Physics GRE GR 1777, Problem 020 Solution

20. Electrodynamics (Magnetic field)

Solution

The magnitude of the magnetic field can be obtained from the Biot-Savart law.
  
where  is the radius of the loop and  is the current.
In this problem, the radius of the circular loop is , the magnitude of the magnetic field at the center of a circular loop is
 

Answer

(D) 

Reference

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/curloo.html