Seeger The Demi Song* - UMD
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Physics 131 11/30/16 Prof. E. F. Redish 1 11/30/16 1 Physics 131 ■ Theme Music: Pete Seeger The Demi Song* ■ Cartoon: S. Harris * A chemistry TA November 30, 2016 Physics 131 Prof. E. F. Redish Foothold ideas: Energies between charge clusters ■ Atoms and molecules are made up of charges. ■ The potential energy between two charges is ■ The potential energy between many charges is 11/30/16 8 Physics 131 U 12 elec = k C Q 1 Q 2 r 12 U 12... N elec = k C Q i Q j r ij i < j =1 N ∑
Transcript of Seeger The Demi Song* - UMD
Physics 131 11/30/16
Prof. E. F. Redish 1
11/30/16 1 Physics 131
■ Theme Music: Pete Seeger The Demi Song*
■ Cartoon: S. Harris
* A chemistry TA
November 30, 2016 Physics 131 Prof. E. F. Redish
Foothold ideas: Energies between charge clusters ■ Atoms and molecules are made up of charges. ■ The potential energy between two charges is
■ The potential energy between many charges is
11/30/16 8 Physics 131
U12elec = kCQ1Q2
r12
U12...Nelec =
kCQiQj
riji< j=1
N
∑
Physics 131 11/30/16
Prof. E. F. Redish 2
How many interactions in the system have an electric potential energy? (Equivalently: How many “1/r” terms will we have to add up to get the total electric PE?)
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-q
-q
r 2q Q
How many of those potential energies change when the charge Q moves to the right?
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-q
-q
r 2q Q
Physics 131 11/30/16
Prof. E. F. Redish 3
Sketch a graph of the extra potential energy from adding Q as a function of position r of charge Q
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-q
-q
r 2q Q
ΔU = kCQ
qi
rQ→qii=1
3
∑ = kCQq1
r1
+q2
r2
+q3
r3
⎛
⎝⎜⎞
⎠⎟
Contour plots/Topographical maps ■ Contour plots
show the topography of a region by drawing lines to show a constant height above sea level.
■ Which path would you prefer to walk from a parking lot to the top of the mountain? Why?
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A B
Physics 131 11/30/16
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11/30/16 13 Physics 131 https://phet.colorado.edu/en/simulation/charges-and-fields
A 3D representation
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