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Problems

Author: E.E. Kimberly

 Problem Answer 1 How many horsepower are required to hoist 4000 lb at the rate of 50 ft per min? 2 How many kilowatts are required to supply the power in Problem 1? 3 What is the power in an electric iron whose resistance is 11 ohms, when the iron is connected to a 115-volt circuit? 4 A bill for electric energy is \$20 for 150 hours. The price of energy is 7 cents per kilowatt-hour. What was the average power? 5 How much heat per hour is produced in the iron of Problem 3? 6 How many hours would be required for a heater of 10 ohms resistance connected to a 220-volt line to produce enough heat to melt 100 lb of ice? Latent heat of ice is 144 Btu per pound. 7 A baseball field is lighted by 40 lamps, each of which draws 2.3 amperes from a 115-volt line. If the field is lighted for 2 1/2 hours and the cost of electric energy is 3 cents per kilowatt-hour, what is the energy cost? \$0.795 8 If power for the field in Problem 7 is furnished by an individual engine-generator unit and the efficiency of the generator at the required load is 82%, what must be the horsepower output of the engine? 17.3 hp 9 A waterfall 72 ft high has a flow of 5000 tons of water per minute, 10 per cent of which is diverted through a water turbine whose efficiency at full load is 72%. The turbine drives an electric generator whose efficiency is 86% at full load. Calculate: (a) the rated horsepower of the turbine; (b) the rated power output of the generator; (c) the dollar value of the yearly energy output if the equipment is operated at full load continuously and the market price of the energy is 1.2 cents per kilowatt-hour. 10 An immersion heater used to maintain the temperature of a plating solution draws 15 amperes from a 120-volt line. How many Btu per hour are delivered to the solution? 11 An elevator with an unbalanced weight of 2000 lb is being driven at 250 ft per min by a motor whose efficiency is 86%. The hoisting-gear efficiency is 72%. Calculate the power input to the motor. How much energy is used in raising the elevator 80 ft? 12 If the voltage of Problem 10 were reduced 10%, what would be the reduction in power in the heater? 13 A 500-watt, 120-volt soldering iron will overheat if left connected to its supply , without use. How much resistance must be connected in series with it to reduce its rate of heat input 20%. 14 In a magnetic field like that shown in Fig. 2-2 a coil of 500 turns is rotating. When the coil is in a horizontal position, it encloses 4104 lines of flux. If the maximum voltage generated is 400 volts, what is the speed of rotation of the coil in revolutions per minute?

Last Update: 2011-02-23