How does the magnetic field becomes 0 outside a toroid? Also how can we explain it with magnetic field lines?

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    $\begingroup$ Draw a toroid with a tightly wound coil around it. Now apply Ampere's law. Why would you think that there is non-negligible field outside the toroid? $\endgroup$
    – hyportnex
    Commented Jul 12, 2020 at 21:37
  • $\begingroup$ @hyportnex What are you saying, is there field present outside a toroid? $\endgroup$ Commented Jul 12, 2020 at 22:02
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    $\begingroup$ You have to consider it as an ideal toroid. $\endgroup$
    – Ken
    Commented Jul 12, 2020 at 22:23
  • $\begingroup$ Related : (1) Does magnetic field depend on z inside a toroidal coil?,(2) Magnetic Induction at the centre of a Toroid. $\endgroup$
    – Frobenius
    Commented Jul 12, 2020 at 22:27
  • $\begingroup$ I ask, have you seen the reasoning for why the magnetic field is considered negligible outside a solenoid? If so you can apply the same reasoning to a toroidal coil. $\endgroup$
    – Triatticus
    Commented Jul 13, 2020 at 0:47

1 Answer 1


You might have heard of amperes circuit law which states that integral of B.dl is equal to current enclosed in the loop and we see that outside a toroid we can't find any current enclosed so magnetic field is 0

  • $\begingroup$ Nope, you can't say that for sure! The looped line integral is 0 (and not the magnetic field). Hence, it can't be said that magnetic field is 0 as the integral is looped. $\endgroup$ Commented Jul 13, 2020 at 13:58
  • $\begingroup$ You may want to describe where you are putting your chosen loop and the symmetry involved. $\endgroup$
    – R.W. Bird
    Commented Jul 13, 2020 at 15:07

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