# I The direction of electric dipole moment of water molecule

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1. Apr 16, 2017

I was watching a video explaining how microwave ovens work when I found that there is a difference between my physics textbook and online images of the electric dipole moment of the water molecule, as well as the one shown in the video.

View attachment 195065

Why do they differ?

2. Apr 16, 2017

### Staff: Mentor

What is the difference?

3. Apr 16, 2017

### pixel

The direction shown for the dipole moment is different.

4. Apr 16, 2017

### Staff: Mentor

They both show the dipole moment as positive towards the H and negative towards the O. One draws the arrow from positive to negative, but they both are labeled correctly.

5. Apr 16, 2017

### nasu

The direction shown in your book is the usual one, as is derived from the definition of the dipole moment $\vec{p}=\sum q \vec{r}$
where $\vec{r}$ is the position vector of the charge q.
Why they show it the other way in that video is hard to say, especially without watching the video.

6. Apr 17, 2017

Where do you place the origins of your coordinate system for drawing $\vec{r}$ vectors?
Somebody said that the direction of the dipole moment in Physics is reverse than what it is in Chemistry, is it true?
Aren't they vectors? So how can we reverse the direction of a vector and not changing it?

7. Apr 17, 2017

### pixel

That is what the OP is asking about, the differently drawn arrows.

8. Apr 17, 2017

### nasu

It looks like indeed in chemistry they choose to represent the dipole moment vector pointing from positive to negative even though the definition formula looks to be the same as the one used in physics.
See here for example (caption to figure 1.1)
https://chem.libretexts.org/Core/Ph...tomic_and_Molecular_Properties/Dipole_Moments

And it does not matter where you place the origin of the coordinate system. The simplest choice is to place it between the two charges but is just the simplest not the only option.