15.2 energy conversion and conservation

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15.2 Energy Conversion and Conservation

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Think about this…

• What happens when you burn a piece of wood?

• The piece of wood gives off heat (thermal energy) and light (electromagnetic energy)

• You are left with ashes.

• This is an example of converting energy forms.

• Chemical energy converted to thermal and electromagnetic energy.

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Energy Conversion

• Energy can be converted from one form to another.

• The process of changing energy from one form to another is energy conversion.

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Examples of Energy Conversions

• Wind-up toys store elastic potential energy in a compressed spring.

• When the spring unwinds, potential energy is converted into the kinetic energy of the toy’s moving parts.

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are needed to see this picture.QuickTime™ and a decompressor


What forms of energy are involved?• Light bulbs

– Convert electrical energy into thermal and electromagnetic energy

• Cars

– Convert chemical energy into thermal and kinetic energy

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Conservation of Energy

• When energy changes from one form to another, the total energy remains the same.

• The only thing that changes is what the energy looks like.

• The Law of Conservation of Energy states that energy cannot be created or destroyed. It can only change forms.

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What about a burning log?

• When we burn a huge log, we end up with a small amount of ashes. Didn’t we lose energy? We lost mass!

• Let’s think about it another way…

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Loosing your planner…Let’s say that you wake up one morning and you can’t find

your planner! You search everywhere, but you still can’t find it.

Would you say that your planner doesn’t exist anymore?No. It would still exist, but it just doesn’t exist in the same

places where you are used to finding it.

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http://urbangardenproject.files.wordpress.com/2009/03/wood-ashes-00.jpghttp://www.thompsonservices.org.uk/images/Scroll%20Log%20rack%20(1358).jpg

What about a burning log?• Losing your planner is a lot like a burning log.

Even though we lose mass, we know that the energy from the mass converted to other forms of energy.

• We didn’t lose energy, we just converted forms!



More Energy Conversions

• When an object falls, it gains more speed; it gains kinetic energy.

• If energy can’t be created, how is the object gaining kinetic energy?

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More Energy Conversions

• Remember that the object has gravitational potential energy before it falls. This potential energy is converted to kinetic energy.



Energy Conversion in Pendulums

• What is the motion of a pendulum like?

• It starts off with no speed up at a higher level.

• Then you release the pendulum and it starts to move to the other side.

• Then, it reaches a certain height on the other side and at the highest point, it momentarily stops.

• Then it starts the cycle over again.

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Think about the Pendulum in terms of energy.

• At the highest points, the pendulum stops. It has no kinetic energy. But it has a lot of potential energy.

• Then that potential energy starts to convert to kinetic energy.

• At the bottom, the pendulum has maximum kinetic energy and no potential energy.

• Then that kinetic energy starts to convert back to potential energy!

• It’s a cycle of conversions.http://www.problogger.net/wp-content/uploads/2007/10/google-pendulum.jpg



Friction

• Eventually, friction causes all moving things to slow down. But this doesn’t mean we lose energy.

• Friction is really the conversion of kinetic energy into thermal energy.

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Energy Conversion Calculations

• Mechanical energy (KE + PE) is conserved.

(KE + PE)beginning = (KE + PE)ending



Practice

At a construction site, a 1.50 kg brick is dropped from rest and hits the ground at a speed of 26.0 m/s. Assuming no air resistance, calculate the gravitational potential energy of the brick before it was dropped.

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PracticeA diver with a mass of 70.0 kg stands

motionless at the top of a 3.0 m-high diving platform. Calculate his potential energy relative to the water and his speed when he enters the pool.

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E=mc2

• This famous equation really tells us that mass and energy are related. One can be converted to the other.

• Energy is released as matter is destroyed.

• Matter can be created from energy.

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15.2 energy conversion and conservation

Documents

energy forms

forms of energy

energy conversionswind

energy states

total energy

heat thermal energy

elastic potential energy

gravitational potential