 
       
      
    Biot-Savart law indicates that the moving electrons produce a magnetic field B such that
a) 
b) 
c) it obeys inverse cube law
d) it is along the line joining the electrons and point of observation
The answer is the option (a)
(a) According to the Biot-Savart law , the magnitude of  is 
Where  is the proportionality constant 'r' is the magnitude of position vector from charge to the point at which we have to find the magnetic field and 
 is the angle between 
B is perpendicular to both v and r.
