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Electromagnetic Theory

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Bob Scott

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I'd like to understand electromagnetics a bit better. What is the definition of electromagnetic flux? I'm attempting to make a better equation for flux density.

Is flux static, like a measure of the magnetic field intensity, as the lines of force that you can see using iron filings over a bar magnet?

Or is it defined as dynamic only, applying to AC fields only? The equation for B (AC flux density) in Gauss in my Dad's old engineering books applies to AC voltages. These books are really terse.

The ambiguity is in the word "flux" itself. When something is in a state of flux it is changing.
 
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Flux is like fluid flow. It is a characteristic of vector fields.

A fluid flowing through a pipe is a vector field, that is there is a velocity vector at each point in the field.
An electric field is a vector field, that is there is an electric field vector at each point in the field.
A magnetic field is a vector field, that is there is a magnetic field vector at each point in the field.
Atmospheric temperature is a scalar field, that is there is a scalar quantity at each point in the field.

Vector fields exert forces on real particles moving through them, scalar fields do not.
 
Flux does not always mean change. There are of course static electric fields, lightning being an obvious example. There are static magnetic fields, any polarized magnetic material is an example. One of the meanings for flux under the physics subdefinition of flux according to dictionary.com is:

A quantity expressing the strength of a field of force in a given area.

Static electric flux is easy to define, the number of electrons in a given area, moveable flux is different as the bulk of electrons in matter are all 'in use' so the net electric flux is zero. Magnetic flux is a bit more complex, I know little to nothing about exactly what gives rise to a static magnetic field, something I should probably look up one day. Electromagnetic theory is based primarily on the interaction of electric and magnetic fields which implies them changing, and non-moving electric or magnetic flux is pretty boring stuff.
 
Thank you very much Papabravo and Sceadwian! That clears that up for me.

Papabravo, thanks for refreshing vector/scalars for me. Thanks Sceadwian for seeing it at a different angle. The more points of view that you can get of a subject, the easier it is to understand intuitively.
 
Flux density equation

Bob Scott said:
I'd like to understand electromagnetics a bit better. What is the definition of electromagnetic flux? I'm attempting to make a better equation for flux density.

Is flux static, like a measure of the magnetic field intensity, as the lines of force that you can see using iron filings over a bar magnet?

Or is it defined as dynamic only, applying to AC fields only? The equation for B (AC flux density) in Gauss in my Dad's old engineering books applies to AC voltages. These books are really terse.

The ambiguity is in the word "flux" itself. When something is in a state of flux it is changing.

Why are you trying to make a better equation for flux density?
 
microtexan said:
Why are you trying to make a better equation for flux density?

I thought it would be easier to design a saturating switching power supply transformer with a different equation for B.
The existing B=V*10^8/(4.44*A*N*f) uses AC voltage and frequency. I think current is more suitable.

So I've come up with some new ones:

B=0.1415*Al*N*I/A

B is in Gauss
Al is the core's inductance in milliHenries at 1000 turns.
N is the number of turns that pass through the core.
A is the core's cross sectional area in cm^2.

I've also come up with:

Al=:mu: *0.89*A/Le

:mu: = the core material permeability.
A is the core's cross sectional area in cm^2.
Le is the core's magnetic path length in cm.

This way you can easily see that the Al value is dependent only on core material and core geometry.

You see I had some trouble verifying the published specifications of an Amidon toroidal core FT240-77 made of type 77 material that has a characteristic permeability of 2000. The specs in Amidon's leaflet said it had an Al value of 3130. Their catalog said it was about 2750. My own test with an oscillator to determine inductance found the real thing to be closer to 1690.

I can elaborate on the derivation of the formulas but only if you're interested.
 
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I don't mean to be impertinent but a saturated inductor in an SMPS sounds like a really bad idea. Are you sure that is what you want to do?
 
Papabravo said:
I don't mean to be impertinent but a saturated inductor in an SMPS sounds like a really bad idea. Are you sure that is what you want to do?

Oh, you would never sound impertinent.

The saturation is part of the oscillation cycle. As soon as the core saturates, the driving transistor turns off. It's been done before in photographic Xenon flash units and... remember the old Mark 10 capacitive discharge ignition? See:
 

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Sorry to post as this is out of my league, but caught my eye, as I need more Theory work to understand everything I am learning. I am a hobbyist so take it easy on me..lol

The example given the Xenon flash, sounds allot like a Florecent starter. Right at saturation it disconnects the current there-by causing the feild to breakdown. Is the math involved based on Maxwell's equations?

Am I close on my assumptions?

The math escapes me, sorry to admit.


-BaC
Oh, you would never sound impertinent.

The saturation is part of the oscillation cycle. As soon as the core saturates, the driving transistor turns off. It's been done before in photographic Xenon flash units and... remember the old Mark 10 capacitive discharge ignition? See:
 
This sites is on my quick list, nicely layout as well. I am sure it has been posted here already so pardon me if it was;

Magnetic Flux

-BaC

I'd like to understand electromagnetics a bit better. What is the definition of electromagnetic flux? I'm attempting to make a better equation for flux density.

