|April 1945 Radio News|
These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio & Television News articles I have already posted. All copyrights are hereby acknowledged.
Not having a full collection of magazines is a real disadvantage when multiple part stories are published and some editions are missing. Such is the case here with Milton Kiver's series on electronics design. I do have other parts of the series, but they have not been posted on RF Cafe yet. However, each installment is pretty much independent of the others. This month's topic is on the fundamental theory of electrical potential and force. The name 'Maxwell' is mentioned, but not in the way that strikes fear in the heart of engineering students being introduced to the integral and differential forms of his eponymous equations (I know first-hand), so it's safe to keep reading. First semester physics books cover the same material, but since you night not have one handy, here you go.
By Milton S. Kiver
Part 10. An explanation of the importance of the electric field theory of Maxwell's equations in describing u.h.f. phenomena. A subsequent article will cover a similar explanation based upon the magnetic principles.
Fig. 1. - Showing the relationship of force vs. distance between electrical charges.
The name of Maxwell seems to be ever present in describing ultra-high-frequency phenomena and this perhaps, is as it should be, since without his work it might have taken a great deal longer to fill out the ideas on wave propagation.
The set of equations that form the basis for the electromagnetic theory are called Maxwell's equations, although he is not solely responsible for most of them. All these equations are expressed either in ordinary differential form or compactly placed by means of vector analysis. Neither will be used here since it is the avowed purpose of these article to use very little mathematics, so (with apologies to Maxwell) an attempt will be made to discuss these important relations without the benefit of the exact science. With this explanation as a background it should be much easier to comprehend some of the properties of wave guides, cavity resonators, antennas, and any other device that depends for its action on the above-mentioned law.
Maxwell's equations are generalized statements on the behavior of electric and magnetic fields. The electric laws are based on the observed behavior of the electron and the influence it exerts on other nearby electrons. It would have been just as easy to have based all our findings on the proton (positive charge) behavior, but since the electron is the more mobile of the two it is easier to deal with its properties. For the magnetic laws it is necessary to go back to the properties of a magnet and see its actions and reactions when brought near other substances that are affected by it. When all these important facts are tabulated, there are four (sometimes given as five) statements that form the starting point. These four (or five) equations are known as Maxwell's equations.
Now to see just what these equations mean. They set down a set of rules by which the science of electromagnetics has been developed. As long as these rules are followed, all is fair; but if any deviations are introduced, then the fundamental principles of the science are being ignored and something else is now being engaged in; another game - so to speak. There is nothing wrong in modifying the rules if it is found that experience dictates such a change. But so far Maxwell's equations have predicted all the observed results so it is safe to assume that they are entirely correct and no attempt should be made to change any of their forms.
These equations may be looked upon as four walls to keep the players within certain confines. While in these boundaries they may make other limitations, such as having the electric or magnetic fields restricted to one direction, but this is still within the game since the general rules are in no way being altered. The rules are just not being used to their fullest extent - that is all. Since the equations work in one, two, or three dimensions, it is possible to use the above restrictions and still arrive at correct results.
Fundamental Electrical Theory
Fig. 2. - Indicating the distribution of force about an electric charge by means of equipotential lines.
Fig. 3. - The use of lines of force and equipotential lines in radio tubes.
Before any discussion of the above equations will be undertaken, it would be advisable to review the foundations of all electric and magnetic theory. The electric field will be dealt with first. Any electric charge, such as an electron, exerts a force upon other charges near itself. This force, while being just as much a force as the gravitational pull or the force exerted by a machine, does not apply to every material body in the universe but only to other electric charges. If these other charges are positive, the force is one of attraction while if they are negative, it is a repelling force. Now, for many people the idea of just showing a charge without indicating its force was rather hard to understand, so whenever an electric charge is shown, lines radiating away from this charge are also drawn and these lines are called lines of force. They are the pictorial representations of what cannot be seen but what is quite definitely there, namely, the force itself. Fig. 5 shows these various lines, both for attraction and repulsion.
In addition to the representation of the electric lines of force, as in the above figure, it is also possible to indicate the distribution of electric forces as in Fig. 2. Here instead of lines of force, we have all the points that have the same force exerted on them, connected by one line. Because of the symmetry of the field about the electric charge, these equipotential lines happen to be concentric circles. However, this is a special case and will not always occur. The circles closest to the center have the greatest force exerted on them, while the farther away we get from the central electric charge, the less the force.
