WHAT IS AMPERE’S LAW.......................................................?

@ sai 
Ampere circuital law is given by ampere as a substitute to the biot savarts law .. and when the situation arises in which biot savarts law will be complicated ,, in that case we have to use ampere circuital law . 
it states that 
for any closed loop path the sum of length element times the magnetic field in the direction of length element is equal to the permeability times the electric current enclosed within the loop . 
MATHEMATICALLY U CAN SAY 
SURFACE INTEGRAL OF B.DL = u0 . I 
where u0 is the permeability of free space . and i is the current enclosed . 

for applying it we need to choose a close surface which is called as amperian loop . 
HOPE IT CLEARS YOUR DOUBT 

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