p240_ct7_f07 - Physics 240 Fall 2007 Lecture #7 Dr. Dave...

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Physics 240 Fall 2007 Lecture #7 Dr. Dave Winn 2405 Randall Lab winn@umich.edu
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One more key idea in electrostatics • We have seen electrostatic force and developed the idea of electric field to quantify it • What about energy? Clearly there is energy stored in separated electric charge, it can come out as light and sound in a spark…
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Review of potential energy • Work changes energy: W= F d x = Δ KE • For conservative forces work is reversible; they can take energy away and completely return it • Recall PE definition: Δ PE = -W conservative • Neg. work by force increases PE, decreases KE • Pos. work decreases PE, increases KE Cedar Point Millennium Force Ride… Examples
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Gravitational PE • Work done by gravity: • Since force acts only downward, Δ PE depends only on change in height h • As objects move higher, energy is ‘stored’ as gravitational potential energy. () h mg r d y mg r d F W PE g g Δ = = = = Δ ˆ G G G Δ h r d G y mg ˆ
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Bouncing electron • A very light electron is fired from a very great distance with initial velocity v 0 towards a much more massive negatively charged ball. What happens? _ _ _ _ _ _ _ _ - v 0 Negative work is done on it by the electrostatic force, converting it’s kinetic energy into potential energy, until finally it comes to rest. Then it reverses course and goes flying out again… - - v 0
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It’s all relative Δ PE between two points is always clearly defined by the definition • Only potential energy changes are defined. • Often we talk about ‘the potential energy at a point’ meaning PE measured relative to a reference point • This reference point is often the ground for gravity, or the unstretched position of a spring. • To talk about PE at a point, we have to define a reference point
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Imagine a positive charge moving from the point marked P I to the point marked P F . Is the change in potential energy Δ PE of this particle: 1. Positive 2. Negative 3. Zero + P I P F + F electric Work done moving from P I to P F is positive, so Δ PE is negative
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Imagine a negative charge moving from the point marked P I to the point marked P F . Is the change in potential energy Δ PE of this particle: 1. Positive 2. Negative 3. Zero + P I P F - F electric Work done moving from P I to P F is negative, so Δ PE is positive!
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p240_ct7_f07 - Physics 240 Fall 2007 Lecture #7 Dr. Dave...

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