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Lecture 17
Changing Forms of Energy
Ozgur Unal
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Consider a hair dryer.
What forms of energy are involved in it?
In a hair dryer, electrical energy is converted into heat,
kinetic energy, sound energy etc.
Electrical energy being used comes from power plants.
Examples of power plants: Thermal power plants, nuclear
power plants, hydroelectric power plants etc.
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More examples of energy transformation:
Light bulbs:
Burning fuel:
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Potential energy (PE) and kinetic energy (KE) can be
converted to each other.
The GPE of a falling object is converted into KE.
When you stretch a rubber band and let it go, you convert
elastic PE into KE.
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In order to understand the conversion between KE and PE,
it is helpful to identify the mechanical energy of a system.
Mechanical energy = Potential energy + Kinetic Energy
ME = KE + PE
In other words, mechanical energy is energy due to the
position and the motion of an object or the objects in a
system.
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Falling objects:
As an object falls down, its PE gets smaller and its KE
increases.
In other words, PE is converted into KE.
How about ME? Does it get smaller? Does it increase?
Total ME does not change. It stays the same!
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How is the energy transformation in projectile motion?
As the object rises up its speed (KE)
decreases, however due to the
increased height the PE gets bigger.
Mechanical energy stays the same!
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Consider the pendulum shown to you.
Analyze the energy transformation taking place in the
pendulum.
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Lecture 18
The Law of Conservation of Energy
Ozgur Unal
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If we ignore the effects of friction, the mechanical energy of
an object or a system of objects is constant.
This is true for other forms of energy.
Energy can change from one form to another, but the total
amount of energy never changes.
The law of conservation of energy states that energy cannot
be created or destroyed.
In other words, the total amount of energy in the universe
does not change.
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Does the law of conservation of energy hold every time?
Is there any exceptions to this law?
Consider the motion of the ball.
Inıtially it has kinetic energy, because it moves. But
eventually it stops.
Where does the energy go? Is it being destroyed?
Similar to the fading motion of the bob in a pendulum, the
ball’s KE is transformed into thermal energy due to friction.
Bend the copper wires continuously for a minute, then
touch the wire by your lowe lips and sense the thermal energy
accumulated in the wire.
DO NOT SWALLOW THE WIRE!
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Example: Consider a simple pendulum. The bob in the
pendulum has a mass of 2 kg. It is released to swing from a
height of 25 cm from the lowest level. What is the maximum
speed of the bob during its motion?
Example: A 4kg-object is thrown up from a height of 10
meters. If the initial speed of the object is 9m/s, what is the
speed of the object when it reaches the ground?
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How do living things get the energy they need?
Where does that energy come from?
Consider the food chain below:
Grass -------> Rabbit --------> Fox
The radiant energy (sunlight) coming from the Sun is used
by plants.
Consumers get the energy they need by eating plants and
other consumers.
Energy is transformed and conserved in a food chain!
How about the Sun?
Is it a limitless souce of energy?
Is energy created in the Sun?
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Have you every seen
this person before?
Have you every seen this equation?
E = m * c2
What does this equation tell you?
c is the speed of light, c = 3 x 108 m/s
Mass can be converted into energy!!
Matter and energy mean the same thing.
This is what fuels the Sun.
Let’s take a closer look at what’s going on in the Sun.
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In a type of nuclear reaction, called nuclear fusion, nuclei of
atoms fuse together to form heavier nuclei.
The mass difference
between the reacting
nuclei and the newly
formed nuclei is
converted into energy.
Sun is fueled by the nuclear fusion of Hydrogen nuclei into
Helium nucleus.
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There is nuclear reaction, called
nuclear fission, in which a heavy
nucleus breaks apart to form smaller
nuclear by releasing enormous
amounts of energy.
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In both nuclear reactions, nuclear fusion and nuclear
fission, the total energy is conserved.
To see this, you need to consider the energy equivalent of
mass according to
Einstein’s famous
formula.
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Example: 4 H nuclei fuse to form a He nuclei in the process
called nuclear fusion. The masses of H nucleus and He
nucleus are 1.674 x 10-27 kg and 6.646 x 10-27 kg, respectively.
What is the amount of energy released in this reaction?
Energy equivalent of one H nucleus: EH = mH * c2
Energy equivalent of one He nucleus: EHe = mHe * c2
Total energy before reaction: 4*EH = 4*mH * c2
Total energy after reaction: EHe + Ereleased = mHe * c2 + Ereleased
Total energy is conserved:
Ebefore = Eafter
4*mH * c2 = mHe * c2 + Ereleased
Ereleased = c2(4mH - mHe)
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