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Agenda

Agenda. Friday&Monday – Problems Ch. 21-23 Tuesday – lab 5 & “Curve” Quiz Can Improve score by 5-20 pts Or replace quiz 2 (not a popular quiz) Today Potential & Potential Energy Chapters 6&7. Potential Energy. Measure of Energy “Stored” in a system Akin to work.

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Agenda

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  1. Agenda • Friday&Monday – Problems Ch. 21-23 • Tuesday – lab 5 & “Curve” Quiz • Can Improve score by 5-20 pts • Or replace quiz 2 (not a popular quiz) • Today • Potential & Potential Energy • Chapters 6&7

  2. Potential Energy • Measure of Energy “Stored” in a system • Akin to work

  3. Gravitational PE [I know, U…] • How much work does it take to raise a mass M to a height H in a gravitational field g? Negative work done by gravity Implies gravitational energy stored Work done by something else (Outside)

  4. Conservative Forces & PE • Energy from Conservative forces can be described in terms of PE • Spring PE (E stored by spring) • Gravitational PE (E stored by gravity) • Electrical PE (E stored in Electric Fields) • Conservative = Path Independent • Conservative = No energy lost • Conservative N.E. to friction

  5. “Mechanical” Energy Conservation • Have • Q = DU + W • Heat, internal energy, Mechanical • Most large systems, Heat irrelevant • Thermal energy of a golf ball? Small! • Need to look closer at macroscopic here • WNC = DE = EF – E0

  6. “Mechanical” Energy ConservationEF = E0 + WNC • WNC = DE = EF – E0 • WNC Work done by Non-Conserved • “Outside” or Friction, etc…. • E = Energy = PE + KE • See how thermal might come in? • Wonder of Energy • No Directions • If no WNC, then no cares about path! • Can often ignore everything but initial & final

  7. Relativity • No – not the extra cool one • Energy is relative • Can you tell what floor I’m on when I drop something? • Gravitational PE comes into play as relative height change, not absolute height.

  8. Potential vs. Fields • Energy ~ Integral of Force • Field from a point C = constant (k, G) S = stuff (Q, M0 r = distance from object emanating field

  9. Potential vs. Fields • Energy ~ Integral of Force • Field from a point C = constant (k, G) S = stuff (Q, M0 r = distance from object emanating field P  Potential Could be gravitational Potential Could be electrical potential (Volts)

  10. Examine GravityCh. 8? • How fast must something be traveling to escape the pull of the Earth’s gravitational field? • Needed • Gravitational Potential Field • Energy Relationship • Beginning “height” • Final height”

  11. Examine GravityCh. 8? • How fast must something be traveling to escape the pull of the Earth’s gravitational field? • Needed • Gravitational PE = PEG = -GmME/r • Energy Relationship  EF = E0 + WNC • Given an initial velocity, no other “NC”  WNC=0 • Beginning “height” RE (~ Surface of Earth) • Final height”  Far Away (infinity)

  12. Find Escape VelocityEF = E0 + WNC • Initial Energy • KE = 0.5mv2 • PE = -GmME/RE • Negative implies object attracted to earth • As r increases, PE becomes less negative • As r increases, h increases, PE increases (mgh) • WNC = 0 • Only force is gravity • Final Energy • PE = ? • PE = 0 [no earth pull]

  13. Find Escape VelocityEF = E0 + WNC • Initial Energy • KE = 0.5mv2 • PE = -GmME/RE • WNC = 0 • Only force is gravity • Final Energy • PE = 0 [no earth pull] • KE=? • KE = 0 [minimum initial energy to escape earth]

  14. Find Escape VelocityEF = E0 + WNC • EF = 0, WNC =0 • E0 = 0 • E0 = PE0 + KE0 • E0 = -GmME/RE + 0.5mv2 = 0 • v2 = -2GME/RE • What does escape velocity depend on? • How does this relate to electricity? • V = kQ/r & PEE = kQ1Q2/r • Same method, gravity easier as no + or -

  15. Reference for PE • When dealing with “points,” what is a good reference for energy? • Hint: Earth (from outside) looks like point source (G Law)

  16. Explore System of Charges

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