Electrical Power and Efficiency Problems
Directions: Show your work and include units. You don’t need to use GRASS to do these questions.
1. Calculate the power of a drill which transfers 2000J of energy in 5 seconds. Show your answer in watts
and kilowatts. Remember: 1OOO W = 1 KW
2. A 750 Watt hairdryer is used for 15 minutes.
a) Calculate the electrical energy used in Joules and in Kilojoules
b)
Calculate the cost to use 15 min. every day for 1 year @ 8 cents/kw▪h
3. A microwave oven operates on 1500 W of power and is used for 20 minutes. How much electrical
energy, in joules is used by the microwave oven?
4. A refrigerator operates on average for 12 hours a day. If the power rating of the fridge is 600 W, how
much electrical energy does the fridge use in one week in joules?
5. A room has a 60 watt, a 100 watt, and a 150 watt light bulb. How much does it cost to use all of the
lamps for 2.5 hr @ 8 cents/ kW▪h?
6. An electric heater draws 1100 W of power. Electricity costs 8¢ per kW•h. How much does it cost
tooperate the heater 3.0 h a day for 30 days?
7
A light bulb takes in 30J of energy per second. It transfers 3J as useful light energy and 27J as heat
energy. Calculate the efficiency.
8
A kettle takes in 2000J of energy per second. It transfers 1500J as useful heat energy and 500J is wasted
as sound energy. Calculate the efficiency of the kettle.
9
Bob has an electric lawn mower which transfers 5kW▪h of energy. The cost of energy is 10 cents/kW▪h.
Calculate the total cost of running his lawn mower for 1 hour.
10 Ingrid’s washing machine has a power of 2kW. She uses it for 2 hours. Each kW▪h costs her 5 cents.
a) First calculate the energy used by the washing machine.
b) Now calculate the total cost of running the washing machine.