Chapter-4|Moving Charges and Magnetism| NCERT 12th Physics:
Application of Ampere’s Circuital Law:
Magnitude of Magnetic Field of a Straight Wire (Infinite) Using Ampere’s Law:
Magnetic field due to a straight conductor at point P, at a distance r from the wire.
Now, We will draw a circle with radius r, passes through point P (called amperian loop).
By using Right Hand Thumb Rule, we can see that magnetic field is tangential with dl at all place, then angle between B and dl is zero.
So by first case of Ampere’s Law we can apply it.