03-09notes
March 09, 2011
THE PROBLEM OF
ACTION AT A DISTANCE
"... hypothesis non fingo..."
Newton only gave the
mathematical
relationship. He made no
guess about what the
mechanism was or why it
worked. That's the
difference between a law
& a theory
But the fact that gravity acted at a distance (Earth can pull you
without touching you) bothered him.
03-09notes
March 09, 2011
CHARGE BASICS
Likes repel; opposites attract
(unlike gravity which only ever attracts)
03-09notes
March 09, 2011
WAYS OF CHARGING
Most items are neutral - equal numbers of protons & electrons.
When charging it is usually the electrons that do the moving
+ means lost electrons
- means gained electrons
1. Charging by Friction.
Some materials tend to take electrons, others tend to
give them up. (Rub a balloon on your hair)
This leads to equal & opposite charges on each item.
2. Charging by Contact
Bring a charged object into contact with a neutral
object, charge will be shared.
"Grounding" means sharing your charge with a much
larger object (like Earth) - none will be left for you.
3. Mysterious Third Way
Bring a charged object NEAR a neutral object; ground
the neutral object. It will get the opposite charge.
03-09notes
March 09, 2011
Charles-Augustin de Coulomb
Measured the force between two
charges with a setup similar to
Cavendish's from 10 years earlier.
It's also an inverse-square law and it's much stronger than gravity.
You don't notice it as much because most objects are neutral.
03-09notes
March 09, 2011
03-09notes
March 09, 2011
POLARIZATION
Bring a charged object near (but not touching) a neutral
object, and some of its electrons will move to one side.
+- +
+ - -+
+- - + - + -+
- -+ - +
+ -+ -
-
-
-
+
+
+
+
- +
- ++ - + - -+
+
-+ --
+
+ ++
++
+
+
- -- -
- - +- + - - + - -+ +-+ --
+
Object is neutral.
Object is still neutral, but
one side is now more
positive and the other
side is more negative