CHAPTER VII
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._
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.
=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.