# Electric dipoles

Homework Statement:
The vector distance (d), what sense does it have, from positive to negative ... Or from negative to positive?
Relevant Equations:
Electric dipoles
What is the convention? Because the field lines leave the positive charges and enter the negative ones, do not they?

BvU
Homework Helper
berkeman
Mentor
What is the convention? Because the field lines leave the positive charges and enter the negative ones, do not they?
@Guillem_dlc -- Have a look at the Wikipedia article on electric dipoles linked to by @BvU and let us know if you have any more questions. In the future, it helps if you can provide links to your reading about your question. That will either answer your question outright, or at least it will show us where your confusion is coming from. Thanks.

Field lines, by convention, come out of positive charges and enter into negative ones ... Am I right?

berkeman
Mentor
View attachment 241346
Field lines, by convention, come out of positive charges and enter into negative ones ... Am I right?
Yes, they are in the direction of the force on a positive test particle.

Yes, they are in the direction of the force on a positive test particle.
So why are they upside down in a dipole?

berkeman
Mentor
So why are they upside down in a dipole?
Who is upside down? Put a positive test particle in that dipole E-field, and which way will it go?

Who is upside down? Put a positive test particle in that dipole E-field, and which way will it go?
Towards the negative. Wait, I think I got it!

berkeman
Towards the negative. Wait, I think I got it!
No, I do not understand...

Will it go towards the negative because the arrow points towards the positive?

berkeman
Mentor
No, I do not understand...

Will it go towards the negative because the arrow points towards the positive?
The E-field (and force) vector arrows in the drawing point from the + charge to the - charge, so a positive test charge placed in that field will be repelled by the + charge and attracted to the - charge.

The E-field (and force) vector arrows in the drawing point from the + charge to the - charge, so a positive test charge placed in that field will be repelled by the + charge and attracted to the - charge.
I found this:
An electric dipole consists of two charges, one positive +q and the other negative -q of the same value, separated by a distance d, usually small.
The main characteristic of the electric dipole is the dipole moment, which is defined as the product of the charge q by the distance between the two charges, d, in the direction of the dipole axis and the direction of the negative charge to the positive. This magnitude is vectorial, and it is written:
p = q · d
When placing an electric dipole in an electrostatic field, the dipole tends to orient itself with its positive charge pointing in the direction of the field lines. The action of the field thus creates a moment of rotation M given by the expression:
M = p · E · sin θ

The arrows point from - to +, and the text says: "from negative to positive". Unlike the field lines, that is what confuses me.

berkeman
Mentor
The arrows point from - to +, and the text says: "from negative to positive". Unlike the field lines, that is what confuses me.
That is the Dipole Moment Vector, not the Electric Field Vector. I see now where your confusion came from.

That is the Dipole Moment Vector, not the Electric Field Vector. I see now where your confusion came from.
Clear!! And is it hard to know why it has that sense?