What is the net charge and why does it matter?

[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?

 

[DrClaude]

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

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.

 

[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.

 

[Doc Al]

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.

 

[AlephZero]

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

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.

 

[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.

 

[Doc Al]

@DOC AL. I agreed sir. An atom is neutral if its net charge is zero.

Right, by definition.

Won't net charge being zero mean no field around that atom?

No.

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.

Well, you are wrong.

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

 

[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.

 

[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.

 

[Doc Al]

@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.

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.

@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.

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

 

[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.

 

[Doc Al]

@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.

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.

 

[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.

 

[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.

 

[Doc Al]

@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.

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

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.

See above.

Sorry Sir, I never said dipole has no field around it (If I have pardon me).

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

 

[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.

 

[ZapperZ]

@DOC AL. Would you please provide examples where dipole can experience force without having net charge. I think you are saying, there will be field around a system with zero net charge. I have given my reason why I say there should be no field around the system with no net charge. Even now you disagree it, if you say why, it helps me a lot. Because, that is where the answer of the question is hidden. Until now, you have given no reasons. You are pointing @sophiecentuar Sir reply I hope. Even there, I don't see proper reason.

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.

 

[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.

 

[ZapperZ]

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.

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.

 

[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.

 

[ZapperZ]

@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.

q1 = 1e
q2 = -1e
Net charge Q = q1 + q2 = 1e + (-1e) = 0

Net charge = 0.

I think you need to clarify why you consider that to be non-zero.

Zz.

 

[CURIE WILLEY]

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?

As posted previously, 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 we say dipole has no net charge, it can't experience any force, if we keep it near electrically charged matter. I hope, dipole (if distance between charges is not zero) experiences force when kept near charged matter. So, I disagree the statement of dipole having zero net charge. Correct me, if I am wrong Sir.

]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.

I should agree your statement also. There is one positive and one negative charge. If we add, we get zero. But, it contradicts the definition of electric charge according to my explanation above. If you say I am wrong, you are indirectly proving definition of charge as incorrect (If my explanation above is agreed by you). In order to make the definition of charge not getting violated and even the rule of algebra, I think we can add or subtract charges if and only if the distance between them is zero. Correct me if I am wrong, Sir.

 

[ZapperZ]

You are wrong because you chose to redefine something that has been established. You are also confusing between "zero net charge" versus "non-zero electric dipole moment"! Somehow, you equate non-zero electric polarization with being non-zero net charge. THAT is what is wrong with your argument. It is like insisting that weightlessness must mean zero gravity. You can have one without automatically having the other!

Zz.

 

[CURIE WILLEY]

As posted previously, 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 we say dipole has no net charge, it can't experience any force, if we keep it near electrically charged matter. I hope, dipole (if distance between charges is not zero) experiences force when kept near charged matter. So, I disagree the statement of dipole having zero net charge. Correct me, if I am wrong , Sir.

Do you disagree anywhere here?

 

[ZapperZ]

Do you disagree anywhere here?

Yes, with your definition of "net charge".

My definition, which is often used, is simple algebra.

Other than the fact that we are arguing about what we call a cow, is there a reason why the cow will be different if you call it something else? Did Pluto change just because it is no longer a planet?

You seem to be extremely concerned that we must not say a dipole has no charge. How did this change the physics? Did the dipole care? Did the mathematical description change? Did the electric field geometry got modified just because you declared it to not be "neutral"?

What exactly is the purpose of this?

Zz.

 

[CURIE WILLEY]

You are wrong because you chose to redefine something that has been established.

I don't think I have redefined anything, if I have please pardon me and mention the factor which I have redefined.

You are also confusing between "zero net charge" versus "non-zero electric dipole moment"!

If the dipole has zero net charge it has zero electric dipole moment (and zero electric field), I agree this.

Somehow, you equate non-zero electric polarization with being non-zero net charge. THAT is what is wrong with your argument. It is like insisting that weightlessness must mean zero gravity. You can have one without automatically having the other!

Zz.

I failed understand your statements here.

By disagreeing my statements :

As posted previously, 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 we say dipole has no net charge, it can't experience any force, if we keep it near electrically charged matter. I hope, dipole (if distance between charges is not zero) experiences force when kept near charged matter. So, I disagree the statement of dipole having zero net charge. Correct me, if I am wrong Sir.

Aren't you proving definition of charge as incorrect?

Yes, with your definition of "net charge".

My definition, which is often used, is simple algebra.

I am not disagreeing with you sir. We can't run from reality, either I must be wrong some where or the other case. I am agreeing with your algebra. What I am saying is that, in order to not violate definition of charge and even the algebra rule. I am saying that, we can add or subtract charge if and only if distance between charges is zero.

Other than the fact that we are arguing about what we call a cow, is there a reason why the cow will be different if you call it something else? Did Pluto change just because it is no longer a planet?

You seem to be extremely concerned that we must not say a dipole has no charge. How did this change the physics? Did the dipole care? Did the mathematical description change? Did the electric field geometry got modified just because you declared it to not be "neutral"?

Little drops combine to form ocean. So, I think, these little drop things matters a lot.

What exactly is the purpose of this?

Zz.

The purpose of this is either to take off my misunderstanding if not, to draw conclusion that system of particles will be neutral if and only if there is no field around it.

 

[Nugatory]

@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.

You are overlooking a subtlety in the definitions. Electric charge is a property of the matter that makes up a system, whereas net charge is a property of the system. It is possible for a zero-net-charge system to be assembled out of pieces of matter that are charged, and depending on the arrangement of charges within the system, there may be a non-zero electric field around the system.

 

[DrClaude]

If the dipole has zero net charge it has zero electric dipole moment (and zero electric field), I agree this.

