Consider the circuit shown in the figure. The curved segments are arcs of circles of radii a and b. We have to find the magnetic field at P, assuming a current in the circuit.

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As the point P is on the extension of the straight-line segments of the circuit, the magnetic field at P due to these segments will be zero. The magnetic field at P due to the arc of circle of radius a will be in the direction of , if the circuit is in the - plane, and .

As the direction of the current flow in the arc of the circle of radius b is in the counter-clockwise direction, the magnetic field at P due to this segment of the circuit will be in the direction . We, therefore, find the magnetic field at P to be given by

The magnitude of the magnetic field at P will be