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For reference, I have read the Textbook of Electrical Technology by Theraja and I don't understand how it just uses the mass symbol as a symbol of magnetic flux in poles. Why is it any different from the original phi symbol? Why does the pole thing matter? Isn't it already covered by the magnetic flux quantity itself.

I don't have convenience right now of using a computer to just copy the parts from the book which I am confused with so I cited the reference. Another question I have is why do some formulas use the phi symbol while the others use the m? Are they different in terms of units somehow? Thank you.

enter image description here enter image description here

Edit: Chapter 6, page 257 onwards. The m thing are on the first parts.

DKNguyen
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AndroidV11
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    Are you just talking about B and H? At least tell us which of the 880 pages it is on. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwj1rbzXir3rAhWSFjQIHVDQAXIQFjABegQIARAB&url=https%3A%2F%2Felectricalanswers.files.wordpress.com%2F2014%2F09%2Fa-textbook-of-electrical-technology-volume-i-basic-electrical-engineering-b-l-theraja.pdf&usg=AOvVaw3JFvnysWtI5HfpYzGB7Ghe – DKNguyen Aug 28 '20 at 04:42
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    Chapter 6, page 257 onwards. It appears in the first parts – AndroidV11 Aug 28 '20 at 04:47
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    I think it's just using m as a placeholder for something to be explained in the future so you don't get overwhelmed. Purely for demonstrative purposes. Just read through it from top to bottom and treat m exactly as it says: "a number whose units will be defined later". As far as I can tell, m does not appear past the pages I edited into your post. – DKNguyen Aug 28 '20 at 04:51
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    But m is used in the intensity of magnetisation. So effectively, can I replace m with phi there? – AndroidV11 Aug 28 '20 at 06:24
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    It is actually also used in the Magnetic Potential in my edited post. Can I replace that with phi too? I think it just became complicated for me that in different situations different symbols are used even though there is no distinction. – AndroidV11 Aug 28 '20 at 06:26
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    Also sorry for the many comments, but I really don't understand well the 6.5 part. It is also connected to what I am asking about phi and m. Yeah guys sorry if I am really confused about the whole m and phi thing but I want to get a more intuitive sense on how it works and right now I still can't settle on an idea. I mean like it is a placeholder, but why bother doing it with that in the first place. – AndroidV11 Aug 28 '20 at 06:32
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    I don't really understand magnetics well enough to answer your question. Best I can do is post the images for you. – DKNguyen Aug 28 '20 at 14:37
  • Yes, I very much agree with you saying "I want to get a more ***intuitive sense*** on how it works and right now I still can't ***settle on an idea***. I hope to try to give an answer on what you want, perhaps in one or two more days' time. Cheers. – tlfong01 Aug 30 '20 at 05:38
  • @AndroidV11, I have made a draft of how I tried to make sense of the magnetic symbols, now summarized in ***Appendix A of my answer***. (1) For me the comparison chart is important, (2) Often it is not that you don't get the intuitive sense, but perhaps there no such thing as intuitive sense, because it is just definitions and algebra. I often tell myself the following: (1) Laplace one told those who don't understand or cannot make sense of the Laplace Transform. "OK, so I don't fully understand my transform,but it works", / to continue, ... – tlfong01 Aug 30 '20 at 08:35
  • (2) Oliver Heavyside once says something similar "Am I to refuse to eat because I do not fully understand the mechanism of digestion?" More examples: 4 figure log table to transform multiplication to addition, slide rule engineers just use, without understanding or forgetting the log and anit log (OK, I know you never heard of log table and slide rule:)) (3) In EE and Physics, there are many concepts which one cannot thoroughly intuitively understand. Examples are: (a) imaginary number***j***, (b) Euler constant ***e***, (c) entropy s. / to continue, ... – tlfong01 Aug 30 '20 at 12:05
  • (d) fourier and laplace transform, (e) nyquist diagram, (f) e = mc**2, (g) f = m*a. In some cases, you might need to do a dimensional analysis to convenience youself that nothing seems to to go wrong. And in case you still worry about your coming ee exam, let me tell you a story: ages ago, even I didn't understand everything thoroughly, I did pass all my exams, including gce al physics with distinction, and got my letters including mscphysics, mieeembcsceng. Good luck to your exam. Cheers. – tlfong01 Aug 30 '20 at 12:07
  • One more perhaps laughable thing about imaginary number ***j*** and ***e***. It is only a year ago I tried to listen BBC In Our Time program talking j and e, then I am happy to know that the j had actually troubled the mathematicians and scientists for centuries because it is hard to understand. But then EE engineers make good use of j. Of course I am not encouraging you to give up going deeper. I actually regret that in my exam days, I dived too deep and forgot to drill exercises enough so I almost failed exam because I could not answer fast in exam because lack of drilling exercises. – tlfong01 Aug 30 '20 at 13:20
  • One more thing about the symbol ***MMF = Ampere Turns*** in the comparison chart. I admit that the comparison chart is not comprehensive enough. For example, there is no comparison between MMF and EMF, and MMF is defined as Ampere Turn there. This reminds me what I read about making a magnet, and the ***ampere turns*** is used is used as a design parameter. DIY an Electromagnet Ampere Turns, Guass - Cool Magnet Man https://www.raspberrypi.org/forums/viewtopic.php?f=37&t=234552&hilit=inductor+tlfong01&sid=67231d78902197e27be6a17d144c518c&start=75#p1437668. More references in Appendix B. – tlfong01 Aug 31 '20 at 00:47
  • So if you skim my long reference list, you might see why I have more intuitive sense on magnetism than you, because I have done perhaps 100+ hobbyist hours studying and do experiments on inductor, back EMF etc And the reason that I am very interested in back EMF because back EMF recently fried a couple of my 5V switching power supplies. – tlfong01 Aug 31 '20 at 00:54

