Direction of Electric Field & Field Due to a Dipole

[warhammer]

TL;DR Summary

EF direction- Taken Positive to Negative
But EF due to a Dipole on Axial Point - Along Direction of Dipole Moment i.e. From Negative to Positive. Why so?

Hi all. I am stuck with a seemingly silly doubt all of a sudden.

The direction of Electric Field is taken from Positive to Negative (because Field Lines originate from a Positive Charge and terminate at Negative Charge).

We know that direction of Dipole Moment is from Negative Charge to a Positive Charge. Now when we consider a axial point for this Dipole case, we find that the Electric Field is along the direction of Dipole Moment (i.e. from Negative to Positive Charge).

I'm not sure how this is possible at all. From the already established convention the Electric Field should be opposite to the direction of Dipole Moment, running from Positive Charge to Terminating at Negative Charge.

In addition to this, what happens in case of Polarisation in Dielectrics, when we take the Internal Dipole Field from Positive to Negative only (in such a way that it tries to mitigate the externally applied field and opposite to it).

I need some help in overcoming this misconception. It's giving me a really really hard time!

 

[vanhees71]

Just have a look at the field lines:

https://commons.wikimedia.org/wiki/...ric.svg#/media/File:VFPt_dipoles_electric.svg

Taken from the Wikipedia article on the electric dipole:

https://en.wikipedia.org/wiki/Electric_dipole_moment

 

[Dale]

Summary: EF direction- Taken Positive to Negative
But EF due to a Dipole on Axial Point - Along Direction of Dipole Moment i.e. From Negative to Positive. Why so?

We know that direction of Dipole Moment is from Negative Charge to a Positive Charge. Now when we consider a axial point for this Dipole case, we find that the Electric Field is along the direction of Dipole Moment (i.e. from Negative to Positive Charge).

Take the upper right diagram that @vanhees71 posted as the prototype. All of the field lines go away from the positive charge.

 

[warhammer]

Just have a look at the field lines:

View attachment 312990

https://commons.wikimedia.org/wiki/...ric.svg#/media/File:VFPt_dipoles_electric.svg

Taken from the Wikipedia article on the electric dipole:

https://en.wikipedia.org/wiki/Electric_dipole_moment

So in essence, one should just think about Field Lines as emanating outwards from a positive charge instead of abovementioned conventions as they lead to confusion?

I think this way I can resolve the three mental conundrums.

1. Field is always treated as outwards from a positive charge
2. Since field is outwards from a positive charge, we see that Dipole Moment and Field are indeed in same direction for an Axial Point
3. Positive Charge Centers are swayed along the direction of External EF in a dielectric material. If we treat Charges hugging at the boundary of a rectangular slab, with positive charges along the direction of external field, then for the "insides" of the slab the internal field would again be directed away from the positive charge (which essentially is opposite to the direction of external field)

Is this a cleaner way to look at things?

 

[robphy]

The "physical dipole" (@vanhees71 's upper right diagram), which consists of a pair of equal-magnitude oppositely-sign charges $q$ and $-q$ separated by a fixed displacement $\vec d$,
is a model of the "pure dipole" (@vanhees71 's upper left diagram).

This model is a good model as $|\vec r|\rightarrow \infty$ ("asymptotically")
(or $|\vec d|\rightarrow 0$ and $q\rightarrow\infty$ while keeping $q\vec d$ fixed),
but not as good otherwise.

 

[Mister T]

But EF due to a Dipole on Axial Point - Along Direction of Dipole Moment i.e. From Negative to Positive. Why so?

Can you provide a sketch of the situation. Is your axial point inside or outside the dipole? Is it closer to the negatively charged side of the dipole, or the positively charged side?