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Transcript

Philosophically:

Counter Example: You are talking to your romantic interest having a great interaction and your parental unit walks up and joins the conversation. You would be an unusual person if the character of your interaction did not change. Why are charges different? ...gravity too.

Why is the specific character of the interaction between a pair of charges (Coulomb's Force) NOT affected by the presence of other nearby charges?

Superposition

If there are more than two charges the core question is how to determine the force on a given charge.

makes the "problem" into a vector problem which brings all of that vector stuff (good and bad) along for a ride.

Coulomb's Law

experimentally it can be demonstrated that the force on a given charge is the vector sum of the forces between each pair of charges -- it can get complicated fast:)

charge is expressed in Coulombs to get Force in Newtons

charge on Object 1

charge on Object 2

usually the magnitude of the charge unless you are using vector form of equation OR are very careful about interpreting the resulting sign of force

q q

1

2

________

F

= k

purpose is to scale the charges and distances to get the observed forces

2

c

scaling constant

r

k = 9*E9 Nm2/C2

most common calculational error is to forget to square the distance

k = 8.99*E9 Nm2/C2

force blows up (mathematically) when r goes to zero

1

since the separation between the charges is in the denominator this means that the force changes rapidly with changes in r

___

k =

= 8.85*E-12

0

4

0

12

the distance between the charges measured in meters (m) if the force is desired in Newtons

from charge 1 on charge 2

Newton's 3rd Law

Newton's 2nd Law

the magnitude of the force on charge 2 from charge 1 is the same as the

magnitude of the force on charge 1 from charge 2 -- since all physical contact is, at its root, a coulomb force from one atom on another then Newton's 3rd Law is a consequence of Coulomb forces.

Coulomb forces are just another non- contact force (like gravity) to put on the freebody diagram you are anxious to do for your physics problems:)

Action at a distance

This is a fundamental philosophic problem. HOW do the charges "know" to feel the force from each other? How long does it take for one charge to notice that the other has moved....is it instantaneous? Same problem exists for the gravitational force and any other non-contact forces.......hmmmm

Basic Charge Ideas

http://prezi.com/2ppfda7yld_e/basic-charge-ideas/

Particularly the "3rd grade" rule!

Concepts and Skills from Newtonian Mechanics

the concepts and skills from first term physics are presumed to be relatively close to the surface of your mind including vector skills, kinematics, graphing and plotting interpretations, freebody diagrams, energy tools and concepts, momentum, torque, center of mass, etc etc etc......ain't it great!

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