Download Ch26 Electric Charges and Forces

講者： 許永昌 老師
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Contents
 Developing a Charge Model
 Charges and Materials
 Charges
 Atoms and Electricity
 Charge Conservation and Charge diagram (P795)
 Insulators and Conductors
 Charging and Discharging
 Charge polarization
 Electric Dipoles
 Coulomb’s law
 The Field Model
 Unit vector notation
 先講電，再講電力，最後講到電場。
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Action I (請預讀P788~P790)
 Purpose:
 Developing a
 Objects:
 2 Plastic taps
 Plastic rods + wool
 Glass rods + silk
 Small metal spheres
 Action:
 (
,

 (
 (





) Quickly pulling a piece of tape  cling to your hand:
New force? Long range force?
 (

(課本P789 ~ P791 的圖很清楚)
, two
of charges) Rub plastic and glass rods with wool and silk.
How can you test whether or not
,
,
?
) Two pieces of tapes repulsive to each other.
and
)
Charge two pith balls by
them with charged plastic and glass rods.
Make sure that
.
They will ___???______.
How to charge a material? (1)….. (2)……
Which one is positive charged? Why?
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The conclusion thus far are:
causes some objects to become charged.
 Charge can be transferred by
.
 A charged object

small
objects.
This is the test for charge.
 There are at least two different kinds of charge.

Like charges
, different charges
.
:
 Suppose there is a third type of charge. What test would reveal that
an object has this third charge?

Charged? Attractive?
 How to make sure that an object is neutral, positive, or negative?
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Action II (請預讀P791~P792)
 Purpose:
 Understand the concept of
and
.
 Objects:
 One metal rod
Charge diagram
 One wood rod
 A small metal ball
 A pith ball
 Action:
 由於不易產生大的靜電，以下的實驗不確定能否展示。
 As this figure shows, check whether the metal ball is charged
or not.

Conclusion: Both insulators and conductors can be charged. They
differ in the
of the charge.
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Charge (請預讀P793~P795)
 Franklin: (簡單定義“正電”)
 A glass rod that has been rubbed with silk is
positively charged.
 Atoms and Electricity:
 An atom consists of a very small and dense
(diameter~10-14 m) surrounded by
much less massive orbiting
.

Electrons and Protons have charges of
opposite sign but exactly equal magnitude.
:
e=1.6010-19 C. (小心，有的書本取負
號。)
 Law of conservation of charge:
 Charge is neither created nor destroyed.
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The Micro/Macro Connection
and
are the basic charges
of ordinary matter.
 Therefore, an object has charge q=Npe + Ne(-e).
 Macroscopic: q
 Neutral: q=0.
: neutral 代表 no charge 嗎？
 Microscopic: Np, Ne, e.
 Neutral atom (remove one electron) positive ion.
 Neutral atom  (accommodate one electron) 
negative ion.
 Plastic rod is charged by rubbing with wool:
 Frictional charging via
works
well with large organic molecules.
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Homework
 Student Workbook
 26.1
 26.4
 26.7
 26.8
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Insulators and Conductors
 Insulators:
 The electrons in the insulator are all tightly
bound to the positive nuclei and
to
move around.
 Conductors:
 There are something highly mobile inside
the conductors. The charges physically
move are called the
.


Metal:
 Charge carriers:
Ionic solutions:
 Charge carriers:
.
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Action III (請預讀P797~P800)
 Purpose:
 Get the feeling about
.
 Actions:
 Polarization:

,
and
----A piece of paper
 Discharging:
 The electric forced exerted by a charged tape will be more and more
weak as time pass.
 If it is contact with you or the earth, it discharges very fast.
 Induction:

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Stop to Think
 How to charge two
metal spheres with
?
 How to charge two
metal spheres with
?
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Homework
 Student Workbook
 26.10
 26.17
 26.19
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Stop to Think
 Plot the force vectors upon these two spheres. (小心，
力怎麼畫關乎到誰當系統，假定以整個球當作一個系
統好了。)
 可利用free-body diagram
與 interaction diagram 來想。
++++
++
 Do this experiment to confirm your thought.
 Different charge numbers.
 Different distances.
+
+
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Coulomb’s law(請預讀P800~P804)
 Coulomb’s Law: (重點在於定出
)
 If two charged particles having charges q1 and q2 are a distance
r apart, the particles exert forces on each other of
F1 on 2  F2 on 1 
k q1 q2
r
2

q1 q2
4 r
2
.
0
where k is called the electrostatic constant.
These forces are
an action/reaction pair, equal in magnitude and opposite in
direction.


Electrostatic constant: k=8.99109 Nm2/C2.
Permittivity constant:  0  1  8.85 10-12 C2 / Nm 2 .
 The forces are
4 k
along the line joining the two
particles. The forces are repulsive for two like charges and
attractive for two opposite charges.
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Using Coulomb’s Law
 仔細看，其實很有限：
 Coulomb’s law applies only to
 Coulomb’s law applies only to
 Electric forces, like other forces, can be
.
.
.
 Exercise: (千萬別亂用屏蔽 “screening” 的概念)
 Draw the individual and net force vectors on the right
one: -2
2
-1
2
1
 Draw the individual and net force vectors on charge B:
+1nC +
-1nC 5cm
(B) +1nC +
5cm
- -2nC
(B) +1nC +
- -2nC
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Homework
 Student Workbook
 26.20
 26.21
 26.22
 26.24
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A Problem of Coulomb’s Law and
Newton’s Law of Gravity
test
 Logical deduction:
 Objects:

One source charge and one test charge.
source
t=0
Fs on t  t  0 
Fs on t  t  t '
t=t’
 Situations:
 1. If both source and test charges are at rest, the force pairs no doubt
obey Coulomb’s Law.
 2. If the source charge suddenly starts moving, as shown by arrow. In
response, the force vector on the test charge must pivot to follow the
source charge.
 Does this happen
? Or is there
?
 Neither Coulomb’s Law nor Newton’s Law of gravity is
dependent on time.  instantly.
 However, How about the case of the distance between these
two particles is about 1,000,000 light years?  some delay?
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The Field Model (請預讀P805~P810)
 Faraday’s Concept:
 The source particle

around it.
(The space around the source particle is altered to
create the
.)
 The test particle then
to the altered
space.

The interaction between the test particle and this
alternation of space is a
rather like
a contact force.
 In
electrodynamics, you will find that
people use
to describe
the EM field generated by the source and the
to describe the interaction
between EM field and the test charge.
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The Electric Field
 Three basic concepts:
1. The field exists
, even though
diagrams may show a few illustrative vectors.
2. The
the field.

(☆)Test charge also creates an electric field. But charges don’t exert
forces on themselves, so the test charge is
.
The field at each point in space is a
3.

E r  
.
Fon qtest at r
qtest
For single particle source:

 1 kqqtest

2
 4
  1 kq

r
1 kq
0
E r   
, away from q   
, away from q  
rˆ .
2
2
qtest
4

r

  4 0 r
direction
0



magnitude



r
Unit vector: rˆ  .
r
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Exercises
 Q1: Plot the electric field created by these two particles.
+
+
 Q2: Which one is the possible electric field?
(a)
(b)
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Homework
 Student Workbook
 26.26
 26.28
 Student Textbook
 58
 自己寫 Terms and Notation.
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