72 ADVANCED ELECTRICITY AND MAGNETISM. 



The primary coil of the transformer has 560 turns of wire. How 

 many turns of wire are there in the secondary coil? The trans- 

 former delivers 250 amperes to the service mains. How much 

 current does it take from the supply mains? A usual allowance 

 in transformer coils is 1,000 circular mils sectional area of wire 

 for each ampere. Find size of wire used in primary coil and in 

 secondary coil of the transformer. Ans. 56 turns, 25 amperes, 

 25,000 circular mils, 250,000 circular mils. 



11. Let it be assumed that the force required to propel a canal 

 boat is proportional to the velocity of the boat. A force of 50 

 pounds is required to maintain a steady velocity of 5 feet per 

 second, (a) Find the value of the coefficient by which the 

 velocity of the boat must be multiplied to give the frictional 

 drag, and specify the unit in terms of which this coefficient is 

 expressed, (b) Find the velocity at which the boat would be 

 propelled by a force of 36 pounds, (c) Find the rate at which 

 work is done by a force of 36 pounds in propelling the boat. 

 Ans. (a) 10 pounds per foot-per-second. (b) 3.6 feet per 

 second, (c) 129.6 foot-pounds per second. 



12. When a force of 50 pounds is applied to the above canal 

 boat, the boat starts from rest and after some time reaches its full 

 speed of 5 feet per second. At a given instant the velocity of the 

 boat is 3 feet per second. At this instant: (a) Find the rate at 

 which work is done on the boat by the propelling force, (b) Find 

 the dragging force which is acting on the boat due to the friction 

 of the water, (c) Find the rate at which work is dissipated in over- 

 coming the friction of the water, (d) Explain what is becoming 

 of the difference between (a) and (c). Ans. (a) 150 foot-pounds 

 per second, (b) 30 pounds, (c) 90 foot-pounds per second. 

 (d) It is being stored as increasing kinetic energy of the boat. 



13. An electromotive force of 50 volts acts on a circuit of which 

 the resistance is 10 ohms. At a certain instant during the time 

 that the current is growing from zero to its full value, the current 

 has an actual value of 3 amperes. At this instant: (a) Find the 



