# help needed - toroid - Calculate the magnetic field strength

#### wertyu

Joined Jul 13, 2020
10
Q - A cast iron toroid of mean length 200 mm is wound with a 4000 turn coil, through which a magnetising current of 0.25A is passed. Calculate the magnetic field strength of the circuit

ok i cannot find anything on this and im stuck , i asked for help but get no reply from online tutors .

does anyone have a equation for this please .

#### Papabravo

Joined Feb 24, 2006
16,822
You need a piece of information that is not given , at least not in your statement. It has to do with the core material. What could that be?
Once you know that it's just Ampere's law. should be in the first page of Google hits.

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#### wertyu

Joined Jul 13, 2020
10
is it permeability of cast iron

#### Papabravo

Joined Feb 24, 2006
16,822
Yes - so what value are you going to use for the relative permeability of cast iron?

#### wertyu

Joined Jul 13, 2020
10
not sure need to think more here so this is where i am .

mmf = F = N/I ampere turns 4000 / 0.25a = 16000at

radius of 200mm mean length = 0.064m

mag field strength H = F / L 16000 / 2pi*0.064 = 16000 / 0.40192 = 39808 H

is that it ? its not asking me for flux or flux density just yet .

#### wertyu

Joined Jul 13, 2020
10
no sorry

mmf = 4000 x 0.25 = 1000at

mag field strength is 1000 / 2pi x 0.064 = 2486 H

or is it just 1000 / 0.2 = 5000 H ?

#### MrAl

Joined Jun 17, 2014
8,479
no sorry

mmf = 4000 x 0.25 = 1000at

mag field strength is 1000 / 2pi x 0.064 = 2486 H

or is it just 1000 / 0.2 = 5000 H ?
What units are you worknig in, amperes/meter or oersteds?

If working in amperes/meter then 4000*0.25/0.2=5000 amperers/meter is correct but not sure why you used "H" there. H is used for mmf sometimes but not for the units themselves.

And yes mmf is considered external not internal to the construction. It is the driving 'force' from the outside in the same way that a voltage or current is an input to some circuit.
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