Note that the equipotential lines never cross each other. If one were to place an electric charge on an equipotential surface or line then it would require no work at all to move this charge along this equipotential line because the charge is neither being moved toward or away from the central electrical charge. Radio engineers, especially those engaged in tube manufacture, use charts illustrating the equipotential lines and fields of force within tubes quite extensively. To illustrate, refer to the diagrams of Fig. 3.
In Fig. 3A, we see that in a simple diode having a cylindrical plate, the lines of force are radial from the plate to the cathode. The equipotential lines are also drawn and an electron leaving the cathode will try to reach the plate by the shortest route. The shortest route will be along the path where the force that is exerted by the plate is greatest. This will always occur along the lines of force or at right angles to the equipotential lines. Hence, the electron will travel in a straight line from cathode to plate.
In Fig. 3B we have the deflecting plates of a cathode-ray or television tube. The electron beam, in speeding toward the fluorescent screen, must pass between these plates and while in this region, Will be subjected to the electric field that exists there. The plate that is more positive will attract the negative electron beam and cause the beam to deflect in this direction. The stronger the voltage, the greater the deflection.
Here we have merely two examples of the use of visualizing electric forces and electric fields and how they are utilized, whether directly or indirectly, in radio apparatus.
The regions that the electric forces act in are called electric fields and, in the literature of the subject, are quite often referred to. These electric fields can be explored by taking other electric charges and placing them under the influence of these fields of force. From the way these outside charges act it is possible to tell the direction of the force in these fields and just how intense the field strength is. By experimenting with electric fields of various strengths and noting different reactions on charges placed in these regions, it is possible to arrive at rules which govern the behavior of all such situations. Thus the first step, experimentation, will lead on to the next point where it is possible to express all the facts in a law or formula and which will cover all data taken under similar conditions. For the case just mentioned there is Coulomb's Law which states that the force acting between two electric charges (or what is the same thing, two electric fields, since fields are produced by charges) is directly proportional to their strengths and inversely proportional to the square of the distance between them. Using the formula notation, it is
q1 is the amount of charge of one unit
q2 is the amount of charge of the other unit
d is the distance between them
F is the force brought on by placing the two charges close to each other.
Before going much further it might be advisable to point out that while the terms electric field and electric intensity are sometimes used interchangeably, they are really separate. The electric field refers to the region or place where the electric intensity or electric force acts and is not actually attached or connected to this force in any way. It is quite analogous to a pitcher or container of water and the water itself. Both are distinct and yet when placed on the dinner table the two terms are used interchangeably. In the same sense, electric field electric field intensity and just plain electric intensity may be considered one and the same as far as it will be used here.
Laws for Electric Fields
Fig. 4. - The number of lines of force leaving any electrical charge is (by definition) equal to 4π x the charge.
Fig. 5. - Configuration of electric lines of force, (A) for attraction and (B) repulsion.
The next phase that interested the scientists after they had formulated the ideas of electric charge and electric force or intensity was to get the exact relationship between the charge and the amount of electric field intensity due to this charge. The problem was this: Suppose there was some charge inside a hollow sphere. How is the number of lines of force or how is the electric intensity related to this charge Q?The answer is known as Gauss' Law and in words it states that the net outward electric flux (or lines of force) from any charge in all directions is 4π times the amount of charge Q. See Fig. 4. Thus there are two fundamental relationships that hold in a region containing electric charge:
1. First is Coulomb's Law and this sets up the idea of electric charges and the forces between them.
2. And second is Gauss' Law which gives the exact relationship between the force set up by any charge and the amount of the charge itself.
It is to be noted that whenever lines of force are drawn they are always shown with arrows (Figs. 2 and 5). These arrows are meant to indicate the direction in which the electric force due to the charge act. The electric theory was first developed by men who postulated that the lines of force should have arrows on them pointing in the direction that a positive charge, if placed near any electric charge, whether positive or negative, would go. This means that if a certain space were filled with protons or positive charges, then all the lines of force would point away from these positive charges since they would repel an exploring positive particle in this field. On the other hand, negative charges would attract this exploring positive charge and so the arrows on the lines of force connected with negative charges point toward the charge itself. All this sometimes tends to be confusing and so it is better to just think of these lines of force as an actual force which will attract oppositely charged particles and repel like charges.
