ELECTRIC FIELD. 



163 



PROBLEMS. 



i. A very small ball b weighing o.i gram and suspended by 

 a very fine fiber of silk stands as shown in Fig. pi. What is the 

 charge on b? Ans. 1.78 X io~ 9 coulombs. 



Note. Take 980 dynes as the weight of one gram. 



B 



20" 



B 



Fig. pi. 



2. Find the maximum charge that can be held on a sphere 

 200 centimeters in radius, the sphere being surrounded by air of 

 which the dielectric strength is, say, 33,000 volts per centimeter. 

 Ans. 0.001469 coulomb. 



Note. The maximum electric field intensity at the surface of the sphere being 

 known, the corresponding flux from the sphere and charge on the sphere can be 

 calculated. 



3. The sphere of the previous problem is surrounded by a 

 dielectric of which the strength is 60,000 volts per centimeter 

 and of which the inductivity is 2.5. Find the maximum charge 

 that can be held on the sphere. Ans. 0.00667 coulomb. 



4. Two flaj metal plates 7 centimeters apart are separated by 

 a layer of oil 3 centimeters thick (inductivity 2.00), and a layer 

 of glass 4 centimeters thick (inductivity 5). An electromotive 

 force of 20,000 volts is connected to the metal plates. Find the 

 electrical stress in the oil and in the glass. Ans. 4,347 volts 

 per centimeter in the oil ; 1 ,739 volts per centimeter in the glass. 



5. Calculate the force of attraction of two parallel flat metal 

 plates 30 centimeters square and one centimeter apart with air 



