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Unit C Energy Flow in Technological Systems
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Every object involved in energy transfers
(system) must use some form of energy
Energy is the ability to do work
Measured in Joules (J) or kilojoules (kJ)
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Open system-energy and matter can flow
into and out of
Closed system-energy but not matter can
flow into
Isolated system-neither energy or matter can
flow in or out
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Total Input Energy
 Energy that goes INTO a system
 Examples
▪ Fuel (chemical potential energy)
▪ Batteries (chemical potential energy)
▪ Springs (elastic potential energy)
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Useful Output Energy
 Desirable form of energy that EXITS a system
 Examples
▪ Heat/thermal (baseboards)
▪ Light (flashlight)
▪ Kinetic (golf club)
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Chemical
Nuclear
Light
Sound
Mechanical
Heat
Electrical
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1.
2.
Energy is the ability to do work
Two categories that all forms of energy can
be put into
Kinetic Energy- energy of motion
Potential Energy-stored energy
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Occurs whenever a force moves an object any
distance
2 conditions must be met
 Movement
 Push or Pull/Force
Work = force x distance the object travels
W=Fd
Joule = newton x metre
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If 10 N of force lift a textbook off the floor 2.4
m, how much work is done?
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What force is applied when a box is moved 15
m and 1450.67 J of work are done?
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Graphs provide visual representation of work and shows
relationship between two variables
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1 type of graph to describe Work
1. Force-Distance Graph
What does it look like?
 Plot Distance in meters on the x axis and Force in newtons on
the y axis
What does it tell us?
 Shows us direct relationship between distance traveled and
force it takes to move it
 The area under the line represents the amount of work done
▪ The area can often be divided into two or more simple geometric
shapes
▪ A = Length x Width (rectangle)
▪ A = ½ Base x Height (triangle)
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Energy due to motion or moving objects
Ek
Examples include
 swinging a golf club
 Walking
 Pushing/pulling
Kinetic energy = ½ (mass) x (velocity)2
Ek = ½ mv2
J = kg x (m/s)2
Determine the kinetic energy of a 1000 kg roller
coaster car that is moving with a speed of
20.0 m/s.
Missy Diwater, the former platform diver for
the Ringling Brother’s circus has a kinetic
energy of 15 000J just prior to hitting the
bucket of water. If Missy’s mass is 50 kg then
what is her speed?
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Latent or stored energy, the potential of an
object to do work due to its position or
condition
Ep
Different forms/Examples
 Chemical –stored in food
 Electrical – stored in electrical appliances
 Stored Nuclear- stored in nuclei
 Gravitational
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Force applied against gravity (usually) the
weight of the object results in stored energy
Weight= force of gravity acting on mass
 Weight - Fg=ma
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Acceleration due to gravity (g) is 9.81 m/s2
Potential energy = mass x acceleration due to
gravity x height
Ep = mgh
J = kg x m/s2 x m
Acceleration due to gravity is a constant and
always equal to 9.81 m/s2
Melody was grabbing a 600 g can of soup off a
1.8m shelf, when it suddenly fell to the floor.
What is the gravitational potential energy of
the can?
A child with a mass of 25.0 kg is at the top of a
slide. The gravitational potential energy of
the child is 981 J. What is the height of the
slide the child is on?
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Law of Conservation of Energy
 Total amount of energy in a given situation
remains constant
Ep = Ek
mgh = ½ mv2
 Energy can be converted from one form to
another but it never changes
Ep + Ek = Total Energy
mgh + ½ mv2
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Potential energy at the start of the swing will
equal the potential energy at the end of the
swing
Kinetic energy in mid swing equal to potential
energy at the start of the swing
A 50.0 kg rock is dropped over the edge of a
cliff, 30.0m above the surface of a lake. What
is the speed of the rock just before it strikes
the surface of the lake?
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Law of conservation of energy assumes that
all machines are perfect and 100% efficient
 In reality there are so many energy conversions
between total energy input and final useful
energy output energy that this is not true
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Means how much of the initial energy going
into a system comes out at the end in the
form we want
Most ‘waste’ energy is lost to the
system/surroundings as heat
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Total Input Energy
 Energy that goes INTO a system
 Joules (J)
 Examples
▪ Fuel (chemical potential energy)
▪ Batteries (chemical potential energy)
▪ Springs (elastic potential energy)
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Useful Output Energy
 Energy at end of conversion that is used
 Joules (J)
 Examples
▪ Heat/thermal (baseboards)
▪ Light (flashlight)
▪ Kinetic (golf club)
Efficiency = final useful energy OUTPUT
X 100 %
total energy INPUT
When a 100 W light bulb is on for 1.0 h, it uses
360KJ of electrical energy. During that time, the
light bulb emits 19 kJ of light. What is the
efficiency of the light bulb in transforming
electrical energy into light energy?
A bobcat uses 393 kJ of chemical potential energy
stored in the fuel to lift 2750 kg of dirt 3.2 m
straight up, to dump it in a dump truck. What is
the efficiency of the bobcat in converting
chemical potential energy into gravitational
potential energy?
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Units, sig digs, scientific notation
Forms of Energy and Energy Conversions
Work
 Math
 Graph
▪ Force-Distance
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Kinetic vs Potential Energy
 Math
▪ Ek
▪ Ep
▪ Total Energy
 Graph
▪ Energy-Time
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Law of conservation of energy and Efficiency of
a system