How is work measured?=
Ans. In foot pounds.
=Ques. What is a foot pound?=
Ans. The amount of work done in raising a weight of one pound one foot or
the equivalent, overcoming a pressure of one pound through a distance of
one foot.
=Ques. What is the electrical unit of work?=
Ans. The _volt-coulomb_.
A volt-coulomb of work is performed when one ampere of current
flows for one second in a circuit whose resistance is one ohm,
when the pressure is one volt.
=The Ampere-Hour.=--A gallon of water may be drawn from a hydrant in a
minute, or in an hour; it is still one gallon. So in electricity, a given
amount of the current, say one _coulomb_, may be obtained in a second or
in an hour.
_The ampere is the unit rate of flow._
What is called the electric current is simply the relation of any quantity
of electricity passed to the time it is passing; that is
quantity in coulombs = current in amperes × time in seconds, or
simply
coulomb = ampere × second.
Again:
10 coulombs = 2 amperes × 5 seconds = 10 amperes × 1 second =
1 ampere × 10 seconds, etc.
One _ampere-hour_ is simply another way of saying 3,600 coulombs. Of
course 3,600 coulombs of electricity may be obtained in any desired time.
It all depends on the rate of flow or the current strength in amperes.
For instance, 2 amperes in 1/2 hour, or 4 amperes in 1/4 hour
will also give one ampere-hour of 3,600 coulombs.
It is well to keep the distinction between coulombs and amperes in mind,
as even in text books very lately published these units are confounded. To
illustrate further the difference between coulombs and amperes, the
following example is given.
It is sometimes estimated that the quantity of electricity in a
flash of lightning is 1/10 coulomb, and the duration of the
discharge 1/20000 part of a second. What is the current in
amperes?
Now since
coulombs = amperes × seconds (1)
solving (1) for the current,
amperes = coulombs/seconds (2)
substituting the given values in (2),
amperes= (1/10) / (1/20000) = 2000
=Power.=--The term power means _the rate at which work is done;_ it is
usually expressed as _the number of foot pounds done in one minute_, that
is
power = (foot pounds) / minutes
_Power exerted for a certain time produces work._
=Ques. What is the mechanical unit of power?=
Ans. The horse power.
=Ques. What is one horse power?=
Ans. 33,000 foot pounds per minute.
The unit is due to James Watt as being the power of a strong
London draught horse to do work during a short interval and used
by him to measure the power of his steam engines. One horse
power = 33,000 ft. lbs. per minute = 550 ft. lbs. per sec. =
1,980,000 ft. lbs. per hour.
=Ques. What is one horse power hour?=
Ans. Work done at the rate of one horse power for one hour.
=Ques. What is the electrical unit of power?=
Ans. The watt.
=Ques. What is a watt?=
Ans. It is the power due to a current of one ampere flowing at a pressure
of one volt. One watt = one ampere × one volt. It is equal to one joule
per second.
=Ques. What is a kilowatt?=
Ans. 1,000 watts.
=The Watt-Hour.=--The elements which may be measured are, however, not
only the volume of current, the unit of which is the ampere, and time, the
unit of which is the hour, but also the _pressure_, the unit of which is
the volt.
It is evident that a perfect system of electrical measurement should take
account of the total amount of energy consumed, and should depend not only
upon the volume of current, but _also upon the pressure_ at which the
current is applied.
The basis of such a system if provided in a unit which is the product of
the two units of current and pressure, and which is termed a _volt-ampere_
or _watt_.
_The watt-hour represents the amount of work done by an electric current
of one ampere strength flowing for one hour under a pressure of one
volt._
EXAMPLE--An incandescent lamp taking one-half an ampere of
current on a circuit having a pressure of 100 volts, or a lamp
taking one ampere on a circuit having a pressure of 50 volts,
would each be consuming 50 watts of energy, and this multiplied
by the number of hours would give the total number of watt-hours
for any definite time.
_The watt, then, is an accurate and complete unit of measurement and is
generally applicable to all forms of electrical consumption._
[Illustration: FIG. 85.--Tyndall’s experiment illustrating the production
of heat by friction. A brass tube about 7 inches in length and 3/4 of an
inch in diameter, is fixed on a small wheel. By means of a cord passing
round a much larger wheel, this tube can be rotated with any desired
velocity. The tube is three parts full of water, and is closed by a cork.
