190 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



plates twice as wide and twice as long. These plates are sepa- 

 rated by a plate of glass (inductivity 5) which is four times as 

 thick as the air space in the first condenser. What is the capacity 

 of the second condenser? Ans. 0.015 microfarad. 



125. Two metal plates, 100 centimeters x 100 centimeters, are 

 separated by 2. centimeters of air. This condenser is charged by 

 a battery having an electromotive force of 2,000 volts. What is 

 its energy in joules ? Ans. 0.000884 joule. 



126. A flat glass plate, inductivity 5, size 100 centimeters x ioo 

 centimeters x 2 centimeters, is slid between the metal plates 

 specified in problem 125, the battery being left connected to the 

 metal plates. What is the energy of the condenser after the 

 glass is in place ? Ans. 0.00442 joule. 



127. The 2,ooo-volt battery is disconnected from the metal 

 plates specified in problem 1 26 after the glass is in place and the 

 metal plates are thoroughly insulated. The glass plate is then 

 withdrawn, the whole charge being left on the metal plates. 

 What is the electromotive force between the metal plates after 

 the glass plate is withdrawn ? What is the energy of the con- 

 denser after the glass plate is withdrawn ? How much has the 

 energy been increased by withdrawing the glass ? How much 

 force was necessary to withdraw the glass, ignoring friction, 

 weight, etc. ? (Assume that the glass is withdrawn sidewise, not 

 cornerwise.) Ans. 10,000 volts, 0.0221 joule, 0.01768 joule, 

 1,768 dynes. 



128. The air condenser specified in problem 125 is charged 

 with 2,000 volts, the battery is disconnected and the plates are 

 then moved to a distance 3 centimeters apart, charge on the 

 plates remaining unchanged. What is the electromotive force 

 between the plates after the movement? What is the increase 

 of energy due to the movement ? How much force was neces- 

 sary to produce the movement, ignoring friction, weight, etc.? 

 Ans. 3,000 volts, 0.000442 joule, 4,420 dynes. 



129. What is the intensity of the electric field between two 



