I'm an EE, not a physicist, so please forgive if this question is dumb.

I learned a bit of magnetics when I took motors 20 years ago, but I don't remember much.

I'm reaching out to the physics community because finding EE's who know the answer to this question can be tough. (Take me, for example.)

I bring 1 kg of iron to a flux density of 1 T. How much energy does that take?


1 Answer 1


Usually we can answer questions like this using $ \frac{U}{V} = \frac{B^2}{2 \mu} $ (magnetic energy stored per unit volume), but since iron is nonlinear and ferromagnetic, we need to use its magnetisation curve.

enter image description here

In this case $ \frac{U}{V} = \int_{0}^{B} H \ dB $, so the energy required per unit volume of iron is equal to the area between this curve, the vertical axis, and the line $ B = 1 $. It's quite linear in this region so you could approximate it as a triangle (which is equivalent to just using the original formula above). I estimate it as

$$ \text{energy per unit volume} = \frac{1}{2} \times {1} \times {2000} = 1000 J = 1 \ kJ. $$

Perhaps this diagram looks familiar - we are looking at the dashed part of the curve (initial charging up) in this analysis.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.