In making the experiment, the tube is pressed between a wooden clamp,
while the wheel is rotated with some rapidity. The water rapidly becomes
heated by the friction, and its temperature soon exceeds the boiling
point. The pressure caused by the formation of steam forces out the cork
and projects it to a height of several yards.]
A watt of electrical energy corresponds to 1/746 of a horse
power of mechanical energy; hence, if a lamp or motor require
energy equivalent to 1/746 of a horse power for one hour, it
might be said to take one watt-hour.
=Mechanical Equivalent of Heat.=--The eminent English physicist, James
Prescott Joule, worked for more than forty years in establishing the
relation between _heat_ and _mechanical work_; he stated the doctrine of
the conservation of energy and discovered the law, known as Joule’s law,
for determining the relation between the heat, current pressure, and time
in an electric circuit.
=Ques. What is heat?=
Ans. A form of energy.
Heat is produced in the agitation of the molecules of
matter--the energy expended in agitating these molecules is
transformed into heat.
[Illustration: FIG. 86.--Joules’ experiment on the mechanical equivalent
of heat, in which he caused paddle-wheels to rotate in a vessel of water
by means of falling weights _W_. The amount of work done by gravity upon
the weights in causing them to descend through any distance _d_ was equal
to their weight _W_ times the distance. If the weights descended slowly
and uniformly, this work was all expended in overcoming the resistance of
the water to the motion of the paddle-wheels through it; that is, it was
wasted in eddy currents in the water. Joule measured the rise in the
temperature of the water and found that the mean of his three best trials
gave 427 gram meters as the amount of work required to develop enough heat
to raise a gram of water one degree. He then repeated the experiment,
substituting mercury for water, and obtained 425 gram meters as the work
necessary to produce a calorie of heat. The difference between these
numbers is less than was to have been expected from the unavoidable errors
in the observations. He then devised an arrangement in which the heat was
developed by the friction of iron on iron, and again obtained 425. This
corresponds to 772 foot pounds, but later experiments show that the
correct value is 778 foot pounds.]
=Ques. How is heat measured?=
Ans. In British thermal units (B.t.u.).
=Ques. What is the British thermal unit?=
Ans. The quantity of heat required to raise the temperature of 1 lb. of
pure water 1° Fahr., at or near 39.1° F., the temperature of maximum
density.
=Ques. What is the mechanical equivalent of heat?=
Ans. The number of foot pounds of mechanical energy equivalent to one
British thermal unit.
Joule’s experiments 1843-50 gave the figure 772 ft. lbs. which
is known as Joule’s equivalent. Later experiments gave higher
figures, and _the present accepted value is 778 ft. lbs._, that
is: 1 B.t.u. = 778 ft. lbs.
=Electrical Horse Power.=--It is desirable to establish the relation
between _watts_ and _foot pounds_ in order to determine the _capacity_ of
an electric generator or motor in terms of _horse power._
One watt is equivalent to one joule per second or 60 joules per minute.
One joule in turn, is equivalent to .7374 ft. lbs., hence 60 joules equal:
60 × .7374 = 44.244 ft. lbs.
Since one horse power = 33000 ft. lbs. per minute, the electrical
equivalent of one horse power is
33000 ÷ 44.244 = 746 watts.
or,
746 / 1000 = .746 kilowatts (K.W.)
Again, one kilowatt or 1000 watts is equivalent to
1000 ÷ 746 = 1.34 horse power.
=The Farad.=--The measure constructed to hold a gallon of water may be
called the gallon measure. The capacity of a condenser which would contain
a charge of one coulomb under one volt pressure is the _farad._ It may
seem strange that there is a unit of quantity and another of capacity to
hold that quantity, when in the case of water the term “gallon” may
suffice for the measure and the liquid it can hold. Electricity in this
respect, however, corresponds to a _compressible fluid_ or a _gas_.
A gallon measure may hold a gallon of gas or ten; it depends entirely upon
the pressure. So a condenser of a certain size may hold any number of
coulombs, according to the electrical pressure.
The farad being inconveniently large for practical use, one-millionth of a
farad, called a _microfarad_, is generally adopted.