22-1 electromagnetism the new science of electromagnetism(the combination of electrical and magnetic...
TRANSCRIPT
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22-1 Electromagnetism
The new science of electromagnetism(the combination Of electrical and magnetic phenomena) wasdeveloped.
22 Electric Charge
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1 Electric Charge is an intrinsic characteristic of the fundamental particles making up those objects.
2 Two kinds of charge: positive charge and negative charge. The object contains equal amounts of these two kinds of charge is said to be electrically neutral.
22-2 Electric Charge
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Charge with the same electrical sign repel eachother, and chargeswith opposite electrical signs attract each other.
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Such as metals,tap water,andhuman body,some of the negative charge can move rather freely.
Conductors:
Insulators: Such as glass,chemically pure water ,and plastic,noneof the charge can move freely.
22-3 Conductors and Insulators
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Semiconductor: Such as silicon and germanium,are material that are intermediate between conductors and Insulators.
Superconductors: They present no resistance to the movement of electric charge through them.
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atom
nucleus
negative charge electrons
positively charge proton
electrically neutralneutron
The properties of conductors and insulates are due to the structure and electrical nature atoms.
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CHECKPOINTCHECKPOINT
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How can we knowthe electrostatic force between small electric chargeq1 and q2 at rest andseparated by adistance r at this 3situations?
22-4 Coulomb’s Law
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221 ||||
r
qqkF
221 ||||
r
qqkF
K is a constant.For practical reasons,theSI unit of charge is derived from the SI unit of electric current,the ampere(A).
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The SI unit of charge is the coulomb (C). It is defined in terms of the unit of the unit of current ,the ampere (A),as the charge passing a particular point in 1 second when there is a current of 1 ampere at that point.
The Coulomb and Ampere
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For historical reasons (and because doingso simplifies many other formulas),the electrostatic constant k of Eq.22-1 is usually written 1/4πε0.The Coulomb’slaw becomes
221
0
||||
4
1
r
qqF
πε
221
0
||||
4
1
r
qqF
πε
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Coulomb’s law describes the electrostatic force between small (point) electric charge q1 and q2 atrest (or nearly at rest) and separated by a distance r
221
0
||||
4
1
r
qqF
πε
221
0
||||
4
1
r
qqF
πε
Here:ε0=8.85×10 - 12C2/Nm2 1/4 πε0=k=8.99×109Nm2/C2
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The force of attraction or repulsion between point charges at rest acts along the line joining the two charges.
How about the situation if there are more then two charges are present?
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A shell of uniform charge attracts or repels a repels a charged particle that is outside the shell as if all the shell’s charge were concentrated at its center.
The two shell theorems for electrostatics are:
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If a charged particle is located inside a shell of uniform charge,there is no net electrostatic force on the particle from the shell.
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Sample Problem 22-1
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N
m
CC
cmN
R
qqF
24
2
1919
229
221
012
1015.1
)0200.0(
)1020.3)(1060.1(
)/1099.8(
||||
4
1
N
m
CC
cmN
R
qqF
24
2
1919
229
221
012
1015.1
)0200.0(
)1020.3)(1060.1(
)/1099.8(
||||
4
1
(a)
Step one:Step one:
Step two:Step two: iNF ˆ1015.1 2412
iNF ˆ1015.1 2412
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(b)
Step one:Step one:
Step two:Step two:
N
m
CC
CmN
R
qqF
24
22
1919
229
231
013
1005.2
0200.043
1020.31060.1
/1099.8
434
1
N
m
CC
CmN
R
qqF
24
22
1919
229
231
013
1005.2
0200.043
1020.31060.1
/1099.8
434
1
iNF ˆ1005.2 2413
iNF ˆ1005.2 2413
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iN
iNiN
FFF net
ˆ1000.9
ˆ1005.2ˆ1015.125
2424
1312,1
iN
iNiN
FFF net
ˆ1000.9
ˆ1005.2ˆ1015.125
2424
1312,1
9.00×10 - 25 and 00
Second key idea
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NR
qqF 24
2
41
014 1005.2
)43
(
||||
4
1 N
R
qqF 24
2
41
014 1005.2
)43
(
||||
4
1
1412,1 FFF net
1412,1 FFF net
(c)
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Method 1
Step one:Step one:
Step two:Step two:
Step three:Step three:
jFiFF ˆsinˆcos 141414
jFiFF ˆsinˆcos 141414
jNiNF ˆ10775.1ˆ10025.1 242414
jNiNF ˆ10775.1ˆ10025.1 242414
jNiN
jNiN
iN
FFF net
ˆ1078.1ˆ1025.1
ˆ10775.1ˆ10025.1
ˆ1015.1
2425
2424
24
1412,1
jNiN
jNiN
iN
FFF net
ˆ1078.1ˆ1025.1
ˆ10775.1ˆ10025.1
ˆ1015.1
2425
2424
24
1412,1
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Method 2
Step one:Step one:
Step two:Step two:
N
NN
FFFFF xxxnet
25
02424
01412,14,12,,1
1025.1
60cos1005.21015.1
60cos
N
NN
FFFFF xxxnet
25
02424
01412,14,12,,1
1025.1
60cos1005.21015.1
60cos
N
N
FFFF yyynet
24
024
014,14,12,,1
1078.1
60sin1005.2
60sin0
N
N
FFFF yyynet
24
024
014,14,12,,1
1078.1
60sin1005.2
60sin0
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Step three:Step three:
Step four:Step four:
N
FFF ynetxnetnet
24
2,,1
2,,1,1
1078.1
0
,,1
,,11 0.86tan
xnet
ynet
F
F 0
,,1
,,11 0.86tan
xnet
ynet
F
F
000 0.941800.86 000 0.941800.86
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“electrical fluid” is also not continuous but is made up of multiples of a certain elementary charge. Any positive or negative charge q that can be detected can be written as and the elementary charge has the value:
q=n e, n=±1,±2,±3,…,
e=1.60×10-19 C.
22-5 Charge Is Quantized
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Particle Symbol Charge
Electron e or e- -e
Proton P +e
Neutron n 0
When a physical quantity such as charge can have only discrete values rather then any value, we say that the quantity is quantized.
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Sample Problem 22-4Sample Problem 22-4
(a)
N
m
CCmN
r
eF
14
100.4
1060.1/1099.8
4
1
215
219229
2
2
0
N
m
CCmN
r
eF
14
100.4
1060.1/1099.8
4
1
215
219229
2
2
0
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(b)
N
m
kgkgmN
r
mGF p
35
215
2272211
2
2
102.1
100.4
1067.1/1067.6
N
m
kgkgmN
r
mGF p
35
215
2272211
2
2
102.1
100.4
1067.1/1067.6
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Initially sphere A has a charge of –50e and sphere B has a chargeof +20e.The spheres are made ofconducting and are identical in size.If the spheres then touch,what is theresulting charge on sphere A?
QuestionQuestion
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This hypothesis of conservation of charge has stood up under close examination, both for large-scale charge bodies and for atoms, nuclei, and elementary particles.
22-6 Charge is conserved