Polar molecule in electric field

In summary, when an electric field is turned on, a gas dipole molecule will rotate to align with the vector of the field and remain in that position due to thermal fluctuations. This is more likely to occur when the potential energy of the molecule in the field is greater than the thermal energy. If the potential energy is comparable or less than the thermal energy, the molecule may oscillate or remain unpredictable in its orientation. Applying a high enough voltage can change the rotation to pendular motion, which may result in the electric field doing net work on the molecules, potentially increasing their translational energy and affecting temperature and pressure.
  • #1
ael65
9
0
what happens to gas dipole molecule when electric field E is turn on ?
a) is going to rotate to lined up with vector E and stay like that ?
b) is going to oscilate, like pendulum, pointing on average toward E ?
c) none of the above.
d) is every collision going to flip molecule unpredictible, or it maintain it's orientation with respect to E ?

-ael
 
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  • #2
ael65 said:
what happens to gas dipole molecule when electric field E is turn on ?
a) is going to rotate to lined up with vector E and stay like that ?
b) is going to oscilate, like pendulum, pointing on average toward E ?
c) none of the above.
d) is every collision going to flip molecule unpredictible, or it maintain it's orientation with respect to E ?

-ael

since it's a gas you can think of one atom at a time, and there is a means of maintaining equilibrium apparently, so the dipole will line up with the field (there is some "damping" to get rid of mechanics oscillations such as in option "b" above). There should be thermal fluctuations of the dipole away from the field direction tho. Thus answer "a" is fairly correct (moreso than b or d).
 
  • #3
what mechanism is providing a "dumping" ?
Is this EM radiation from oscilating charge ?

-ael
 
  • #4
probably something like that.
 
  • #5
ael65 said:
what happens to gas dipole molecule when electric field E is turn on ?
a) is going to rotate to lined up with vector E and stay like that ?
b) is going to oscilate, like pendulum, pointing on average toward E ?
c) none of the above.
d) is every collision going to flip molecule unpredictible, or it maintain it's orientation with respect to E ?

-ael
Due to collisions, the polarization will be thermalized.
The probability of polarization along the field will be ~exp(p.E/kT).
 
  • #6
Thx Meir,

I already found article on "STARK EFFECTS ON RIGID ROTOR WAVEFUNCTIONS" which explained how probability of molecule orientation will depend on kinetic enrgy, dipole moment and strength of electric field. There are QM states to be considered, but simple classical approximation can be obtained based on:

<max rotation angle> ~= arccos( <1 axis thermal energy ~ 0.5kT> / <dipole moment> * E)

I also tried to find an apprximate strength of the E field to force a molecule of cyanamide (long polar molecule) to limit its rotation to +-15deg. I got something like 150V/um at room temp. This is 50 times more then voltage causing air breakage.
 
  • #7
The deciding factor will be the strength of the electric field .
If the potential energy of the dipole molecule in the electric field is very much grater than the thermal energy of the molecule,then option (a) is your answer.
If the potential energy of the dipole molecule in the electric field is comparable to the thermal energy of the molecule,then option (b) is your answer.
If the potential energy of the dipole molecule in the electric field is less than the thermal energy of the molecule,then option (d-unpredictability) is your answer.

mail me if anyone doesnot stand to this explanation.
 
  • #8
A followup question...

If one applies a sufficiently high voltage to change rotation to pendular motion about the electric field axis, what are the thermodynamic consequences? IE:

1) does the electric field do net work on the molecules?
2) if yes, a) how much and b) does the energy go end up in increased translational energy resulting in higher temperature/pressure?

Thanks
 

1. What is a polar molecule?

A polar molecule is a molecule that has an uneven distribution of charge, with one end having a slightly positive charge and the other end having a slightly negative charge. This is due to the unequal sharing of electrons between atoms in the molecule.

2. How does a polar molecule behave in an electric field?

In an electric field, a polar molecule will experience a force that aligns its dipole moment (the separation of positive and negative charges) with the direction of the field. This can cause the molecule to rotate or move towards the source of the electric field.

3. What factors determine the strength of the electric field on a polar molecule?

The strength of the electric field on a polar molecule depends on the magnitude of the electric field, the dipole moment of the molecule, and the angle between the dipole moment and the direction of the field. The larger the dipole moment and the closer the angle is to 90 degrees, the stronger the force on the molecule will be.

4. How does a polar molecule interact with other molecules in an electric field?

In an electric field, polar molecules can interact with other polar molecules through dipole-dipole interactions. This can cause the molecules to align with each other and form clusters, or to repel each other if their dipole moments are oriented in opposite directions.

5. Can a nonpolar molecule be affected by an electric field?

Yes, although nonpolar molecules do not have a dipole moment, they can still experience a force in an electric field. This is due to the unequal distribution of electrons within the molecule, which can create temporary dipoles that interact with the electric field.

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