136 ADVANCED ELECTRICITY AND MAGNETISM. 



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 withdrawn, the whole charge being left on the metal plates.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 the glass plate is withdrawn? 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 Assume that the glass is withdrawn sidewise, not cornerwise.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, 0.01768 joule, 1,768 dynes. 



Note. The increase of energy of the condenser is equal to the mechanical 

 work done in withdrawing the glass plate. 



13. The air condenser specified in problem 10 is charged with 

 2,000 volts, the battery is disconnected and the metal 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 necessary 

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

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



