15.2 Energy Conversion and Conservation

15.2 Energy Conversion and Conservation

Page 453-459

Energy ConversionEnergy can be CONVERTED from one form to anotherThe process of changing energy from one form to anotherEx: When a spring unwinds goes from Elastic Potential Energy TO Kinetic EnergyLight bulb: Electric (the battery creates electricity) TO Thermal and Electromagnetic (heat of the bulb and the light it shines)

Energy conversion examples

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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)

Law of Conservation of EnergyThe law of conservation of energy states that Energy cannot be created or destroyedIn 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 endTherefore, 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 overallFriction can slow thingsSo the pendulum would eventually stop moving QuickTime™ and a

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Friction effects machineryFriction within machinery reduces efficiencyFriction is a major cause of energy consumption in cars and factories because all the moving parts are subject to frictionYou 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 oysterSea gulls use PE to obtain foodAs the gull brings the oyster up into the air PE increasesAt its highest point the gull releases the oyster at max PEAs it falls PE is converted to KE as the oyster increases is velocity toward the rocksAs it hits the rocks the oyster is at max KE and it shatters into the gulls lunch

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Energy Conversion: pole vault

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

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http://www.popsci.com/how-it-works/article/2008-07/how-it-works-pole-vault

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Energy Conversion CalculationsMechanical 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 problemAt 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 kgVelocity = 26 m/s

But we know thatKEb + 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

So now we know KEend therefore we also know PEb

PEb = KEend = 507 J, so PEb = 507 J

Cannot find PE with only this info

Step 3

Step 2

Step 1

Final answer

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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.

E = mc2

m = mass and C = speed of light

The End

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