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AIMS
 State and apply the law of conservation of energy
Fixed amount in closed systems
Change form not create or destroy
 Understand need to transform energy
Explain any losses
 Use systems diagrams to account for energy changes
 Identify energy forms and changes within a system
 Calculate energy transfers
The Law of Conservation of Energy
 The conservation of energy is a fundamental concept of
physics.
Along with the conservation of mass and momentum.
Derived from first law of thermodynamics.
 Within a closed system, the amount of energy remains
constant and energy is neither created nor destroyed.
Energy can be converted from one form to another but the total
energy within the domain remains fixed.
Energy Transformation
 How energy can be converted to other forms is important
to technologists
 Some forms are directly interchangeable
Dropping a stone
Potential  Kinetic
 Others require several stages
Coal burnt in a power station to produce electrical power
Chemical  heat  kinetic  electrical
Systems Approach
 Systems diagrams can be used to summarise energy changes
 Consider a light bulb (simplified)
ELECTIRCAL
LIGHT BULB
LIGHT
 Produce system diagrams for an electric motor and an electric
generator
Energy Transformation Examples
A
State the energy form at
points A, B, C and D
D
A: Potential energy
B: Kinetic Energy (linear motion)
B
C
C: Kinetic Energy (rotary motion)
D: Electrical Energy
Energy Transformation Examples
1.
2.
State the energy form at points
A to H in the diagram opposite.
A
Describe the energy changes that
take place within the system
MASS
ELECTRIC MOTOR
B
A: Potential
B: Electrical
C: Sound
D: Electrical
E: Light
F: Electrical
G: Electrical
H: Potential
ELECTROLYTIC CELL
(WATER)
H
EXPANSION
CYLINDER
G
-
+
D
MICROPHONE
F
SOLAR CELL
E
ELECTRIC BULB
C
LOUDSPEAKER
Energy ‘losses’ during transformation

We accept that energy cannot be created or destroyed

This tells us that the energy output of a system equals the energy
input

HOWEVER, not all the energy is used to do USEFUL work


When a conversion takes place there is always a loss
 Examples are sound, friction or heat
Go back to the energy conversion diagrams for the bulb, motor
and generator and add any losses to the output side
Energy Losses in a Wind Turbine

A turbine can be used to generate electricity. The generator can
be connected to it in two ways.
coupled directly to vanes
coupled via shafts and gears
Energy ‘losses’ during transformation
1.
List the energy conversions that take place during its operation
2.
Describe the energy losses in both systems
3.
Which do you think is more efficient?
Calculating Energy Transfers: A Falling Ball
E P1
EP2
EK1
EK2
E = EP1

E = (EP2 + EK1) = EP1
E = EK2 = (EP2 + EK1) = EP1
If the mass is 5kg and the building is 25m tall calculate the
final velocity and the kinetic energy at impact
Worked Example
 A body of mass 30 kg falls freely from a height of 20 metres. Find
its final velocity and kinetic energy at impact.
First calculate the initial potential energy.
EP = mgh
= 30  9.81  20
= 5.88 kJ
 This potential energy is converted or transferred into kinetic energy,
which means that the kinetic energy at impact is equal to 5.9 kJ.
 To calculate the final velocity of the body we begin by taking EK =
5.9 kJ.
EK = ½mv² 5.88  10³ = ½  30  v²
v² = 392.4
v = 19.8 m/s
Pupil Problems

A 5 kg mass is raised steadily through a height of 2 m. What
work is done and what is the body’s potential energy relative to
the start?

A body of mass 30 kg is projected vertically upwards with an
initial velocity of 20 m/s. What is the initial kinetic energy of the
body and to what height will it rise?

A mass of 20 kg is allowed to fall freely from a certain height
above a datum. When the body is 16 m above the datum, it
possesses a total energy of 3,531 J. What is the starting height of
the object?
Efficiency
Calculating efficiency
The efficiency of an energy transformation is a
measure of how much of the input energy appears
as useful output energy.
The efficiency of any system can be calculated
using the equation:
Efficiency,  = Useful energy output
Total energy input
Worked Example
An electric lift rated at 110 V, 30 A raises a 700
kg load a height of 20 m in two minutes.
 By considering the electrical energy input
and the potential energy gained by the
mass, determine the percentage efficiency
of this energy transformation.
Worked Problem
Ee = ItV
= 30  120  110
= 396 kJ
Potential energy gained is calculated as follows.
EP = mgh
= 700  9.81  20
= 137.3 kJ
Percentage Efficiency = Useful Energy Output  100%
Total Energy Input
= 137.3 100% = 34.7%
396
Pupil Problems
(1) An electric kettle is rated at 240 V, 10 A. When switched on it
takes three minutes to raise the temperature of 0.5 kg of water
from 20C to 100C.
Determine:
The electrical energy supplied in the three minutes
The heat energy required to raise the temperature of the water
The efficiency of the kettle.
Pupil Problems
Boxes in a factory are transferred from one floor to another using a
chute system as shown above. The boxes start from rest at the top of
the chute and during the decent there is a 40 per cent loss of energy.
The boxes weigh 10 kg each.
Calculate the velocity of the boxes at the bottom of the chute.
CHUTE
7m
dm
Energy Audits
 Your Teacher will show you how to
construct an energy audit