SNC1DO
Electrical Efficiency and Percent Efficiency
1) A 60-W fluorescent light bulb uses 2000 J of electrical energy, but produces only 400 J of light energy. What is the
percent efficiency of this light?
2) Calculate the percent efficiency of an electric motor that uses 15 000 J of energy , but produces only 11 500 J of useful
energy.
3) Calculate the percent efficiency of an incandescent light bulb that produces 2500 J of light energy from 50 000 J of
electrical energy.
4) When would the release of thermal energy from an electrical device NOT be considered to be a form of wasted energy.
Give an example.
5) A light bulb takes in 30J of energy per second. It transfers 3J as useful light energy and 27J as heat energy (per
second). Calculate the efficiency.
6) A kettle takes in 2000J of energy per second. It transfers 1500J as useful heat energy and 500J is wasted as sound
energy (per second). Calculate the efficiency of the kettle.
7) Calculate the efficiency of a hair dryer which takes in 3000J of energy per second and transfers 600J as useful heat
energy. Express your answer as a decimal and not as a percentage.
8) Calculate the efficiency of a TV which takes in 5000J of energy per second and transfers 1000J as useful light energy
and 1500J as useful sound energy. The remaining 2500J is wasted as heat energy. Express your answer as a decimal and
not a percentage.
9) Calculate the power of a drill which transfers 2000J of energy in 5 seconds. Show your answer in watts and kilowatts.
10) Convert the following powers into kilowatts
i) 3000W
iv) 20000W
v) 1W
ii) 20W
iii) 900W
11) Calculate the energy transferred by a computer with a power of 2kW which is left on for 1 hour.
12) Calculate the energy transferred by a hairdryer with a power of 500W left on for 15 minutes (Remember units!!)
13) An oven has a power of 400W and is left on for 30 minutes. Calculate the energy transferred.
14) Bob has an electric lawn mower which transfers 5 kWh of energy. Each kWh of energy costs 10 cents. Calculate the
total cost of running his lawn mower.
15) Ingrid’s washing machine has a power of 2kW. She uses it for 2 hours. Each kWh costs her 5 cents.
a) First calculate the energy transferred by the washing machine.
b) Now calculate the total cost of running the washing machine.
16) Julie uses her hair dryer for 30 minutes. The hair dryer has a power of 500W. Calculate the cost of Julie drying her
hair if each kWh costs 8 cents.
17) Calculate the efficiency of a light bulb that gives of 40J of light from 200J of electrical energy.
18) Calculate the percentage efficiency of a motor that does of 60J of work from 240J of electrical energy.
19) Calculate the efficiency of a radio that gives of 30J of sound from 360J of electrical energy.
20) Calculate the efficiency of a runner who produces of 500J of kinetic energy from 2500J of chemical energy.
21) Calculate the percentage efficiency of a student who takes 2 hours to do 30 minutes of homework.
22) What is the useful light output from a bulb with an efficiency of 25% when supplied with 600J of energy?
23) What is the useful work output from a motor that is 60% efficient when supplied with 1200J of energy?
24) What is the useful sound output from a radio with an efficiency of 42% when supplied with 20J of energy?
25) What is the useful work done by a runner that is 30% efficient when supplied with 200J of energy?
26) How long does it take a student of efficiency 40% to do 30 minutes homework?
27) A light bulb is rated as 40W
(a) How much electrical energy does this bulb use in one second?
(b) Calculate the efficiency of the bulb if it gives out 5J of light in one second.
(c) What is the percent efficiency of the bulb?
(d) What is the useful power output of the light bulb if it is supplied with 750 J of energy?
(e) How much light energy should the bulb give out in 3 minutes?
(f) How much heat energy should the bulb give out in 3 minutes?
SNC1DO
Electrical Efficiency and Percent Efficiency
ANSWERS
1) A 60-W fluorescent light bulb uses 2000 J of electrical energy, but produces only 400 J of light energy. What is the
percent efficiency of this light? (ANS: 20%)
2) Calculate the percent efficiency of an electric motor that uses 15 000 J of energy , but produces only 11 500 J of useful
energy. (ANS: 76.6%)
3) Calculate the percent efficiency of an incandescent light bulb that produces 2500 J of light energy from 50 000 J of
electrical energy. (ANS: 5%)
4) When would the release of thermal energy from an electrical device NOT be considered to be a form of wasted energy.
