Question

why the current outside the toroid is zero? ​

Answer 1

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Place your circular Amperian loop outside the toroid. By definition, Ampere's Law states that integration of magnetic field over an amperian loop is equivalent to constant μ multiplied by current enclosed. Therefore, current enclosed by Amperian loop is zero. Thus, magnetic field outside the toroid is zero.

@MissTranquillity

Answer 2

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You will observe that current that flows at outer circumference and inner circumference of toroid are in opposite direction to each other. Therefore, current enclosed by Amperian loop is zero. Thus, magnetic field outside the toroid is zero