The magnetic field is zero at mid point of AB and at P as shown

A long straight wire carrying a current of 30 A is placed in an external uniform magnetic field of induction 4×10 −4 T. The magnetic field is acting parallel to the direction of current. The magnitude of the resultant magnetic induction in tesla at a point 2.0 cm away from the wire is

A square loop PQRS of side 'a' and resistance 'r' is placed near an infinitely long wire carrying a constant current I. The sides PQ and RS are parallel to the wire. The wire and the loop are in the same plane. The loop is rotated by 180∘ about an axis parallel to the long wire and passing through the mid points of the side QR and PS. The total amont of charge which passes through any point of the loop during rotation is:

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A battery is connected between two points A and B on the circumference of a uniform conducting ring of radius r and resistance R as shown in Fig. One of the arcs AB of the ring subtends angle θ at the centre. Show that that the magnetic field at the centre of the coil is zero and independent θ.