Give an example.
5) A light bulb takes in 30J of energy per second. It produces 3J as useful light energy and 27J of heat energy (per
second). Calculate the efficiency. (ANS: 10%)
6) A kettle takes in 2000J of energy per second. It transfers 1500J as useful heat energy and 500J is wasted as sound
energy (per second). Calculate the efficiency of the kettle. (ANS: 75%)
7) Calculate the efficiency of a hair dryer which takes in 3000J of energy per second and transfers 600J as useful heat
energy. Express your answer as a decimal and not as a percentage. (ANS: 20%)
8) Calculate the efficiency of a TV which takes in 5000J of energy per second and transfers 1000J as useful light energy
and 1500J as useful sound energy. The remaining 2500J is wasted as heat energy. Express your answer as a decimal and
not a percentage. (ANS: 0.5)
9) Calculate the power of a drill which transfers 2000J of energy in 5 seconds. Show your answer in watts and kilowatts.
(ANS: 400 W / 0.4 kW)
10) Convert the following powers into kilowatts
i) 3000W = 3 kW
ii) 20W = 0.02 kW
iii) 900W = 0.9 kW
iv) 20000W = 20 kW
v) 1W = 0.001 kW
11) Calculate the energy transferred by a computer with a power of 2kW which is left on for 1 hour. Your answer should
be in kJ. (ANS: 7200 kJ)
12) Calculate the energy transferred by a hairdryer with a power of 500W left on for 15 minutes (Remember units!!).
(ANS: 450000J or 450 kJ)
13) An oven has a power of 400W and is left on for 30 minutes. Calculate the energy transferred in J and kJ. (ANS:
720000J / 720 kJ)
14) Bob has an electric lawn mower which transfers 5 kWh of energy. Each kWh of energy costs 10 cents. Calculate the
total cost of running his lawn mower. (ANS: $0.25)
15) Ingrid’s washing machine has a power of 2kW. She uses it for 2 hours. Each kWh costs her 5 cents.
a) First calculate the energy transferred by the washing machine in kJ. (ANS: 14 400kJ)
b) Now calculate the total cost of running the washing machine. (ANS: $0.20)
16) Julie uses her hair dryer for 30 minutes. The hair dryer has a power of 500W. Calculate the cost of Julie drying her
hair if each kWh costs 8 cents. (ANS: $0.02)
17) Calculate the efficiency of a light bulb that gives of 40J of light from 200J of electrical energy. (ANS: 20%)
18) Calculate the percentage efficiency of a motor that does of 60J of work from 240J of electrical energy. (ANS: 25%)
19) Calculate the efficiency of a radio that gives of 30J of sound from 360J of electrical energy.
(ANS: 8.3%)
20) Calculate the efficiency of a runner who produces of 500J of kinetic energy from 2500J of chemical energy.
(ANS: 20%)
21) Calculate the percentage efficiency of a student who takes 2 hours to do 30 minutes of homework. (ANS: 25%)
22) What is the useful light output from a bulb with an efficiency of 25% when supplied with 600J of energy?
(ANS: 150J)
23) What is the useful work output from a motor that is 60% efficient when supplied with 1200J of energy? (ANS: 720J)
24) What is the useful sound output from a radio with an efficiency of 42% when supplied with 20J of energy?
(ANS: 8.4 J)
25) What is the useful work done by a runner that is 30% efficient when supplied with 200J of energy? (ANS: 60 J)
26) How long does it take a student of efficiency 40% to do 30 minutes homework? (ANS: 1 hour and 15 minutes)
27) A light bulb is rated as 40W
(a) How much electrical energy does this bulb use in one second? (ANS: 40 J)
(b) Calculate the efficiency (NOT PERCENT)of the bulb if it gives out 5J of light in one second. (ANS: 0.125)
(c) What is the percent efficiency of the bulb? (ANS: 12.5%)
(d) What is the useful power output of the light bulb if it is supplied with 750 J of energy? (ANS: 93.75 J)
(e) How much light energy should the bulb give out in 3 minutes? (ANS: 900 J)
(f) How much heat energy should the bulb give out in 3 minutes? (ANS: 6300 J)