Chapter-4|Moving Charges and Magnetism| NCERT 12th Physics:
Ampere’s Circuital Law:
According to this law, the line integral of a magnetic field B around any closed path in vacuum is µ0 times the net current Inet enclosed by the curve.
It is another method to calculate magnetic field strength due to current like Biot Savart’s law.
Ampere’s law is applicable only for an Amperian loop (closed or circle).
This law is useful only in highly symmetric case, but valid everywhere.
The choice of an Amperian loop has to be such that at each point-
(I) B is tangential to the loop (B.dl= Bdl)
(II) B is normal to the loop (B.dl= 0)
(III) B vanishes (B=0) = (B.dl= 0).