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Transcript
講者: 許永昌 老師
1
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。
2
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?
3
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?
4
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.
5
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.
6
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.
7
Homework
 Student Workbook
 26.1
 26.4
 26.7
 26.8
8
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:
.
9
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:

10
Stop to Think
 How to charge two
metal spheres with
?
 How to charge two
metal spheres with
?
11
Homework
 Student Workbook
 26.10
 26.17
 26.19
12
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.
+
+
13
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.
14
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
15
Homework
 Student Workbook
 26.20
 26.21
 26.22
 26.24
16
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?
17
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.
18
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
19
Exercises
 Q1: Plot the electric field created by these two particles.
+
+
 Q2: Which one is the possible electric field?
(a)
(b)
20
Homework
 Student Workbook
 26.26
 26.28
 Student Textbook
 58
 自己寫 Terms and Notation.
21