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# What does the m symbol for magnetic flux mean? Why is it needed when the phi symbol exists?

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.

Edit: Chapter 6, page 257 onwards. The m thing are on the first parts.magneticselectromagneticmagnetic-fluxshareedit  follow  flagedited 2 days agoDKNguyen28.3k11 gold badge3737 silver badges7979 bronze badgesasked 2 days agoAndroidV114122 bronze badges

• 1Are you just talking about B and H? At least tell us which of the 880 pages it is on. google.com/… – DKNguyen 2 days ago
• 1Chapter 6, page 257 onwards. It appears in the first parts – AndroidV11 2 days ago
• 2I 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 2 days ago
• 1But m is used in the intensity of magnetisation. So effectively, can I replace m with phi there? – AndroidV11 2 days ago
• 1It 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 2 days ago
• 1Also 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 2 days ago
• 1I don’t really understand magnetics well enough to answer your question. Best I can do is post the images for you. – DKNguyen 2 days ago
• 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 16 hours ago
• @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 14 hours ago
• (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 numberj, (b) Euler constant e, (c) entropy s. / to continue, … – tlfong01 10 hours ago
• (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 10 hours ago
• 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 9 hours ago

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_potentialshareedit  follow  flagedited 10 hours agoanswered 2 days agoTony Stewart Sunnyskyguy EE7596.1k22 gold badges3434 silver badges140140 bronze badges

• 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 2 days ago
• 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 2 days ago
• 1They 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 Sunnyskyguy EE75 2 days ago
• 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 15 hours ago
• 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 15 hours ago
• 1sometimes 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 Sunnyskyguy EE75 10 hours ago
• 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 9 hours ago
• 1Induction cooking only works on metal bases not water or oil based ingredients. – Tony Stewart Sunnyskyguy EE75 9 hours ago
• 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 9 hours ago
• 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 9 hours ago
• 1It’s others you could be misleading – Tony Stewart Sunnyskyguy EE75 9 hours ago
• 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 9 hours ago
• Let us continue this discussion in chat. – tlfong01 just now   Edit

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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.

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.

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.

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 –

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

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)

/ to continue, …

This is not the end of answer. I plan to write at least a couple of more pages. Stayed tunedshareeditdeleteflagedited 14 hours agoanswered 2 days agotlfong0152044 silver badges66 bronze badges

• 1Wow thanks for the large amount of effort. – AndroidV11 2 days ago
• 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 2 days ago