2 Answers2

3

m is the mass of one ferromagnetic material which was equated from two possible masses m1,m2 into one. The Magnetic potential,M has been defined by the energy per pole of magnetic flux per unit in the magnetic H field. The flux phi can be derived as the mass drawn into that measured H field both controlled by the gap, r.

A relevant Wiki definition follows but is more relevant to Maxwell's equations than mass.

That is for static permanent magnets, similar to static charges in insulators=dielectrics which have an E field and forces inverse to radius r for parallel plates, cylindrical or flat.

E fields are for voltage/m and H fields are for currents/m.

The energy equivalence is 1/2 CV^2=1/2 LI^2, which in ideal LC components causes perpetual motion of charges at a resonant frequency. Yet ideal parts never exist due to a series R. For electromagnets, coils, chokes and inductors https://en.wikipedia.org/wiki/Magnetic_scalar_potential

Tony Stewart EE75
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  • Oh my goodness, I though EE75's answer is concise and should be good. So I read the first sentence, and so far so good. Then big trouble came when I read the second sentence. I know what is electrical potential, but what the hack is "magnetic potential"? Yes, it defined by "energy per pole" of "magnetic flux per unit" of "magnetic field", all of which are Greek to me, no wonder denoted by Greek symbols Anyway, I dared not to read the third sentence when I sneak previewed the term "mass" which I guess is what the OP is asking about. Another scary term is "gap", again, what the hack is it? – tlfong01 Aug 28 '20 at 07:04
  • Yesterday I bought a RFID reader and the spec says it is only near field NFC, not far field. But I want to detect the RFID tag farther. So I googled and read that NFC is mainly Electrical field, very little Magnetic field. I was surprised, because I always (wrongly) thought that E & M always go together. My conclusion is that I know too little magnetism to properly use the RFID reader. . I googled and found the book the OP is reading and started reading Chapter 6. / to continue in my answer below.. – tlfong01 Aug 28 '20 at 14:35
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    They always go together but the mutual impedance is sqrt[L/C] and the RFID uses both to send and receive with different source impedances to load. Geometry and gaps play a big role in mutual inductance and capacitive coupling of E fields. A scope probe picks up 50/60 Hz E fields from your finger while an arc nearby from a current transient can couple into a shorted probe loop in the RF spike. – Tony Stewart EE75 Aug 28 '20 at 17:52
  • Thank your for your explanation. So I now understand that E and M waves/field do always go/exist together. It is only the probe/antenna that prefers to read/pick either E or M, or more E, less M, or vice versa etc.By the way, your saying of ***geometry and gaps*** reminds me of my experience of my tech collage days when I DIY AC200 to AC12V transformers. I went to a scrap metal shop and bought 2 catties of E shape and I shape iron plates, and placed one plate on top of the other, interlacing the E directions, leaving little gaps (not sure) of the assembled core, preventing ***eddy current***.. – tlfong01 Aug 30 '20 at 07:26
  • At that time, I know that the ***gap*** is to prevent the ***eddy current*** flowing inside the "core". I was happy some years ago when I bought my first induction cooker and realized that if there is no gap, therefore eddy current flows freely and heats up the metal pot above the cooking surface, and boils my egg. I also now realize that some rice cookers use induction current to heat up the rice grains very evenly, because they using induction current to evenly heat up the rice pot side walls, not just the bottom part of the pot. Cheers. – tlfong01 Aug 30 '20 at 07:36
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    sometimes there are too many false assumptions in your statements to correct. These ought to go in the chat room. Eddy Currents only occur in your thicker laminates not the thinner insulation – Tony Stewart EE75 Aug 30 '20 at 12:22
  • Ah, I very much agree. Actually I was hesitating to say that I actually knows or remember very little about gap, except that gap should increase flux flowing through the core. I know when I studied my EED diploma, my classmates taking the "heavy current" should know many times more heavy current stuff. Abut the eddy current and induction cooker thing, / to continue, ... – tlfong01 Aug 30 '20 at 12:59
