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Conservation of Energy

Conservation of Energy. Mr. Finn Honors Physics December, 2011. Key Questions. What does “ conservation ” mean? What is “ energy ?” Is this law the result of “ circular reasoning ?” “ Misconceptions ” about energy. Conservation. Use sparingly or carefully Avoid waste, economize

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Conservation of Energy

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  1. Conservation of Energy Mr. Finn Honors Physics December, 2011

  2. Key Questions • What does “conservation” mean? • What is “energy?” • Is this law the result of “circular reasoning?” • “Misconceptions” about energy.

  3. Conservation • Use sparingly or carefully • Avoid waste, economize • Saving for future use • Preservation from loss, damage, neglect • Protection of natural resources • Maintenance of a physical quantity during a change Scientific usage of the term!

  4. Conservation Laws As a system(collection of physical objects interacting with each other) undergoes a change of state(configuration or arrangement of the objects), a physical quantity remains constant. Or the initial value of the quantity is the same as the final value of the quantity: Qi = Qf

  5. Classical Conservation Laws • Linear/Translational Momentum • Angular/Rotational Momentum • Energy • Mechanical energy • Mass • quantity that determines gravitational forces • Electric Charge • quantity that determines electromagnetic forces Recall - Noether Theorem and Symmetry

  6. Energy • What IS energy? • Is this a scientifically valid question? • Science is concerned with observable, measurable behavior of objects in the natural world. • It is a conserved quantity that measures how objects interact or their ability to do “work” on each other and change their states of motion. • It partially replaces Newton’s idea of a force. • It is a scalar quantity measured in the units of a J (N m)

  7. Analogy Richard Feynman • Dennis the Menace • Older version of Bart Simpson • Messy Playroom • 18 wooden blocks • Mother believes there should always be 18 blocks in the room • Every day she counts them

  8. Missing Blocks? • One day – only 14 blocks? • 4 blocks appear to be missing • Mother notices a box • Mass of box increased by mass of missing 4 blocks • What remains constant:

  9. More Missing Blocks? • Next day – only 6 blocks? • 8 more blocks appear to be missing • Mother notices the bath tub • Level of water increased by amount equivalent to volume of 8 blocks • What remains constant:

  10. All Blocks Gone? • Next day – no blocks? • Last 6 blocks appear to be missing • Mother notices a bag hanging in the closet • Length of cord increased proportional to weight of missing blocks • What remains constant:

  11. Forget the blocks! • Mother notices the following expression remains constant: A M + B h + C x = constant • Original reference to blocks is irrelevant • Energy discovered in much the same way! M = mass of box h = water level in tub x = length of cord A, B, C = constants

  12. Energy found the SAME way! Galileo: • ball roll down ramp: height = speed at bottom • regardless of angle of ramp • ball rolls up 2nd ramp to same height • regardless of angle of ramps • ball “remembers” its initial height Galileo GPE  KE  GPE

  13. More Data … Christiaan Huygens Huygens • balls colliding on a level, horizontal table, quantity v2 remains constant KE  KE

  14. Yet More Data … James Joule Joule: • object falling a certain height produces a corresponding change in temperature of a certain mass of water GPE  TE

  15. Put it together … • Combination of quantities remains constant: • Or a system of objects in one state has the same “energy” as in another state: Ei = Ef

  16. Work No Resistance W = ∆KE W = F∆x W = ∆GPE Gravity Resistance (Fnet = 0) W = ∆EPE Elastic W = heat Friction

  17. Circular Reasoning • Have we simply defined a concept whose total remains constant by definition? • Is this science or not? • Question: • Can we find/define a set of “state functions” whose sum is always constant for closed, isolated systems? This condition avoids circularity - repeated tests against new experimental conditions.

  18. Misconceptions • On a piece of paper, indicate whether the following statements are TRUE or FALSE. • Briefly justify your answer. • Do so QUIETLY! Do not blurt out any answers. • There is NO need to ask any clarifying questions. The statements are fully self-explanatory.

  19. True - False? • Energy is a force. • Potential energy can be stored in an object, but is not really energy until it has been released. • Some energy is consumed or lost when converted from one form to another - so we need to “conserve” it. • Objects that are not moving do not have energy unless they are alive.

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