A wire carrying current I has the shape as shown in the adjoining figure. Linear parts of the wire are very long and parallel to the X-axis while the semicircular portion of radius R lies in the YZ-plane. Magnetic field at point O is:
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• Semicircle at centre: B=(mu₀ I)/(4R)
• Long straight wire: B=(mu₀ I)/(2π r)
Always apply right-hand thumb rule for direction.