Question

An ideal toroid has inner radius a and outer radius b. The toroid has turns and carries a current I. At which distancer from the center con the toroid is the magnetic field different from zero?

Answer 1

Magnetic field outside is zero. It is non zero only inside the core of a 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. Now, the 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.

Hence, the answer is a < r < b.