Download 15.2 Energy Conversion and Conservation

15.2 Energy Conversion
and Conservation
Page 453-459
Energy Conversion
Energy can be CONVERTED from one
form to another
The process of changing energy from
one form to another
Ex: When a spring unwinds goes from
Elastic Potential Energy TO Kinetic
Energy
Light bulb: Electric (the battery creates
electricity) TO Thermal and
Electromagnetic (heat of the bulb and
the light it shines)
Energy conversion examples
Chemical
energy (in
battery)
Electrical
energy
(through
the wires)
Electrical energy
(from the wall socket
to the electric stove)
Electromagnetic
and thermal
energy (from the
light bulb)
Thermal
energy (the
hot stove coil)
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Law of Conservation of Energy
The law of conservation of energy states
that Energy cannot be created or destroyed
In a closed system (where nothing can
enter or leave) the amount of energy
present at the beginning of a process is the
same as the amount of energy at the end
Therefore, the pendulum in the picture
would always reach the exact same height
on either side and would never come to rest
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Law of Conservation of Energy
(cont.)
In the real world, energy can be lost to
frictional forces (kinetic to thermal energy
which is then lost) If you account for
thermal energy lost to friction energy will be
conserved overall
Friction can slow things
So the pendulum would
eventually stop moving
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Friction effects machinery
Friction within
machinery reduces
efficiency
Friction is a major
cause of energy
consumption in cars
and factories because
all the moving parts
are subject to friction
You can reduce
friction but you cant
avoid it.
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Engine block over
heating can be caused
by excess friction. If a lot
of energy is lost as heat
more fuel is needed to
do less work.
Energy Conversion
The GPE of an object is
converted to KE of motion as it
falls.
Ex: Clock Pendulum
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Energy Conversion: gull vs oyster
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Sea gulls use PE to obtain
food
As the gull brings the
oyster up into the air PE
increases
At its highest point the gull
releases the oyster at max
PE
As it falls PE is converted
to KE as the oyster
increases is velocity
toward the rocks
As it hits the rocks the
oyster is at max KE and it
shatters into the gulls
lunch
Energy Conversion: pole vault
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Pole-vaulter sprints =
kinetic energy
Pole bends = elastic
potential energy
Pole springs back into
shape = kinetic energy
Pole-vaulter rises quickly
into the air = kinetic to
gravitational potential
energy
Reaches highest point
and begins to fall, max PE
to KE
http://www.popsci.com/how-it-works/article/2008-07/how-it-works-pole-vault
Energy Conversion Calculations
Mechanical energy = KE + PE
When friction is small enough to be
ignored mechanical energy at the
beginning equals the mechanical
energy at the end which means
mechanical energy remains constant
Conservation of Mechanical Energy
(KE + PE)begin = (KE + PE)end
Conservation of mechanical energy problem
At a construction site, a 1.5 kg brick is dropped from rest and hits the
ground at a speed of 26 m/s. assuming air resistance can be
ignored, calculate the GPE of the brick BEFORE it was dropped.
Given info:
Mass = 1.5 kg
Velocity = 26 m/s
But we know that
Cannot find PE with only this info
KEb + PEb = KEend + PEend
So if…..
KEb= 0 and PEend = 0
Then…
0 + Peb = KEend + 0
And finally….
PEb = KEend
So we solve for KEend to find PEb
KEend = 1/2 mv2
KEend = 1/2 (1.5 kg) (26m/s)2
KEend = 1/2 (1.5 kg) 676 m2/s2
KEend = 507 kgm/s2m
KEend = 507 N m
KEend = 507 J
Step 1
Step 2
Step 3
So now we know KEend therefore we also know PEb
PEb = KEend = 507 J, so PEb = 507 J
Final answer
Energy and Mass
Einstein’s equation, E = mc2, says that energy
and mass are equivalent and can be converted
into each other
Meaning energy is released as matter is destroyed,
and matter can be created from energy
Therefore, the law of conservation of energy has
been modified to say that mass and energy together
are always conserved.
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E = mc2
m = mass and C = speed of light
The
End
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