# Are atoms really neutral?

1. Jan 30, 2014

### CURIE WILLEY

What is electric charge?
Electric charge is the physical property of matter that causes it to experience a force when close to other electrically charged matter.

So, if the particle has electric charge, there is an electric field around it.

What is a Neutral particle?
In physics, a neutral particle is a particle with no electric charge.

So, a particle will be neutral, if it has no electric charge. In other words, a particle will be neutral, if there is no electric field around it.

If we consider a system of oppositely charged particles as a dipole. Magnitude of electric field (E) due to an electric dipole at a distance r from its center in a direction making an angle $$\theta$$ with the dipole is given by the equation,$$E=\frac{1}{4\pi\epsilon}.\frac{p\sqrt{3\cos^2\theta+1}}{r^3}$$
where, p=2aq (2a is the distance of separation of the charges q).

From the above equation it follows that, electric field around the dipole will be zero if and only if the distance between the two oppositely charged particles is zero.

So, a system of two opposite charges will have no electric field around it, if the the distance between opposite charges is zero. In other words a system of two opposite charges will be neutral if and only if, the distance between the opposite charges is zero.

If we consider hydrogen atom for example, we can consider electron and proton separated by a distance as a dipole. From my above view it follows that atom is not neutral, because there is a separation between the electron and proton. But, always I have heard atom to be neutral. Why?

2. Jan 30, 2014

### Staff: Mentor

I would qualify that by adding "net": a neutral particle is a particle with no net electric charge.

Atoms can be polarized. Dipolar interactions are the main reason why gases of atoms can hold together. But they are neutral.

Also, I would like to point out that in your analysis, you are treating the nucleus and electrons as classical particles having defined positions. In the quantum mechanical description, the electron in the hydrogen atom is delocalized in a spherical orbital around the nucleus. Therefore, the expectation value of the position of the electron, $\langle x \rangle$, is exactly the same as that of the nucleus.

3. Jan 30, 2014

### CURIE WILLEY

Thank you for the reply, sir. Yes, I agree that atom is not classically viewed these days. What about bohr model? It was also considered as nuetral. Isn't it? Then, we can say that bohr model was not able to explain atom's neutral behavior. Correct me, if I am wrong sir.

4. Jan 30, 2014

### Staff: Mentor

I think you are reading too much into the definition of "neutral". An atom or molecule is neutral if its net charge is zero, not because it has no field.

5. Jan 30, 2014

### AlephZero

An atom is not a "particle". Even a non-quantum model of an atom consists of protons, neutrons, and electrons. A "particle" is an idealized model of something with no physical size, which is why they are sometimes called "point particles". Of course even protons neutrons and electrons are not "really" point particles, but they are a closer approximation to point particles than a complete atom.

As you correctly said, a collection of particles at different points in space can have a non-zero electric field even if the total charge on the particles is zero. That fact has nothing particularly to do with atoms or quantum mechanics.

6. Jan 30, 2014

### CURIE WILLEY

@DOC AL. I agreed sir. An atom is neutral if its net charge is zero. Won't net charge being zero mean no field around that atom?
Here in the atom, there is a field around it. So, I feel net charge is not zero. It will be zero if and only if, distance between proton and electron is zero.

7. Jan 30, 2014

### Staff: Mentor

Right, by definition.

No.

Well, you are wrong.

Read AlephZero's post: An atom is not a particle.

8. Jan 30, 2014

### CURIE WILLEY

@AlephZero. I agreed sir, an atom is not a particle. That is the reason why we have used the term net charge. I didn't say anywhere that, if total charge is zero, there will be electric field around it. We can say total charge as zero, if and only if there is no electric field exists around it. Correct me, if I am wrong sir.

9. Jan 30, 2014

### CURIE WILLEY

@DOC AL. I think net charge being zero, mean no field around it. You disagreed it, sir. It would be appreciable if you provide reason.

10. Jan 30, 2014

### Staff: Mentor

That's obviously wrong by your own example of a dipole field. A dipole has a total charge of zero, yet it most definitely has a field.

Net charge being zero simply means that the total charge (just add 'em up) is zero: $\Sigma q_i = 0$

11. Jan 30, 2014

### CURIE WILLEY

@DOC AL. I agreed sir. Net charge being zero simply means that the total charge is zero. I think, if the system has net charge zero, it has no field around it. I replied the same before, but I didn't get satisfactory reply. So, I am replying once again. You have disagreed my view that there will be no field around a system with net charge zero. Please provide reason, Sir.

12. Jan 30, 2014

### Staff: Mentor

No, you provide a reason why you think that having a net charge of zero implies no field around it. Please explain why you think a dipole has no field around it.

13. Jan 30, 2014

### CURIE WILLEY

@DOC AL. Thank you for the reply Sir. If we have a system with no net charge, it can't experience any force when kept near other charged matter. The reason for not experiencing the force should be because, it has no field around it. This is the reason, why I think that having net charge zero mean no field around it. Please at least now provide reason why you disagree this.

Sorry Sir, I never said dipole has no field around it (If I have pardon me). I would like to say that, dipole has no field around it, if the distance between the charges is zero. I have provided mathematical proof for this at the begining.

14. Jan 30, 2014

### sophiecentaur

This thread has gone in a complete circle, it seems. Being neutral does not imply 'no field'. Is there anything more to say about it? There is no paradox or contradiction - you just need to read the definitions carefully.

15. Jan 30, 2014

### Staff: Mentor

That's not true. A dipole has no net charge yet it can certainly experience forces due to other charges.

See above.

But do you agree that a dipole has zero net charge?

16. Jan 30, 2014

### CURIE WILLEY

@DOC AL. Definition of charge says, Electric charge is the physical property of matter that causes it to experience a force when close to other electrically charged matter. If you say a system has no net charge, how can it experience force. I say this because you are saying dipole experiences force even if net charge is zero. Your statement contradicts.

No, Sir. I don't agree that dipole has zero net charge.

Last edited: Jan 30, 2014
17. Jan 30, 2014

### ZapperZ

Staff Emeritus
Check out Page 9 if this presentation:

http://www.physics.rutgers.edu/ugrad/227/L2 Electric Field, dipoles.pdf

I will also remind you (in case you didn't see it), that this is a COMMON topic in undergraduate E&M course, and it is covered in texts such as Griffith's E&M.

Zz.

18. Jan 30, 2014

### CURIE WILLEY

Oops I have edited the previous reply before seeing your reply Sir. It's ok, its there in your reply. Would you please see the edited one.

19. Jan 30, 2014

### ZapperZ

Staff Emeritus
You really need to use the QUOTE function. It's confusing to know who you are referring or answering to.

Why do you not agree that a dipole has no net charge? An electric dipole, BY DEFINITION, has one positive and one negative charge, of equal magnitude. So under what rule of algebra do you consider that to not have a net charge?

Zz.

20. Jan 30, 2014

### CURIE WILLEY

@ZapperZ. Thank you for your precious time, Sir. I saw the link which you provided, I think I failed to see how it answers my question. I think, COMMON topics are important than the topics built from them. These topics may be common to you, sir. Please clarify my doubt here. Where am I going wrong? If you say it. I will try to understand my mistake.