With this in mind, no confusion should result. These lines of force are continuous, starting out from positive charges and ending up on negative charges. Should the space in question contain only positive charges, then the lines of force due to these charged particles will continue indefinitely out into space toward infinity, and a force would everywhere be felt. This is theoretically true but in an actual case the effect of its electric intensity would be confined to the immediate vicinity since from Coulomb's Law it can be seen that force varies inversely as the square of the distance. At a distance of say 8 centimeters from a charge the force would be 1/16 of what it is at 2 centimeters from the same charge, so it is obvious that no great distance is needed before the overall effect of the electric field is negligible. A clear idea of the way these forces decrease with distance can be obtained from Fig. 1.
Turning now from the static case, let us put an electric charge (for example, an electron) into motion and see if any new facts are discovered. The easiest method of accomplishing this end is to use a conductor, that is, a substance that contains a large number of free electrons. Since the free electrons will experience a force when any electric field is brought to bear on them, they will be forced to move and the number that will flow past any point in this conductor will be determined by the strength of the electric force that is causing them to move plus the ease with which they can travel through this conductor. Does the last statement sound familiar? It should, for although it is never stated this way, it the good old formula E = IR which is Ohm's Law. Note that a steady electric field or electric force is used here - a situation that is true for direct-current circuits.
Since the electron has a charge it will produce an electric field. But it has been found on investigation that when put into motion the electron will likewise give rise to a magnetic field. In 1819 the Danish physicist Oersted discovered that current in a wire affected a compass that was held near this wire. Since compass needles will only move under the influence of magnetic fields, Oersted concluded that there must be a magnetic field about a wire carrying a current.
A little while later Ampere carried this one step further and showed that two wires with currents in them exerted forces on each other. With the two wires separate and distinct from each other it must have been the magnetic field that reacted. It is from the above that the first ideas on the relationship between electric and magnetic fields were brought into existence and slowly started the trend that ended with Maxwell's formulation of the electromagnetic theory.
The type of magnetic field produced depends on the type of electric current that is flowing in the wire. A steady flow of electrons will produce a magnetic field that is likewise constant in value, while changing magnetic fields are the product of changing electric currents.
This idea of electric currents giving rise to magnetic fields was proved time and again and was accepted without question in the 19th century. However, in the theories that Faraday and, a little later, Maxwell envisioned about electrodynamics, there was more to the story than just the above. Must currents always be present for magnetic fields to occur? - questioned Maxwell. Was it not possible to deal only with a varying or changing electric field and derive magnetic effects from this? Maxwell claimed that it was possible and so added this revision to the existing electric field equations that were accepted at that time. He postulated two types of currents; one was called conduction current and this was our ordinary flow of electrons along any good conductor. The second type of current, due not to actual moving electrons but rather changing electric fields, he called displacement currents. Both, he said, gave rise to magnetic fields.
A classical example to illustrate the above is given with a fixed condenser in a setup such as shown in Fig. 6. Closing the battery switch will cause current to start flowing in the circuit. Since electrons do not flow across the space between the plates, it might be said that the circuit is open at this point. However, since the number of electric lines of force between the plates are changing, due to the charging effect of the condenser, then, according to Maxwell, the circuit is now no longer open at this point. Instead of electrons or conduction currents flowing across this space, we now have a displacement current and the circuit is continuous. It would even be possible to detect a magnetic field produced between the condenser plates during that portion of the time when the condenser is charging up and the electric field is varying in this region. The entire process ceases, of course, when the condenser becomes charged.
The above was all that was needed to allow Maxwell to set up his basic equations. From these he developed the idea that electromagnetic waves travel through space at. a finite or measurable velocity which we know to be approximately 186,000 miles per second.
Electric and Magnetic Fields
If the reader is a bit puzzled as to why Maxwell needed this added idea of changing electric fields giving rise to magnetic fields, let him pause for a minute and stop to consider that in the space between the transmitting antenna and the receiving antenna there is no flow of electrons at all.
Posted November 10, 2014