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    Induction cooking only works on metal bases not water or oil based ingredients. – Tony Stewart EE75 Aug 30 '20 at 13:04
  • I confess I have not googled or wiki enough about induction cooker so my guess are very likely just wishful thing, just a reflection of what EE stufff I learnt in college can apply in daily lives of induction cooker and microwave oven. I suggest we take a break, and let me studied more before I ask you again for help. BTW, your advice on three phase electricity distribution is very useful to my knowledge in smart city projects. / to continue, ... – tlfong01 Aug 30 '20 at 13:04
  • I recently joined a community project on home solar energy and found that my knowledge in home solar energy and how to join it to the grid is almost zero. So I need to also study more before I ask questions again in EE SE. Just thinking aloud. Sorry for all the typo errors. About induction cooker - yes I know it only works for "magnetic" metal pot. Sorry if I have mislead you about water or oil. Cheers. – tlfong01 Aug 30 '20 at 13:07
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    It’s others you could be misleading – Tony Stewart EE75 Aug 30 '20 at 13:19
  • Ah, I am sorry if I misled others about water and oil with induction cooker. I did not recall anything about this part. Let me see if I can go back to search about oil and water. Many thanks for pointing that out. I always feel sorry that others think that I know a lot about computer and EE and do not try to point out my mistakes, even if they are very sure. Once I was a computer teacher and I always told me students that things go too fast and I could not catch up fast enough to teach, I knew mainframe computer, but mini came along, and then microprocessor, then mcu, then .OMG, Bed time.. – tlfong01 Aug 30 '20 at 13:29
  • Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/112457/discussion-between-tlfong01-and-tony-stewart-sunnyskyguy-ee75). – tlfong01 Aug 30 '20 at 22:36
1

Question

The OP seems to be getting stuck in the following page of Theraja' book Chapter 6 Electricity and Magnetism. Let me see if I can help.

magnetism

Answer

Update 2020aug28hkt2247

My apologies for my earlier too casual reading and learning notes. So I deleted my irrelevant paragraphs and made a summary. My progress is summarized below.

1. Notes on learning permeaabity μ

I found the Magnetism and Electricity comparison chart (Section 6.25, Part A) very useful to understand the ideas which are new to me. Comparing corresponding terms in both M and E immediately clarifies my mind in μ*, when comparing to ρ. For now I just compare μ to ρ, but I am pretty sure that now I can give and understanding examples μ.

2. Notes on learning Magnetic Force F and Magnet Field Strength H

I found Section 6.2 and 6.3 is useful to understand Magnetic Force and Field Intensity H. (Section 6.3 is what the OP is asking). I guess once I understand F, then I understand H, and from now on, only focus on H and forget F.

And I found that I only need to try to understand the first basic equation that firstly introduced. For example, it is not necessary to understand the vector form of the equation, basic form is enough to move on to next topic. I can always come back later in the second pass to look at the vector form of the basic equation. Another thing is don't bother to ask why the constants come up, they are just constants as in calculating the area of a circle, and surface area and volume of the sphere. I can always come back to study the derivation of the equation and constants (See References 5, 6). Moreover, the definitions for F and H are just definitions, there is not much theory behind. But of course you need to have an intuitive idea of Force and Field, and here the comparsion helps a lot to grasp of the intuitive area, eg. MMF corresponds to EMF, Flux corresponds to Current (with some complication, again see the comparison chart remarks).

In short, the Magnetism and Electricity comparison chart is my friend.

/ to continue tomorrow.

Part A - Electricity and Magnetism Comparison and Contrast Chart

So I flipped the pages, looking for something to help me remember things. I found the following very good. It is a compare and contrast of Magnetism and Electricity.

