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.