Chapter 21. Electric Charges
& Electric Field
Electromagnetism (전자기학)
= Electricity (전기) + Magnetism (자기)
+ Electromagnetic wave (전자기파)
=
Light (빛)
Optics (광학)
Electric charge (전하)
Electric field (전기장)
.
.
.
Electric current (전류)
Magnetic field (자기장)
.
.
.
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* 전자기학의 기본 방정식: 4 Maxwell equations
(Faraday)
예: 휴대폰
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 Electric Charge (전하)
Fur
Plastic ruler
Objects can be charged by rubbing.
3
Empirically it was known since ancient times that if amber is
rubbed on cloth, it acquires the property of attracting light
objects such as feathers. This phenomenon was attributed to
a new property of matter called “(electric) charge”. (electron is
the Greek name for amber.) More experiments show that
they are two distinct type of electric charge: Positive &
Negative. The names “positive” and “negative” were given by
Benjamin Franklin.
When we rub a glass rod with silk cloth both objects acquire
electric charge. The sign on the charge on the glass rod is
defined as positive.
In a similar fashion when we rub a plastic rod with fur both
objects acquire electric charge. The sign on the charge on the
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plastic rod is defined as negative.
Plastic
Plastic
Glass
Glass
Charges of the same sign repel
each other. Charges of
opposite sign attract each other.
Two types of (electric) charge:
+ (positive), - (negative)
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How is the atom arranged?
– Visualize a football
stadium as an atom.
Electrons would be
garden peas in the highest
seats with charge of −1.
Protons would be
basketballs or melons with
charge of +1, and
neutrons would reside
about the protons with no
charge. All of the protons
and neutrons could be in a
small basket on the 50yard line.
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Electrically neutral (중성):
3(+) + 3(-) = 0
Nucleus(핵)
p
n
p
n
p
n
proton (양성자): +
neutron (중성자): 0
Atom(원자) =
Nucleus(핵) (small, massive, positive charge)
+ Electron(전자) cloud
(large, very low density, negative charge)
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Nucleus(핵)
p
n
p
n
p
n
proton (양성자): q=+1
neutron (중성자): q=0
Quark(쿼크)  u quark: q=+2/3
d quark: q=-1/3
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“원자”는 전기적으로 중성 (electrically neutral)
문지르면 “전자”가 한 물체에서 다른 물체로 이동하여
“대전”됨 (charged)
-
9
Metal objects can be charged by “conduction (전도)”:
10
“Induction (유도)”:
positive
negative
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“charged (대전)” vs. “induced charge (유도전하)”
Induced
charge
(중성 구리막대)
charged
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Polar molecule (극성분자, 極性分子):
neutral overall, but charge not evenly distributed
 “Induced charge” (유도전하, 誘導電荷)
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* Summary: Electric Charge (전하)
electron (전자),
proton (양성자)
광자 e- + e+
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 Conductors (도체) & Insulators (절연체)
 Conductor:
 Insulator:
Charge flows freely.
Almost no charge flows.
Metals
Most other materials
 Some materials are semiconductors (반도체).
 Superconductor (초전도체): No resistance (저항)
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 The Electric Force  Coulomb’s Law
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: Coulomb’s law
permittivity constant (유전률 상수)
* cf.
: Newton’s law of Universal
Gravitation
(중력법칙, 만유인력의 법칙)
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* Principle of superposition (중첩 원리)
The net force on a charge is the vector sum of
all the forces acting on it.
Example:
Net force on q2
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* Example 21.1 Electric force vs. Gravitational force
수소 원자 속에 있는 양성자와 전자 사이의
“전기력”과 “중력”의 크기 비교
electron (전자)
+
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proton (양성자)
* Solution:
21
* Example 21.3 Vector addition of electric forces on a line
* Solution:
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* Solution:
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* Example 21.4 Vector addition of electric forces in a plane
* Solution:
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