Is flux static, like a measure of the magnetic field intensity, as the lines of force that you can see using iron filings over a bar magnet?

Or is it defined as dynamic only, applying to AC fields only? The equation for B (AC flux density) in Gauss in my Dad's old engineering books applies to AC voltages. These books are really terse.

The ambiguity is in the word "flux" itself. When something is in a state of flux it is changing.
 
The example given the Xenon flash, sounds allot like a Florecent starter. Right at saturation it disconnects the current there-by causing the feild to breakdown. Is the math involved based on Maxwell's equations?
That's correct. The rule comes from Faraday's and Lenz's Laws. Faraday's Law says if a voltage on a wire loop varies in time: then so will the flux through that wire loop. And vice versa.

Lenz's Law states if a field expands or breaks down, a new current will appear and oppose the change in flux. This current moves in the opposite direction from the one that energized the coil in the first place. That's why you have to put reverse-protection diodes on relays.

Take a generator for example. Turning it produces a voltage to power your stuff. But at the same time, you are changing the flux and introducing the opposition current, which in turn opposes your attempts to change the flux (in a recursive relationship). If the generator is not connected to anything, the opposition current can't flow and the generator is easy to turn. But if you short the generator with a wire, the opposition current can flow strongly and the generator gets harder to rotate. Electric motors do it too. If you want to stop a motor fast, you short its terminals.
 
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Thank you DigiTan:)

-BaC
 
Sorry to post as this is out of my league, but caught my eye, as I need more Theory work to understand everything I am learning. I am a hobbyist so take it easy on me..lol

Hey- I'm still confused. I extrapolated on the available equations for B and H, and L etc WHICH HAPPEN TO BE LINEAR EQUATIONS. But the B-H curve is far from linear. There is definitely a whooshing sound over my head.:(

The math escapes me, sorry to admit.
-BaC

Ditto
 
Simple inductive circuit question

Sorry to barge in on your topic, but it's kinda related.
Here follows a quiz, one I'm not quite solving yet:
So you connect a 220V 125Hz supply across a coil, the current in the coil reaches 692mA after 3.2ms, and then the circuit is switched off, probably using a switch between the supply and coil - hehehe......
What would the final steady value of current be?
Any educated guesses?
 
Sorry to barge in on your topic, but it's kinda related.
Here follows a quiz, one I'm not quite solving yet:
So you connect a 220V 125Hz supply across a coil, the current in the coil reaches 692mA after 3.2ms, and then the circuit is switched off, probably using a switch between the supply and coil - hehehe......
What would the final steady value of current be?
Any educated guesses?

this is what i think
if the power is switched off than there is no current flowing so the created field is dying out
but if the curent is flowing and you don't change the frequency and or windings of the coil than it will stay the same only the curent is out of phase with the voltage

Robert-Jan
 
Thanks Robert-Jan
My thinking leans towards this.
Since the circuit is made up of basically two components, namely the AC supply and the coil, as soon as the supply is taken out no circuit exists. There is thus no potential to maintain the charge that was already stored in the coil, and as previously mentioned no circuit means no current flow.
I'm just not sure exactly how the potential stored in the coil will manifest itself after the supply is removed, will it go out with a bang, e.g. arc at switch on instant of switch-off, or bleed off at a slower rate.
I've been told it will definitely arc when the supply is switched off, at the switch.
The thing that puzzles me is that twice a reference is made to the amount of current after the supply was switched off.
This is an Electrical Engineering university question.
 
You haven't supplied enough information, there are a LOT of questions about how to determine what you ask. Namely, the inductance value you're switching on and off, and the switch itself. After the supply is removed the magnetic field will collapse and create voltage across the inductor. If the inductor after being switched off is at it's peak current, the DC resistance is low and the overall isolation of the circuit is high the voltage can be incredibly high. If the switch is triggered during a zero crossing (current through the inductor not AC voltage) exactly nothing will happen.
 
Thanks Sceadwian

Unfortunately that is all the information that is provided, just the supply voltage at freq. and the current after certain interval.
Nothing more. I've assumed there to be a switch to disconnect the supply from the coil.
I also think something is wrong with this question, but still have to provide calculations regarding some values about it, e.g. R of coil, L of coil and E stored in coil.
It is an assignment question for my studies.
I've contacted the tutor about this, but seems she is just as dumbstruck as I am.
I'm starting to think that I'm to old to be wanting to study something at a University again.
 
Badly worded question.

Steady state current with no supply is always zero. There's always a few milliohms of resistance in the best of traces and they'll eventually bring down any circuit to a no-flow condition if there's no supply to keep things moving.

Just about any inductor will arc when contact is yanked suddenly. How much depends on the amount of current it was carrying at the time and the inductance. A big enough inductor with a large enough current will fry switch contacts if you don't have some sort of kickback protection in place (say a zener diode across the inductor with a breakdown voltage 30% higher than expected load).

Thinking on the question, the only way it would make any sense is if the questioner considered "switched off" to mean a short circuit across the supply. Then you would have a case to make for a steady state current, but only if completely ignoring resistance (which is, sadly, often the case in classroom environs).
 
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