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Dipole moment (Read 5886 times)
Gerrit-Jan Linker
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Dipole moment
02.04.09 at 11:13:15
 
Dipole moment
 
Molecules having two equal and opposite charges separated by certain distance are said to possess an electric dipole. The centre of negative charge does not coincide with the centre of positive charge. The extent of polarity can be described by the term Dipole moment.
 
Dipole moment can be defined as the product of the magnitude of the charge and the distance of separation between the charges: dipole moment (m) = charge (e) x distance of separation (d).
 
It is expressed in the units of Debye and written as D  
 
The dipole moment is dependent of the choice of origin.
 
The electric dipole of a molecule, μ, can also be seen as the first derivative of the energy with respect to an applied electric field.  
 
It is a measure of the asymmetry in the molecular charge distribution and is defined by the relation
μ = -e integral ρ(x,y,z) r dV + e suma Zara
where,
r is the radius vector of electrons,  
ra is the vector from the origin to the nucleus of atomic number Za,
ρ (x, y, z) is the electron density.  
 
The dipole moment of a complex molecule can be represented approximately in the form of the vectorial sum of the moments belonging to the individual bonds. The dipole moment for an uncharged molecule is independent of origin but it is origin dependent for an ion.
 
The direction of a dipole moment is towards the positive charge.
 
Source:
GLOSSARY OF TERMS USED IN THEORETICAL ORGANIC CHEMISTRY
http://old.iupac.org/publications/pac/1999/71_10_pdf/7110mirkin_1919.pdf
Dipole Moment - A Measure of Degree of Polarity
http://www.tutorvista.com/content/chemistry/chemistry-iii/chemical-bonding/degre e-polarity.php
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« Last Edit: 17.12.10 at 08:30:34 by Gerrit-Jan Linker »  

Gerrit-Jan Linker
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Energy of an Electric Dipole
Reply #1 - 25.11.10 at 21:38:00
 
Energy of an Electric Dipole
 
U = EpcosΘ
 
where
U = potential energy
E = |E| = Electric field
p = |p| = dipole
Θ = angle between E and p  
 
Source:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/diptor.html#c2
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Gerrit-Jan Linker
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Dipole moment - dependence on the origen
Reply #2 - 16.03.12 at 07:14:01
 
Dipole moment - dependence on the origen
 
Dipole moments are dependent on the choice of the origin when the system has an overall charge. When a system is neutral the dipole moment is not affected when another origin is chosen.
 
Examples with discrete charges.
 
2 charges seperated at a distance 2:
 
charge q1=-1 at r1=(x,y)=(-1,0)
charge q2=+1 at r2=(x,y)=(1,0)
Dipole moment m = q1*r1 + q2*r2 = -1*(-1,0)+ 1*(1,0) = (2,0)
 
Now, shift the origin by a translation vector (2,0):
charge q1=-1 at r1=(x,y)=(1,0)
charge q2=+1 at r2=(x,y)=(3,0)
Dipole moment m = q1*r1 + q2*r2 = -1*(1,0)+ 1*(3,0) = (2,0)
 
Now, shift the origin again by a translation vector (0,2):
charge q1=-1 at r1=(x,y)=(1,2)
charge q2=+1 at r2=(x,y)=(3,2)
Dipole moment m = q1*r1 + q2*r2 = -1*(1,2)+ 1*(3,2) = (2,0)
 
Clearly the dipole moment of a neutral system is not affected by the choice of the origin.
 
When the system is not neutral overall this does not hold.
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« Last Edit: 16.03.12 at 07:18:54 by Gerrit-Jan Linker »  

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