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.

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

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    \$\begingroup\$ Are you just talking about B and H? At least tell us which of the 880 pages it is on. google.com/… \$\endgroup\$
    – DKNguyen
    Aug 28, 2020 at 4:42
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    \$\begingroup\$ Chapter 6, page 257 onwards. It appears in the first parts \$\endgroup\$
    – AndroidV11
    Aug 28, 2020 at 4:47
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    \$\begingroup\$ 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. \$\endgroup\$
    – DKNguyen
    Aug 28, 2020 at 4:51
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    \$\begingroup\$ But m is used in the intensity of magnetisation. So effectively, can I replace m with phi there? \$\endgroup\$
    – AndroidV11
    Aug 28, 2020 at 6:24
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    \$\begingroup\$ 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. \$\endgroup\$
    – AndroidV11
    Aug 28, 2020 at 6:26

2 Answers 2


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

  • \$\begingroup\$ 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? \$\endgroup\$
    – tlfong01
    Aug 28, 2020 at 7:04
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    \$\begingroup\$ 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. \$\endgroup\$ Aug 28, 2020 at 17:52
  • 1
    \$\begingroup\$ 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 \$\endgroup\$ Aug 30, 2020 at 12:22
  • 1
    \$\begingroup\$ Induction cooking only works on metal bases not water or oil based ingredients. \$\endgroup\$ Aug 30, 2020 at 13:04
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    \$\begingroup\$ It’s others you could be misleading \$\endgroup\$ Aug 30, 2020 at 13:19


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.

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, ...


(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


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

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    \$\begingroup\$ Wow thanks for the large amount of effort. \$\endgroup\$
    – AndroidV11
    Aug 28, 2020 at 14:39
  • \$\begingroup\$ 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. \$\endgroup\$
    – tlfong01
    Aug 28, 2020 at 14:43

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