I think if I know Electricity well, this comparison/contrast chart should help me learn and understand magnetism faster.

me cheat 1

One important comparison is the following:

(a) Electricity's resistance corresponds to Magnetism's reluctance.

(b) Electricity's conductance ρ corresponds to Magnetism's premeance μ.

Part B - Magnetism Learning Plan

I am now looking at the topics chart.

topics selection

I found that it is important to know the topics in the first four lines of the contents of Chapter 6, and grasp the meanings of the concepts.

H, B, μ, μr, I, K

It is important to remember the following:

(1) Magnetism is more complicated than Electricity.

(2) The topics to learn should be in this sequence. μ, H, B, I, K (μ is already learnt in the comparison chart.

Part 3 - Learning H -

H 1

H 2

H 3

Learning notes

  1. Comparing use of π in magnetism and circle/sphere equations.

In this equation for circle, perimeter = 2πr, area = πr**2

π is just an universal constant, the same π used in the magnetism equation.

/ to continue, ...

References

(1) A Text Book of Electrical Technology (2005 pdf version) - BL Theraja, AK Theraja, 2005

(2) Magnetic Scalar Potential - Wikipedia

(3) Maxwell's equations - Wikipedia

(4) Basic Electrical Engineering Questions and Answers – Ohm’s Law for a Magnetic Circuit - Sanfoundry

(5) Why is a sphere's surface area four times its shadow (4πr2)? - 2018dec02, 3,323,464 views

(6) Why is the Volume of a Sphere ((4π/3) r**3) - 2014spe28, 544,314 views

Appendices

Appendix A - How to get the intuitive sense of the magnet symbols and settle on an idea Draft 0.1 tlfong 2020aug3001

1. Introduction

I am trying to describe how to get an intuitive sense of the Magnetic symbols, using Electricity to compare and contrast.

Part A - Electricity symbols I, R, V, and C (conductance) and their intuitive senses.

(a) We start with Ohm's Law, which actually applies to both Electricity and Magnetism alike, with some variations.

(b) We know if a wire has high resistance, and for a constant "force" (voltage, or EMF) then low current flows.

(c) So we know current is inversely proportional to resistance, or I = V/R

(d) Now by definition, conductance C = 1/R, so I = V*C

(e) You already have an intuitive sense of the symbols I, V, R, C because you once learnt the analog of I = water flow, R = diameter of pipe

Part B - Magnetic symbols

We now need to agree (not try to remember now) the following based on the comparison cheat sheet (Section 6.25)

(a) Flux F in Webers (Wb) compares with Current I in Amperes

(b) MMF (ampere turn) compares with EMF

(c) Flux density B (Wb/m2) compares Current density A/m2 (yes, no symbol, this is cause of confusion)

(e) Permeance P = 1/Reluctance compares to resistance R = 1/pA

(g) Permeability compares Conductivity

(d) Reluctance S = 1/uA compares resistance R = 1/pA ??? 6.25 point 5 seems problematic

Notes - (a) to (g) seem reasonable, I got stuck in (d)

Appendix B - How to get the intuitive sense of the magnetism by doing experiments

tlfong01's Inductance Learning Notes

(1) Buzzer and And EMF

(2) LC Tank Oscillator and Hartley

(3) Mutual Inductance and Oliver Heaviside

(4) Current Voltage in Inuductor

(5) Energy Stored in Inductor

(6) Measuring Inductance

(7) Imaginary Number j and Euler Constant e

(8) How to measure Inductance - Rose-Hulman University

(9) Using 100mH Inductor and LM2596 to make a switching regulator

(10) Inductor Current and Voltage Maesurement - Electronics Tutorials

(11) Finding Inductance of Electromagnet ZYE1-P20/15 DC6V 0.5A (Coil Resistance = 11.7Ω)

(12) Solenoid and Relays

(13) Inductance Tutorials - Electronics Tutorials

(14) DIY an Electromagnet Ampere Turns, Guass - Cool Magnet Man

/ to continue, ...

This is not the end of answer. I plan to write at least a couple of more pages. Stayed tuned

tlfong01
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    Wow thanks for the large amount of effort. – AndroidV11 Aug 28 '20 at 14:39
  • Thank you for your nice words and encouragement. I am just learning as a newbie. As I said, I know quite a lot of Electricity, but very little in Magnetism. So you see I am polishing my learning notes again and again, because whenever I learn a new topic, I found my description of the old topic not clear at all. – tlfong01 Aug 28 '20 at 14:43