Electrical Efficiency Problems
1.
Calculate the percent efficiency of an electric motor that uses 1.5 x 104J of electrical energy to
generate an output energy of 1.15 x 104J. (77%)
2.
If an incandescent light bulb generates 2.5 x 103J of light energy from an energy input of 5.0 x
104J of electrical energy, determine the bulb’s electrical efficiency. (5%)
3. The alternator in a car’s engine generates 1.1 x 105W h of electrical energy. If that electrical
energy is converted into 6.4 x 104W h of mechanical energy, calculate the percent efficiency of
the car’s engine. (58%)
4. A water heater has an efficiency rating of 64%. If the heater receives 6.5 kW h of electrical input
energy, what output energy should the water heater generate? (4.2 kW h)
5. A 12V battery has a current flow of 1.5A and is operated for 270 seconds.
a) Calculate the Input Energy that the battery is capable of generating. (4860J)
b) If the battery has an efficiency rating of 83%, calculate the output energy of the battery. (4034J)
6.
A furnace has an efficiency rating of 87%. If it can generate 1.73 x 107J of heat (thermal)
energy, calculate the amount of input (electrical) energy required. (1.99 x 107J)
7.
A higher-efficiency furnace is capable of generating 2.73 x 109J of heat (thermal) energy from an
electrical input energy of 2.88 x 109J. Calculate the efficiency rating of this device. (94.8%)
8.
A family is considering two different clothes dryers for purchase. Dryer A (white colour, frontloading”) can generate 3.7 x 104J of heat (thermal) energy from 4.2 x 104J of electrical energy.
The second choice, Dryer B (“blue flame colour with chrome accents”) uses 5.6 x 104J of
electrical energy to generate 4.1 x 104J of heat (thermal) energy. How do the efficiencies of the
two choices compare? (A – 88%; B – 73%)
9. Calculate the electrical energy required for a set of speakers with an efficiency rating of 62.5% to
generate 37.5 W h of sound energy. (60 W h)
10. A kettle has a power rating of 1500W and is in operation for 2.5 minutes.
a) Calculate the input electrical energy available (2.25 x 105J).
b) Calculate the output heat energy generated if the kettle has an efficiency of 71% (1.60 x 105J)