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We can define a vector called the dipole moment ( 电矩 ). NOTE : This p has nothing to do with momentum!. +q. -q. s. Magnitude: Direction : from the negative (-) charge to the positive (+) charge. http://phet.colorado.edu/en/simulation/balloons. - PowerPoint PPT Presentation
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-q +q
s
p
We can define a vector called the dipole moment (电矩 ).
Magnitude:
Direction: from the negative (-) charge to the positive (+) charge.
qsp
NOTE: This p has nothing to do with momentum!
304
1
r
pE
30
axis
2
4
1
r
pE
p
http://phet.colorado.edu/en/simulation/balloons
http://phet.colorado.edu/en/simulation/balloons
Why does the charged balloon stick to the neutral wall?
http://phet.colorado.edu/en/simulation/balloons
Neutral atom
+-
The atom becomes polarized.
++-
The atom becomes polarized.
++-
We can approximate the polarized atom as a dipole.
+ +-chargeE
p
chargeEp
This is the polarizability, which is different for every material.
The dipole makes an electric field at the charge.
+ +-chargeE
p
chargeEp
dipoleE
What is the force on the charge?
+ +-chargeE
p
dipoleE
rr
qE ˆ
4
12
0charge
+ +-chargeE
p
dipoleE
rr
qE ˆ
4
12
0charge
3
charge
03
0dipole
2
4
12
4
1
r
E
r
pE
Substitute…
+ +-chargeE
p
dipoleE
rr
qE ˆ
4
12
0charge
rr
qE ˆ
2
4
15
2
0dipole
Substitute…
+ +-chargeE
p
dipoleE
rr
qEqF ˆ
2
4
15
22
0dipolecharge
The charge is attracted to the polarized atom, no matter if the charge is positive or negative!
Wall
Balloon------
--
+ -
+ -
+ -
+ -
+ -
+ -
+ -
+ -
+ -
+ -
(1) The charges on the balloon polarize the wall.
Wall
Balloon------
--
+ -
+ -
+ -
+ -
+ -
+ -
+ -
+ -
+ -
+ -
(2) The polarized atoms in the wall attract the balloon.
The wall and the balloon are both insulators (绝缘材料 , or 电价值 ).
In an insulator, the charges are not free to move around. Atoms can be polarized, but they do not change their position.
A material in which charges are free to move around is called a conductor (导体 ).
Example: Salt water (ionic solution 离子溶液 )
At equilibrium, the net field in a conductor must be zero.
The ions move with a speed v = uE, where u is called the mobility.
Example: A metalIn a metal, one or two electrons from every atom is free to move around. They form a sea of electrons.
appliedE
onpolarizatiE
The metal becomes polarized.
Example: A metalIn a metal, one or two electrons from every atom is free to move around. They form a sea of electrons.
Net field inside is zero.
The metal becomes polarized.
We will prove these later, using Gauss’ Law
Permanent dipoles in an electric field
+
-
Water is a permanent dipole
OH
H
E+-
p
F
F
In a uniform field, the net force on a dipole is zero:
0 FF
E+-
p
F
F
But the net torque around the dipole’s COM is not zero:
Ep
E+
- p
F
F
This torque will rotate the dipole until it is parallel with the field.
E+
- p
F
F
The dipole-field system has a potential energy:
EpU
dipole
Water
Balloon------
--
+-
+-
+-
+-
+-
+-
+-
+ -
+-
+-
Water
Balloon------
--
+ -
+ -
+ -
+ -
+ -
+ -+
-
+ -
+ -
+ -
Water is polarized by rotating the molecules.
