energy 2 - eduBuzz.org
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Transcript energy 2 - eduBuzz.org
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?
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 9.8 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 5.88 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 ms-1
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 9.8 20
= 137.2 kJ
Percentage Efficiency = Useful Energy Output 100%
Total Energy Input
= 137.2 100% = 34.65%
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 20C to 100C.
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
Revision Questions
A load is raised through 6m. The load is 1kg in mass. What is the load’s
potential energy at its final height?
What work is done in lifting the above load?
If the lifting system is only 60% efficient. What input energy is
required to lift the load?
A kettle boils water in 1 minute. If the kettle uses 100KJ of energy
boiling the water, what is the current rating of the kettle? (240V supply
to kettle)
A process raises the temperature of water from 6 0C to 70 0C in 12.6
seconds. If the initial mass of water is 2kg. What energy is used in the
process?
If the energy of a moving body is 20MJ and it has a mass of 300kg,
what is the bodies velocity?
ENERGY USE IN
HOUSEHOLDS
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