TBR: Dipole Moment Definition

[justadream]

TBR defines a dipole moment (p) as: p =qL

Where q is the “product of the charge” and L is the distance separating the charges



TBR doesn’t really explain intuitively what a dipole moment is. Can someone explain what it is and why it INCREASES with distance? I thought that electrostatic forces (if that is related to what a dipole moment exactly is) tend to DECREASE with distance.

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[Cawolf]

A dipole is pair of particles separated by a distance L with charges of equal and opposite magnitude abs(q).

The dipole moment p = qL can be used to calculate the torque a dipole experiences in an electric field.

t = p x E

The dipole moment just quantifies the magnitude and direction of the dipole. It isn't a measure of force - it describes the polarity (direction and magnitude).

 

[justadream]

@Cawolf

Why do they define "q" as the product of charges?

Does that mean like p = Q1 * Q2 * L?

Also, how do you know when a dipole is oriented "parallel" vs. "antiparallel" to the field?
TBR Physics Book II page 129 shows that if the field is pointing left to right, "parallel" arrangement would be " + -"

How do you know it would not be "- +" instead?

 

[V_K]

So a dipole moment basically expresses the magnitude of a molecules polarity, and a polar molecule will have a separation of charge...partial positive/partial negative.
The simplest example is water, the two H--O bonds have a partial negative charge on the oxygen, and a partial positive charge on the hydrogen. Oxygen is significantly more electronegative than the hydrogens, which have an electronegativity of about 2.1, and oxygen has one of about 3.5 (it is useful to know that across the periodic table from C to F, the electronegativities are 2.5 to 4.0, increasing by about 0.5, only for that particular row and set of 4 elements)

Thus, oxygen being more electronegative, strongly draws the electrons from the hydrogens, and because it has a bent structure (unlike CO2 which is linear), the two bonds do not "cancel out", and have a net dipole towards the oxygen atom. That being said, the dipole moment being charge x distance, water has one. As the distance between the partial positive and partial negative atoms increases, the dipole moment will increase because the relationship is a direct one (on opposite sides of the equal sign in the equation, if that helps)...
So take for example an amino such as glycine, which at neutral pH is a zwitterion and has both a positive and negative end, and because oxygen is more electronegative than nitrogen, the electrons will be drawn towards the carboxylate group, and because the electrons charges are separated by the length of the molecule, that accounts for the large dipole of glycine. If you take into account a larger distance, the dipole will be larger.

Electrostatic forces have to do with the charges on a molecule, if the charges are alike, the force would be repulsive, different charges would be attractive. However this is a force, and the dipole moment is the magnitude of the distribution of charge across the molecule, and not the interaction of the charges.

 

[Cawolf]

I am pretty sure it is just the magnitude of the charge. I am not sure what they mean by that. I always use p = qL.

It is by convention that the dipole moment is a vector that points from the positive to the negative.

So if the field points from left to right, then if the dipole is parallel with +-, it is sitting with it's dipole moment aligned with the direction of the field (which points to the negative).

 

[justadream]

@Cawolf

With regard to the orientation, if a field points from left to right, that means (assuming 2 charges are the source of the field) that:

On the left side you have a positive source charge. On the right side you have a negative source charge.

So if the dipole is "+ -" then it's parallel because it's orientation matches that of the 2 source charges?

 

[Cawolf]

Technically you are correct, but you made it a little more complicated.

By convention, a single charged particle creates an electric field that points away from a positive charge or towards a negative charge.

The dipole is parallel when it's dipole moment has the same direction as the field.

Again, you are right - but you only need one charge to create an electric field.

 

[justadream]

@Cawolf

Thanks! I wish TBR had explained that better.