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Understanding Efficiency
Human Circuit
Learner Outcomes:
 Apply the concepts of conservation of
energy and efficiency to the analysis of
energy devices.
 Compare energy inputs and outputs of a
device, and calculate its efficiency.
 Describe techniques for reducing energy
waste in common household devices.
Power: Rate at which a device converts
energy.
Tokyo's Toshima Incineration Plant
 Burns 300 tons of garbage a day, turning it into electricity,
hot water and a kind of recyclable sand.
The unit of power is:
The Watt (W) – Named for James
Watt
A watt is equal to one joule per
second.
The faster a device converts energy,
the greater its power rating.
Power Ratings:
Most small appliances: 1500 W or
less
Stove: 7000 W
Calculator: 0.4 mW
Review…
Ohm's Law…
 One the most important and fundamental laws
of electronics is that of ohms law.
 This law defines the relationships between
current, voltage and resistance.
Ohm's Law…
 States that the current through a conductor
between two points is directly proportional to
the potential difference across the two points.
Introducing the constant of proportionality, the
resistance, one arrives at the usual
mathematical equation that describes this
relationship- it’s a powerful predictor…
Mathematically:
Power is equal to the current
multiplied by the voltage.
Power (P); current (I); voltage (V)
Energy: Measured in Joules (watts x
seconds)
 You can use the power rating of a device to figure out
the amount of energy the device uses.
 Remember that power is the rate at which a device
converts energy. You can find the amount of energy by
multiplying this rate by the length of time the device
operates.
Kilowatt Hours
 People use A LOT of joules of energy in their
homes/businesses, so kilowatt hours are often used as a unit
for energy.
 Calculation is the same, except hours are substituted for
seconds, and kilowatts (kW) are substituted for watts.
 Electricity meters measure the energy used in kilowatt hours.
Remember what Mr. Edwards taught
us?/Watch video below:
Law of Conservation of Energy
 Energy cannot be created or destroyed.
 It does not just appear or disappear… it
must be transformed from one form to
another.
Energy Dissipation
 However… A lot of the time we find that
the energy output of a device or system is
smaller than the input energy.
 Sometimes, a lot smaller???
Heat…
 Most often the missing energy is lost or dissipated (scattered
in all directions) as heat.
 Hot plate/beaker with water.
 The hot plate radiates some heat to into the surrounding air
instead of into the water, or into nearby objects, including
you.
 Heat is “lost”.
Mechanical Devices
All mechanical systems dissipate
some energy, so their useable energy
is always less than their input energy.
Irrigation System/Pumping System
 You can hear it running, so some energy is dissipated as
sound.
 If the motor has been running for a while, if you touch it,
it will be hot.
 Current flowing through wires always produces some
heat, and the friction between moving parts generates
some heat as well.
 All of the “missing” input energy has been transformed
into energy you cannot use.
Understanding Efficiency
 The efficiency of a device is the ration of useful energy
that comes out of a device compared to the total energy
that went in.
 Efficiency is usually calculated as a percent:
So…
The more input energy that
a device converts to useable
output energy, the more
efficient the device is.
Easy Bake Oven? Remember that they use the “wasted”
energy of a light bulb to bake a cake or cookies!
For each device, try calculating the percent of its input energy that it
converts to useful output energy. Which device is the most efficient? Which
is the least efficient. What do you think causes the difference between the
most and least efficient?
Comparing Efficiencies
By comparing efficiencies of devices
we can judge both their energy cost
and their environmental impact.
Which is more efficient? Hint- Which one
do you see used the most in our school?
Check and Reflect:
Page 338, #s 1-9
We will correct in class on
Wednesday.
Reducing the Energy Wasted by Devices
Limits to Efficiency
An electric heater can be 100%
efficient in converting electricity
to heat, but devices that convert
electricity to other forms of
energy can never be 100%
efficient.
Moving parts…
Generate a certain amount of
thermal energy that is not useful
output.
Friction is one cause of energy lost in
engines.
Increasing Efficiency?
 How to increase efficiency depends on its
purpose… Many devices are made to
convert electrical energy to mechanical
energy, where the worst energy waste
“offender” is friction.
The easiest way to increase efficiency in
these devices:
 Decrease friction as much as possible- use
improved bearings and lubricants.
 In devices where heat is produced, heat loss is a
major concern:
 Heat escapes- add more insulation (for example,
around a stove), or refrigerator- reduces the
amount of heat that transfers into the fridge.
 You need less energy to keep the fridge cold.
Efficiency has increased over time with
better technology:
Check & Reflect:
Check & Reflect, page 342, #1-7
We will correct in class on
Wednesday.