Solenoid
A solenoid is a closely wound helix of insulated copper wire . Magnetic field at a point well inside a long solenoid is given by
B = μ o nl
where , n = number of turns per unit length and I = current flowing through the solenoid
Magnetic field at a point on one end of a long solenoid is given by B = μ o nl / 2
Toroid
A toroidal solenoid is an anchor ring around which is large number of turns of a copper wire are wrapped .
A toroid is an endless solenoid in the form of a ring .
Magnetic field inside the turns of toroid is given by B = μ o nl
Magnetic field inside a toroid is constant and is always tangential to the circular closed path .
Magnetic field at any point inside the empty space surrounded by the toroid and outside the toroid , is zero , because net current enclosed by these space is zero .
Magnetic Field Due to a Current Carrying Long Circular Cylinder
Outside the cylinder ( r > R ) B = μ / 2 π l / r Inside the cylinder when it is made of a thin metal sheet ,