1/25/2008 j.velkovska 1 phys117b: lecture 8 more about electric potential equipotential lines ...
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1/25/2008 J.Velkovska 1
PHYS117B: Lecture 8
More about electric potential Equipotential lines Relation between E and V
Capacitance
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Electric field lines
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Electric potential of a point charge
Potential is = 0 at infinity E goes as 1/r2 … V ~ 1/r ( last lecture we integrated
E to find V)
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Electric field and potential
, ,x y z
V V VE E E
x y z
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Topographic Maps and …pictures
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In Electrostatics, the peaks and valleys are created by charged objects
Density of lines indicates strength of E
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Electric potential inside a || plate capacitor
Uniform field =>
Equipotential lines are equally spaced
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More complex cases
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Calculating E from equipotential map
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Calculating E from V
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Capacitors and capacitance
Two conductors separated by an insulator or vacuum charged to +Q and –Q form a capacitor. Net charge on the capacitor is 0
Once the charge is established: potential difference between the two conductors
Potential difference means – you can store energy on the capacitor
Capacitance: measures the ability of the capacitor to store energy
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Capacitors can have any shape
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Capacitance depends on shape of the conductors and the medium
0 0
0
,
ab
QE
A
QdV Ed
A
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When we put a dielectric between the plates: E –weaker, V – smaller, C - larger
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Capacitors in series
1 21 2
1 21 2 1 2
,
1 1,
ac cb
ab
Q QV V V V
C C
Q Q VV V V
C C Q C C
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Capacitors in parallel