A single-turn circular loop of radius 6 cm is to produce a field at its center that will just cancel the earth's field of magnitude 0.7 G directed at 70 below the horizontal north direction. Find the current in the loop.

The current is  

Explanation:

From the question we are told that  

     The radius of the loop is  

     The  earth's magnetic field is

      The  number of turns is  

Generally the magnetic field generated by the current in the loop is mathematically represented as

Now for the earth's magnetic field to be canceled out the magnetic field generated by the loop must be equal to the magnetic field out the earth

=>    

     Where   is the permeability of free space with value  

=>    

i think the answer is a, hope this : )

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