[" (a) Using Biot-Savart's law,derive the "],[" expression for the magnetic field in the "],[" vector form at a point on the axis of a "],[" circular current loop."],[" (b) What does a toroid consist of ? how "],[" that for an ideal toroid of closely wound "],[" turns,the magnetic field "],[" (i) inside the toroid is constant."],[" (ii) in the open space inside and exterior."],[" to the toroid is zero."]