PHYSICS 112 Dr. Anatoli Frishman [email protected].
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Transcript of PHYSICS 112 Dr. Anatoli Frishman [email protected].
Physics
Mechanics Thermal properties Electromagnetism Optics Atoms & particles
•Electrostatic•Electric current•Magnetism
•Geometrical optics•Wave optics
Condensed Mater High energy Biophysics
Classical physics Quantum physics
Relativistic physics Quantum relativistic physics
Electromagnetism(electric and magnetic phenomena)
1. Electric charge 1a. Qualitative description
I. Electrostatics
This is about: electric charges, electromagnetic forces, and electromagnetic fields
This is about: non-moving electric charges, electrostatic forces, and electrostatic fields. (With very good approximation, electrostatics could be applied to slow moving charges. Slow means that the speed of the considered charge is much smaller then speed of light: v<<c)
•There are two and only two types of electric charges•Charges of the same type repel, and charges of different type attract (this is already qualitative description of electrostatic forces)•These two types are referred to as positive and negative (+q and –q)•Charges can be treated algebraically, and the net amount of electric charges in an isolated system is conserved (is not changed in any process). This is the law of conservation of electric charges
1c. Electric properties of different materials (very brief, qualitative description)
•Microscopic picture (atom)•Insulators•Conductors•Semiconductors•Superconductors
1d. “Games” with electric charges
•Charging by contact•Charging by induction•Electroscope•Electrometer
1b. Elementary charge:
Ce 19106.1
Positive
Negative
Neutral
disk
Gold leaves (or vane)
The electroscope
Positive
Negative
Neutral
repulsion
Charged rod
Inducted charge
Inducted charge
Positive
Negative
Neutral
Stronger repulsion
Charged rod (closer)
Positive
Negative
Neutral
No repulsion
If we ground the electroscope while the rod is there, the charges in the electroscope that were “escaping” from the rod flow to the ground.
Positive
Negative
Neutral
No repulsion
Then we cut the grounding…
Positive
Negative
Neutral
Repulsion
And remove the rod…
The electroscope is now charged.The charge spreads now all over the object.
Electroscope charged by induction
2. Electrostatic forces (Coulomb’s law)
221
r
QQkF
Q1 Q2r
constantty permittivi /1099.84
10
229
0
CNmk
Units:
AI
sACQ
1][
111][
tIQ
t
QI
Example:
?
10.0
100.3
100.26
2
61
F
mr
CQ
CQ
NF
m
CCCNmF
4.5
1.0
100.3100.2/100.9 2
66
229
r
r
r
QQkFF 12
221
2112
Q1 Q2r
Q1 and Q2 have the same sign Q1Q2 >0
21F
12F Q1 Q2r 21F
12F
Coulomb’s law in vector form:
Principle of superposition: ...21 FFFnet
Q1 and Q2 have opposite signs Q1Q2 <0
N
CC
m
CCNm
r
Qk
r
QQk
r
QQkF
9.04
100.12100.2
1.0
100.1/100.9
42
66
2
6
229
322
1231
2
211
Example:Q1 Q3r
13F
12F
Q2 r
13121
13121
FFF
FFF
?
10.0
0.12
0.2
0.1
1
3
2
1
F
mr
CQ
CQ
CQ
NNNFFF
Nm
CCCNm
r
QQkF
Nm
CCCNm
r
QQkF
9.07.28.1
7.21.02
100.12100.1/100.9
2
8.11.0
100.2100.1/100.9
13121
2
66
2292
3113
2
66
2292
2112
Example:
Q
Q Q
Q
a
321
2
2
3
2
2
21
2
FFFF
a
QkF
a
QkFF
tot
1F
2F
3F
Nm
CCNmF
a
Qk
a
Qk
a
QkFFF
tot
tot
7.110.0
101.0 /1099.8
2
12
2
12
222
2
26-229
2
2
2
2
2
2
31
?
10.0
0.1
F
ma
CQ
Example: Compare the gravitational attraction and the electric repulsion of two electrons
19
31
1.6 10 C
9.1 10 kg
e
m
2
2
r
ekFE
2
2
r
mGFG
422312211
219229
2
2
102.4101.9107.6
106.1100.9
kgkgNm
CCNm
Gm
ke
F
F
G
E Big!
Example: One of your friends can resist a force of 100 lb (450 N) with his arms apart. You give him two charged balls with charges Q and Q to hold on each hand. How large a charge Q can he hold outstretched?
Q -Q
r ~ 1.5 m
2
2
r
QkF
24
max max 3.35 10 Ce
rQ F
k
-314 -15
-19
9.1 10 kg1 electron3.35 10 C 2 10 kg
1.6 10 C 1 electron
Less than a cell in your body!
