Question

# Inside a long straight uniform wire of circular cross - section there is a long cylindrical cavity (bore) of radius r whose axis is parallel to the axis of the wire. Axis of cavity is displaced from the axis of wire by a distance ‘L’ as shown in figure. A direct current J flows through the wire. Then I) Magnetic field inside the cavity is non-uniform II) Magnetic field inside the cavity is uniform III) Field inside the cavity is directly proportional to L IV) Magnetic field inside the cavity is independent of L

A
I and IV
B
I and III
C
II and IV
D
II and III

Solution

## The correct option is A II and III¯B=μ02(¯j×¯L)

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