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Unit D – Electrical Principles and
Technologies - Concept 3
Energy Efficiency
Concept 3 – Energy Efficiency
 Energy
 Direct current
 Alternating current
 Power
 Watt (W)
 Kilowatt hour
 Law of Conservation of
Energy
 Efficiency
 Joule (J)
Concept 3 – Energy Efficiency
Learning Concept
 Identify the forms of energy inputs & outputs in a device or
system
Concept 3 – Energy Efficiency
 Recall that energy is the ability to do work
 Four common forms of energy are
 Chemical
 The energy stored in chemical bonds
 Electrical
 The energy of charged particles
 Mechanical
 The energy of an object because of its motion or potential to move
 Thermal
 The total kinetic energy of all the particles in a substance
Concept 3 – Energy Efficiency
Electrical Energy
 Some motors run on direct current (DC)
 Electricity flows in only one direction
 Other motors run on alternating current (AC)
 Electricity flows back and forth 60 times per second
 This is the electricity in household circuits
AC / DC current
Concept 3 – Energy Efficiency
Review:
Device
Electric Kettle
Battery-Operated Toy
Car
Electric Blanket
Cordless Telephone
Input Energy
Output Energy
Concept 3 – Energy Efficiency
Learning Concept
 Apply appropriate units, measures and devices in energy
transformations
 Measure amperage and voltage
 Calculate watts consumed using the formula P=I×V
 Calculate quantity of energy in joules using the formula E=P×t
Concept 3 – Energy Efficiency
 Power is the rate at which a device converts energy
 The faster a device can convert energy, the greater its
power rating
Power Video
www.eastmarinedrive.com
Concept 3 – Energy Efficiency
 Mathematical relationship between power (P), current (I) and
voltage (V) is P=I×V or watts = amperage × volts
 Unit of power is the watt (W)
 Named for James Watt
 Equal to one joule per second
http://static.howstuffworks.com
Concept 3 – Energy Efficiency
Example
 A hair dryer has a power rating of 1000 W. It is plugged into a
120-V outlet. What is the current flowing through the hair
dryer?
I = 8.33
www.electronics-tutorials.ws/
Concept 3 – Energy Efficiency
Practice:
A coffee maker that is plugged into a 120V outlet has 6A of
current flowing through it. What is the power rating of the
coffee maker.
Concept 3 – Energy Efficiency
 Power ratings on devices can be used to determine the
amount of energy the device uses
 Energy consumption of a device is the input power
multiplied by the time the device is used
 Energy is calculated with the formula E=P×t
Concept 3 – Energy Efficiency
 James Joule showed that both mechanical work and
electricity can produce heat, and vice versa
 Therefore, the unit of energy is named the joule (J)
www.magnet.fsu.edu
www.physics.ie
Concept 3 – Energy Efficiency
 A kilowatt hour is also a unit of energy that is used
instead of the joule(J).
 Energy calculation is the same but hours are
substituted for seconds
 Kilowatts (kW) are substituted for watts
http://blog.mapawatt.com
Concept 3 – Energy Efficiency
Example
 A microwave oven has a power rating of 800 W. If you cook a
roast in the oven for 30 minutes at high, how many joules of
electrical energy are converted into heat?
 Important Reminder – joules are measured as (watt ×
second)
E= 1440000 J or 1.4MJ
Concept 3 – Energy Efficiency
Practice:
A hand dryer is converts 129000J of electrical energy into heat
energy in 60 seconds. What is the power rating of the hand
dryer.
Concept 3 – Energy Efficiency
Learning Concept
 Apply the concepts of conservation of energy and efficiency
to the analysis of energy devices
Concept 3 – Energy Efficiency
 The Law of Conservation of Energy is a fundamental
principle
 Energy cannot be created nor destroyed
 It can only be transformed from one form to another
www.tutorvista.com
http://hrsbstaff.ednet.ns.ca
Concept 3 – Energy Efficiency
 However, usually it is found the
output energy is smaller than the
input energy.
 Most often the missing energy is
lost as heat
 All mechanical systems
dissipate some energy, so their
usable output energy is always
less than the input energy
http://indianapublicmedia.org/
Concept 3 – Energy Efficiency
Learning Concept
 Compare energy inputs and outputs of a device
 Calculate efficiency using the formula
energy output
% efficiency 
100
energy input
Concept 3 – Energy Efficiency
 The efficiency of a device is the ratio of useful energy that
comes out of a device compared to the amount that went
in
 The more input energy that is converted to useable
output energy, the more efficient the device
www.toronto.ca
Concept 3 – Energy Efficiency
Example
A quad is 20% efficient at producing useful output energy from
input fuel. How many joules of input fuel energy will this quad
need to produce 1000 J of useful output?
Concept 3 – Energy Efficiency
Example
 Calculate the efficiency of each of the following devices
Device
Input Energy
Output
Energy
Gas-power SUV
675 kJ
81 kJ
Gas-electric hybrid vehicle
675 kJ
195 kJ
Natural gas furnace
110 MJ
85 MJ
Electric baseboard heater
9.5 kJ
9.5 kJ
Alkaline dry cell
84.52 kJ
74.38 kJ
Fluorescent light
12.5 kJ
2.75 kJ
Incandescent light
780 J
31 J
Efficiency
Concept 3 – Energy Efficiency
 It is possible for an electric heater to be 100% efficient in
converting electricity to heat
 Devices that convert electricity to other forms of energy
can never be 100% efficient
 Any sort of movement generates a certain amount of thermal energy
www.hielscher.com
Concept 3 – Energy Efficiency
Learning Concept
 Investigate and describe techniques for reducing waste
energy in common household devices
Concept 3 – Energy Efficiency
 Increasing efficiency depends on the purpose of the device
 Worst waste ‘offender’ is heat from friction
 Decrease friction as much as possible by using improved bearings and
lubricants
www.prolubricants.com
Concept 3 – Energy Efficiency
 Increasing efficiency
depends on the purpose
of the device
 Can add layers of
insulation to either
 Reduce the amount of heat
lost to the surroundings
(oven)
 Reduce the amount of heat
being transferred in
(refrigerator)