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Hello Mr Nelson,

I am having a problem calculating the field strength of an electromagnet and I was wondering if you could give me some guidance.

The formula I have used for calculating the magnetic field, in telsa is

permeability x turn density x current = magnetic field

The (hypothetical) electromagnet I want to measure is 0.000023 meters long with 0.75 turns, giving it a turn density of 32,608.69565.

The current is 0.00486 milliamps

I have read that the relative permeability of magnetic iron is 200.

So, I did the following calculation

permeability x turn density x current = magnetic field

200 x 32,608.69565 x 0.00000486 amps = 31.69 tesla

Does this seem right to you? It seems like quite a high value of tesla.

I have also used some online calculators to check my calculations but I have got some different results from each.

This one gives me the same results as my calculations; 31.6957 tesla

http://www.calctool.org/CALC/phys/electromagnetism/solenoid

but this one gives me the field strength as 0.0000398 tesla

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html

Have I done something wrong? Are my calculations correct?

Your help would be much appreciated.

Thank you and best regards,

Eddie

Well, that's a ridiculous magnet, but you forgot something really important aside from its ridiculous dimensions. A solenoid is long and thin with many turns, so that it's really long compared to its width. You have a 3/4 turn magnet that's reeeeally tiny. Not a solenoid. End effects make Ampere's law inapplicable to use as an approximation. You used the relative permeability. You forgot the factor of 4*pi*10^-7 (permeability of empty space), which really brings the number down. :) You have to multiply the relative permeability by the permeability of empty space, makes your answer agree with the calculator. It's the little things...

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I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.**Education/Credentials**

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