Do You Know the Law?
November 1963 Radio-Electronics

November 1963 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

This quiz from a 1963 issue of Radio-Electronics magazine tests readers' familiarity with key electrical laws beyond the well-known Ohm's Law, including Kirchhoff's voltage and current laws for circuit analysis, Joule's law for heat in resistors, Poynting's theorem for energy flow, Lenz's law for induced forces, Neumann's formula for transformers, Wien's displacement law impacting infrared electronics, Helmholtz's theorem for vector fields, the Wiedemann-Franz-Lorentz law connecting conductivity types, and Faraday's law of induction, challenging engineers to recall these foundational principles and their discoverers. Well, it is not exactly a quiz as much as questioning what you know about each of the laws and their founders. I only got about half of them.

Do You Know the Law?

By Tom Jaski

Just about everyone who ever soldered two resistors together knows Ohm's Law. But in electronics we deal daily with many relationships expressed as laws and attributed to their discoverers, meaning they are considered to be laws of nature - incontrovertible truths about the world around us. Although some laws have been found inadequate, and some will no doubt be found to be actually wrong, the laws in this little quiz have so far stood the test of time. Here they are. Do you know the laws underlying the electronics you are using in your work?

1. Kirchhoff was a German physicist born in Konigsberg in 1824. He is credited with what we call the two "Kirchhoff's laws" used in electronics. Do you know what they are?

2. When you have a current through a resistor and wish to express the losses in it, you know that this is represented by FR. But do you know whose law says it is?

3. Ever hear of Poynting and his law?

4. Do you know what Lenz' law is?

5.Neumann had something to say which concerns you every time you deal with an if transformer. Can you figure out what it is?

6. Again the man and the work he is most well-known for are not the law named after him. Wien, of Wien bridge fame also has a law, not dealing with bridges. Unless you know illumination, you probably don't know it. But Wien's law now also enters electronics via infrared advances.

7. Helmholz is most widely known for his research and essays on human hearing. Yet Helmholtz' law had nothing to do with hearing, but dealt with charges instead. Know it?

8. Three physicians got together and named a law after all of them, Wiedeman-Franz-Lorentz law. If you know this one, you qualify as a science professor (or you're just lucky).

9. Let's not forget the great Faraday. His law is very important in electromagnetic phenomena, and shows how this great scientist was well ahead of his time.

It is no disgrace if you do not associate these effects with their laws. Most engineers know the effects, but have all but forgotten who discovered and expressed them in the form of a law. They are useful just the same.

Quizzes from vintage electronics magazines such as Popular Electronics, Electronics-World, QST, Radio-Electronics, and Radio News were published over the years - some really simple and others not so simple. Robert P. Balin created most of the quizzes for Popular Electronics. This is a listing of all I have posted thus far.

Answers

1. In electronics we recognize two Kirchhoff laws. The first states that "the sum of currents flowing toward any junction is equal to the sum of currents flowing away from that junction." The second law goes like this: "The algebraic sum of voltages around any closed path in a circuit is zero."

2. The relationship between current, resistance and dissipated energy - the familiar I²R expression - is known as Joule's law.

3. Poynting's law, relatively obscure to most of us, deals with effects of transfer of energy in electromagnetic fields and is important in microwaves. It states that the transfer of energy can be expressed as the product of the values of the magnetic field and of the components of the electric field which are perpendicular to the magnetic field, and that the flow of energy at any point is perpendicular to both fields.

4. Lenz' law is important wherever alternating current flows, from 60 cycles to rf. So fundamental is it that it might almost be called the alternating-current counterpoise to Ohm's law. The law deals with the change in flux caused by the motion of one of two mutually coupled coils, and the direction of current in one of them, as follows: "If a constant current flows in a primary circuit (A) and if by motion of A or the secondary circuit (B), a current is induced in B, the direction of induced current will be such that, by its electromagnetic action on A, it tends to oppose the relative motion of the circuits." In other words, "Counter electromagnetic motive force." But if you don't like the way this was stated, blame J. Clerk Maxwell, the famous physicist, for that is how he expressed it. Why the ac counterpoise to Ohm's law? Simply that if a man knows Ohm's law, he knows dc; if he knows Lenz' law, he knows ac.

5. Neuman's law deals with mutual inductance (hence the reference to i.f. transformers) when no magnetic material is present. It states that M (mutual inductance) is a constant for a given relative physical position of the coils, and independent of the fact that the current flows in one or the other coil, and of frequency, current and phase. It is an "appendix" of the law of conservation of energy, which will be overthrown when we move through space at greater speed than light!

6. Wien's law, better known as Wien's displacement law, shows that the wavelength of maximum radiation intensity is inversely proportional to the absolute temperature of a black body, and that the intensity of radiation at this maximum wavelength varies as the fifth power of the absolute temperature. Infrared detection techniques, hinging on the facts known about "black bodies," are intimately concerned with Wien's displacement law.

7. Helmholtz' law describes the curve of increase of a current when a circuit with R and L is suddenly closed. Thus: I = I_o(1 - e^-tR/1), which is a familiar curve to most electronics workers. (It is also related to the discharge curve of a capacitor, but this was not stated by Helmholtz.)

8. Wiedeman, Franz and Lorentz, all physicists, have a law which states that the ratio of electrical conductivity and thermal conductivity at a certain temperature is independent of the conductor material! Sounds hard to believe, but it is true.

9. Faraday, most famous of historical experimenters, stated in his law that if there is any closed linear path in space, and the magnetic flux in this path (surrounded by the path) varies in time, the emf induced in the path is equal to the negative rate of change of the flux in lines per second. This can only be expressed with calculus in formula form, so we will skip that part. But it is a fact that the importance of this discovery was not practically applied in radio for almost a century!

All of which just goes to show that there are many facts about nature and the world around us that we can use to good advantage in our daily work with electronics, without giving much thought to the great discoverers of science, who had powers of observation and deduction, and the ability to state them as "laws of nature."