Download Energy - 11SCP

Energy
Energy
Wednesday, 24 May 2017
• Objective:
– Identify the different forms of energy
– Explain work and power
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Energy
Energy is the capacity to do work
Feynmann Definition
'There is a fact, or if you wish a law, governing all natural
phenomena that are known to date. There is no
exception to this law - it is exact so far as is known. The
law is called conservation of energy. It says that there is a
certain quantity, which we call energy, that does not
change in the manifold changes which nature undergoes.
That is a most abstract idea, because it is a mathematical
principle; it says that there is a numerical quantity, which
does not change when something happens. It is not a
description of a mechanism, or anything concrete; it is
just a strange fact that we can calculate some number
and when we finish watching nature go through her tricks
and calculate the number again, it is the same.'
– (R.P. Feynman, Tbe Feynman Lectures on Physics Volumes 1-3)
In other words…
• Energy is not a physically
measurable quantity but rather a
mathematical idea.
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Energy
• There are two types of energy forms. One
is active energy, the other is potential
energy.
• Active energy is energy that has effects
that are easily seen or detected.
• Potential energy is energy that is stored
and only have effect when released.
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Types of energy
Active energy
Potential energy
Kinetic Energy
Chemical potential energy
Solar energy
Elastic potential energy
Electrical energy
Gravitational potential energy
Sound energy
Nuclear energy
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Some
meanings…
• Kinetic energy
– The energy of an object when it is moving
• Chemical potential energy
– Energy released by chemical reactions, eg
from food, fuels and explosives
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More meanings…
• Elastic potential energy
– Energy stored when a spring or rubber
band is stretched or compressed
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More meanings…
• Gravitational potential energy
– The energy an object gains when it is
lifted up. This energy is released when it
falls down again.
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More meanings…
• Nuclear Energy
– Energy released in nuclear reactors or
atomic bomb explosions. Solar energy is
also produced from nuclear energy in
the sun.
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Measuring Energy
• Energy is measured in Joules (J), or
Kilojoules (kJ). 1000J= 1kJ
Kinetic
e.g. moving objects
Kinetic
Sound
e.g. music
Kinetic
Chemical Potential
e.g. batteries, food
Potential
Heat
e.g. hot stuff
Kinetic
Light
e.g. traffic lights
Kinetic
Elastic Potential
e.g. spring
Potential
Nuclear
Potential
e.g. atomic bomb,
inside the sun
Gravitational
Potential
e.g. object held high
Potential
Electric Potential
e.g. electricity
Potential
Magnetic Potential
e.g. magnets
Potential
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Calculating Energy
• Ep=mgh
• Gravitational potential energy = mass x gravity x
height
• Ek=1/2mv2
• Kinetic energy = 1/2 x mass x velocity
squared
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Energy Conservation
Energy cannot be created or destroyed.
It can only be changed from one form
to another
For example…
• When climbing on a diving board…
– You use your chemical potential energy and
turn it into kinetic energy as you move
– The kinetic energy become gravitational
potential energy when you are off the ground
– When you dive your gravitational potential
energy is transformed into kinetic energy
again.
– The kinetic energy is turned into heat and
sound.
Energy Interactive Cloze
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Work
• Work is done when a force moves an
object.
• If work is done to an object then the
object gains energy. The energy
gained equals the work done.
• So… Both work and energy are
measured in joules
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Calculating work done
• Work = Force x Distance
• W=Fd
• Where
– W is the work in Joules
– F is the force in Newtons which moves the object
– d is the distance the object moves in metres
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Calculating work
W
F
d
Write the formula for…
• W=
• F=
• d=
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An example question
• A toy car is pushed 2 m. The work done on
the car is 800J. Calculate the size of the
force which moved it.
– d= 2m
– W=800J
– F=?
• F=W/d
=800/2
=400N
Power
• Power is the rate at which work is
done.
• Power is measured in watts, W
Power
• Power is calculated using:
Power= Work done
Time taken
P=W/t
Sample Problem (power)
• A runner can do 12000J of work in
one minute. What is the power
output?
P=W/t
W= 12000J
t=60s
P=?
P=12000/60
P=200W