Wednesday, February 6, 2013
LESSON 1:
CHARGE
Announcements
HW #1 due Thursday
Science Saturdays!
 Extra credit in AP Physics B for 2/9
 Possible ORNL internship for participants
 See Dr. Perkins for more info
AP Physics B Course Objectives
III.A.1. Charge and Coulomb’s Law
a) Students should understand the concept of
electric charge, so they can:
(1) Describe the types of charge and the attraction and
repulsion of charges.
(2) Describe polarization and induced charges.
Students will be able to:
1)
2)
Describe how insulating materials can receive net charge
Describe interactions between a charged rod and an
electroscope.
Demonstration #1
1. Demonstrate how you can pick up the circles
of paper without touching it in any way with
your body.
2. What is occurring on the atomic level that lets
you do this?
The atom
 The atom has positive charge in the nucleus, located in
the protons. The positive charge cannot move from the
atom unless there is a nuclear reaction.
 The atom has negative charge in the electron cloud on
the outside of the atom. Electrons can move from atom
to atom without all that much difficulty.
Question
You charge the balloon by rubbing it on hair or
on a sweater, and the balloon becomes
negative. How can it pick up paper circles?
This is a simple electroscope
Pole
The electroscope is made from a metal or
other conductor, and may be contained
within a flask.
The vanes are free to move.
When the vanes have excess positive
or negative charge, they swing
outward
Vanes - uncharged
This is a simple electroscope
Pole
The electroscope is made from a metal or
other conductor, and may be contained
within a flask.
The vanes are free to move.
When the vanes have excess positive
or negative charge, they swing
outward
Vanes - charged
A more complicated version appears in this videoclip.
Demonstration #2
1. Rub the black rod with the
fur. Bring the rod toward the
pole of the electroscope but
do NOT touch.
What happens to the vanes?
2. Come up with an atomiclevel explanation for your
observations.
Demonstration #3
1. Rub the glass rod with the
silk. Bring the rod toward the
pole of the electroscope.
What happens to the vanes?
2. Come up with an atomiclevel explanation for your
observations.
Demonstration #4
1. What happens when your
touch the electroscope with
the glass rod?
Charge
 Charge comes in two forms, which Ben Franklin
designated as positive (+) and negative(–).
 Charge is quantized.
 The smallest possible stable charge is the magnitude
of the charge on 1 electron or 1 proton.
 A proton has charge of e, and an electron has a
charge of –e, where e is referred to as the
elementary or fundamental charge.
 e = 1.602  10-19 coulombs (C), which is the SI unit of
charge.
Conductors v. Insulators
Conductors are materials (mostly metals)
that allow electrons to move freely through
the material.
Insulators are materials that do not allow
electrons to move freely.
Do you think the rods used in today’s
demonstrations are conductors or
insulators?
Sample Problem: A certain static discharge delivers -0.5 Coulombs of
electrical charge. How many electrons are in this discharge?
Sample Problem: The total charge of a system composed of 1800 particles, all
of which are protons or electrons, is 31x10-18 C.
•
How many protons are in the system?
•
How many electrons are in the system?