This statement is completely wrong. If you want to progress, you'll have to accept that you are wrong, and try to understand why.

Take a system made up of $n$ point charges $q_i$ located at positions $\mathbf{r}_i$. The net charge of the system, as ZapperZ pointed out, is
$$
Q = \sum_i q_i,
$$
whereas the electric dipole moment is
$$
\mathbf{p}(\mathbf{r}) = \sum_i q_i (\mathbf{r}_i - \mathbf{r}).
$$
Clearly, you can have $Q=0$ while $|\mathbf{p}| \neq 0$. Indeed, take two charges with $q_1 = -q_2$, with $\mathbf{r}_1 \neq \mathbf{r}_2$, and this is exactly what you get.

Even if the configuration of charges is such that the net charge and dipole moment are zero, you can still have non-zero higher moments, such as the quadrupole.

 

[Doc Al]

I don't think I have redefined anything, if I have please pardon me and mention the factor which I have redefined.

You have redefined the meaning of 'zero net charge'. You think it means "there is no field"; all it really means is that the total charge is zero.

You seem to think that something with zero net charge is equivalent to something with no charges at all and thus no fields. Not so.

If the dipole has zero net charge it has zero electric dipole moment (and zero electric field), I agree this.

Wrong.

Since you are so confused about what "net" means, I wonder what you think of an object that experiences a net force of zero. Do you think that that implies there are no forces acting on it?

 

[sophiecentaur]

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

In that case, you are using your own definition of net charge. That definition will not take you anywhere useful and your problems are only just beginning.

 

[Dadface]

As posted previously, 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.

This discussion is all about definitions and I suppose that it's important that we get the definitions right. Different sources are likely to define charge in slightly different ways with some definitions being more detailed than others.
Your definition above is not necessarily a good definition but keeping closely to it I think it can be improved as follows:

Electric charge is the physical property of "certain particles" that causes "those particles" to experience a force when close to other electrically charged "particles".

I know the definition needs improving but the point I'm trying to make is that it's better to think of charge not as a property of matter (or atoms whatever) but as a property of certain particles within that matter.

 

[sophiecentaur]

This discussion is all about definitions and I suppose that it's important that we get the definitions right. Different sources are likely to define charge in slightly different ways with some definitions being more detailed than others.
Your definition above is not necessarily a good definition but keeping closely to it I think it can be improved as follows:

Electric charge is the physical property of "certain particles" that causes "those particles" to experience a force when close to other electrically charged "particles".

I know the definition needs improving but the point I'm trying to make is that it's better to think of charge not as a property of matter (or atoms whatever) but as a property of certain particles within that matter.

Far too much so, I think. The 'definitions' thing gets in the way of understanding all too often. If one's interpretation of a definition leads to a paradox then the thing to do is to re-examine the definition rather than to obsess about the apparent paradox. Science is greater than the limited definitions that people try to impose on its parts.

 

[CURIE WILLEY]

In that case, you are using your own definition of net charge. That definition will not take you anywhere useful and your problems are only just beginning.

No, Sir. I am not using my own definition.

What is charge?
Electric charge is the physical property of matter that causes it to experience a force when close to other electrically charged matter.(This definition has been extracted from wikipedia)

Do you agree this?

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

Do you agree this?

What is a Neutral particle?
In physics, a neutral particle is a particle with no electric charge. (This definition has been extacted from wikipedia)

Do you agree this?

So, a particle will be neutral, if it has no electric charge.

Do you agree this?

In other words, a particle will be neutral, if it has no electric charge, then it implies that it has no field around it.

Do you agree this?

 

[sophiecentaur]

A dipole is not a particle, so you will, of course, get nonsense out of an argument that assumes it is.
I know that definitions can give you a warm, secure feeling because you can get exam marks by quoting them but that particular definition -

Electric charge is the physical property of matter that causes it to experience a force when close to other electrically charged matter.(This definition has been extracted from wikipedia)

is not well thought out. (It's only from Wiki and I wouldn't want to fly in an aeroplane that had been designed only on the information found in Wiki) It actually says nothing useful at all because it is a circular definition, using 'charge' in both cause and effect. It is not a serious definition at all; it is wide open and not fit for taking further into the subject, at any rate.

What are your motives here? Are you on a crusade to discredit all EM theory or do you want to understand it better? If you want to understand it better then don't waste time hanging onto statements that are not well founded. Use a proper textbook and do it the mainstream way. You call people "sir" but you are not giving them, or the subject the respect that word should imply. Do you really think you are right?

 

[Cthugha]

No, Sir. I am not using my own definition.

Yes, you are.

What is charge?
Electric charge is the physical property of matter that causes it to experience a force when close to other electrically charged matter.(This definition has been extracted from wikipedia)

That one is ok, but not really a good wording.

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

Ok, but not exhaustive.

What is a Neutral particle?
In physics, a neutral particle is a particle with no electric charge. (This definition has been extacted from wikipedia)

This is at best sloppy wording. At worst it is plain wrong. A neutral particle has no NET electric charge. There can be as many charges inside it as you want to. Having no net charge does by no means imply that there is no charge at all inside. Please: NEVER think that wikipedia is a good source for thorough science.

So, a particle will be neutral, if it has no electric charge.

Yes, it will be neutral if there are no charges inside, but this is not a necessary condition for neutrality. For a particle being neutral, it is sufficient that net charge is zero. In that case there will be charges inside which create fields. The superposition of all of these fields will just cancel if you place all the charges at the same position, but this is not a prerequisite for neutrality.

In other words, a particle will be neutral, if it has no electric charge, then it implies that it has no field around it.

No, this conclusion is not warranted at all.