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Action at a Distance
Physics 100
Chapt 12
+ -
Coulomb’s law
q1
M1
q2
M2r
Fc = kq1q2
r2
FN = Gmemp
r2
Newton’s law of Gravitation
All possible
distances!
Moon-earth
FN=GMMME
r2
How does the Moonknow that the Earth
is where it is (& what ME is)?
Electroscope
How do the electronsin the foil know thatsomeone is moving a charged object up here?
& that it’s negativelycharged?
& how much it ischarged by?
Ball on an empty trampoline surface
The ball feelsno net force
Ball on a “loaded” trampoline surface
The ball is “attracted” to the girl’s force
Luminiferous aether & E-Fields
+
Faraday’s method for computingE fields
+
Imaginary small positive “test” charge qtest
Ftest=coulomb force on test charge
Ftest
E = Ftest/qtestx+
Example
+ +
E-field from 2 plus charges
E-Field is the dir of force on + charge
+ -
E-Field for opposite charges
Better picture
+
-
RulesE-field lines start on + charges and end on - charges
Direction of E-field lines = direction of force on a + charge placed at that point (opposite for – charge)
Where E-field lines are close together, the E field is large; where they are far apart, the E-field is small.