8 TREATISE ON ALTERNATING CURRENTS. 



coatings have unit difference of potential when unit quantity of 

 electricity is imparted to one of them. 



The practical unit of capacity is called a Farad, and is 10~ 9 

 times the C.G.S. Electromagnetic Unit. 



The difference of potential, V, of the coatings of a condenser 

 is proportional to the total charge Q of electricity; hence, if C 

 is the capacity of the condenser 



Q = CV (11) 



It is to be noted that one of the coatings of the condenser may 

 be the earth itself, in which case V is the actual potential of the 

 other coating. 



WORK DONE IN CHARGING A CONDENSER. 



11. The work done in charging a condenser so that the 

 difference of the potential of its coatings may be V with a charge 

 Q is given by 



W = ( Q VdQ 



J 



by equation (11) = 



(12) 



PROBLEMS ON CHAPTER I. 



1. Find the force in dynes due to two magnet poles of strengths 4 and 10 

 respectively, placed at a distance of 5 centimetres apart. 



Answer. 1-6 dynes. 



2. Find the equation to the line of force passing through a given point due to 

 two equal and opposite magnet poles. 



Answer. Cos 6 1 cos 2 = constant, where X and 2 are the angles between 

 the line joining the two poles, and the lines joining the respective poles to any 

 point in a line of force. 



3. Find the equation to the line of force passing through a given point due to 

 two magnet poles of strength m and m' respectively. 



