I want to compute the magnetic field inside an infinite solenoid. I know how to do this if we suppose that outside the solenoid, the magnetic field is equal to zero.

But how do you do this if it isn't equal to 0? what does it change ?

I heard that you have to use the Biot–Savart law but I don't know how.

Thank you !


1 Answer 1


The magnetic field produced by an infinite solenoid is zero outside the solinoid. To see this, place an Amperian loop outside the solenoid such that no current passes through and you will find that the magnetic field must be a constant outside the solenoid. In particular, infinitely far away, this $B=0$ so $B=0$ everywhere outside the solenoid.

If some magnetic field $B_{ext}$ is produced by external agents, just add the two fields $B=B_{ext}+B_{solenoid}$

  • $\begingroup$ no. That's not my question. My question is how do you compute the field inside taking into account that there is some outside. You are saying this like if it was easy but I don't succeed in doing this... I think it is more difficult than what you are just saying $\endgroup$ Commented Jun 19, 2018 at 10:08
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    $\begingroup$ It is note like some infinite solenoids has field outside and some does not. If the solenoid is not infinite, then you will have to use Biot–Savart law. But if it is infinite, there is no field outside. $\endgroup$
    – Jasoba
    Commented Jun 19, 2018 at 10:37
  • $\begingroup$ but the fact is I say that the field outside isn't equal to zero... I don't understand sorry. I apply on my solenoid a magnetic force $\endgroup$ Commented Jun 19, 2018 at 11:05

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