CURRENTS IN INDUCTIVE CIRCUITS. 17 



2. A closed circuit whose coefficient of self-induction is 0*5 henry carries a 

 steady current of 100 amperes : what is the energy stored up in the magnetic 

 field due to the current ? 



Answer. 0'25 x 10 11 ergs. 



3. Two closed circuits, whose coefficients of self-induction are respectively 

 - 5 and 0*75 henry, and whose coefficient of mutual induction is 0*6 henry, carry 

 steady currents of 10 and 20 amperes respectively : what is the energy of the 

 magnetic field due to the currents ? 



Answer. 2'95 x 10 ergs. 



4. What becomes of the energy of a magnetic field when the current which 

 creates it is interrupted ? 



5- The axle of a railway carriage wheel, 5 feet 3 inches long, moves at the 

 rate of 60 miles an hour in the earth's vertical field [ = 0'47 C.G.S. units]. What 

 E.M.F. is induced in it? 



Answer. 0*002 volt, nearly. 



