Electronics > Beginners

Relationship between flux density, MMF and core area

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ZeroResistance:
Lets say i have a core area cross section of 1cm^2
And MMF I give is 100AT and due to this MMF in the core area I get a magnetic flux to flow which inturn causes a flux density B in the core.
Now if I double the core area to 2cm^2, but would keep the MMF same that is 100AT, how much would the flux density B change to?

My original guess was the B will halve, but I guess increasing the core area also reduces the reluctance of the core so more flux would flow for the same MMF.

So I would like to know the relationship between these 3 quantities?

spec:
+ ZeroResistance 

Bit of a head scratcher, but I will have a go at answering your question :)

The reluctance (Rm) of an inductor is, l/µA
where:
l is the length of the magnetic circuit in the core
µ is the permeability of the core material
A is the cross section area of the core.
So, if you doubled the cross section area of the core, you would half the reluctance, as you say.

It you half the reluctance you double the flux which means that, as the core cross section area has also been doubled, the flux density in the core would be unchanged, as you thought.

There is a further consequence of doubling the core cross sectional area: the inductance would also double, which means that the reactance, XL, would double (XL= 2ϖfL), which means that if you were pushing an AC constant current through the inductor, the voltage across the inductor would also double.

One thing that often causes confusion with inductors is that reluctance resists the magnetic flux in the core, but reactance resists the flow of electrical current in the inductor's coil. The two parameters are not the same. In fact, far from it: if you half the reluctance you double the reactance.

T3sl4co1l:
What mu and path length?

If everything stays the same, you're just stacking two cores, using the same magnetization for each.  Doesn't matter if they're stacked or alone, you're just asking if it's two together, so the total flux is double of course. :)

Tim

spec:
Take a cold shower Tim- you will feel better :)

T3sl4co1l:
You covered it in much better detail so I don't have much to add... I just wanted to get my shoe in :-DD

Tim

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