# Question about the existence of Charge

1. Jul 8, 2012

### arul_k

Question about the existence of "Charge"

Is it necessary to have a concept of charge apart from mass and electric field?
What I mean to ask is for example in the case of an electron, it can be consisdered to have mass and a negative electric field, where is the need to introduce the concept of charge?

mathematically it may be necessary to quantify charge but then is charge only a mathematical concept?

2. Jul 8, 2012

### Simon Bridge

Re: Question about the existence of "Charge"

In order:
1. yes
2. can you write down the electric field of an electron without including the size of the electron charge? How do you account for the shape of the electric field of a dipole in relation to the individual fields for the individual charges? Howe do you formulate a theory of electrodynamics (eg electromagnetism, electricity, light etc) without the concept of charges?
3. no - it is an empirical concept. The math is just a way of describing it.

Of course, bottom line is that these are all just models which we use to predict phenomena in nature. When the model does very well we tend to think we understand the phenomena. If you are not careful it will boil down to how you know that anything is real.

3. Jul 8, 2012

### arul_k

Re: Question about the existence of "Charge"

Thanks Simon for your reply.
The shape of any 2 fields interacting with each other is purely a field interaction, I don't see the need for charge. The theory of electromagnetism is a vast topic, but there again much of electromagnetism could be explanined on the basis of varying electric and magnetic fields.
Its hard to break free of the old idea of charge giving rise to a field, how ever as I stated in my question its possible that there exists only mass and E/M fields. Its quiet possible that another model may describe just as well our observations in nature.

4. Jul 8, 2012

### Simon Bridge

Re: Question about the existence of "Charge"

That's a very general statement - it is always possible that some new physics will be discovered. The main purpose of Physics Forums is to teach and discuss the current state of knowledge in science, math, and technology.

Can you come up with a description of the electron's electric field without having some charge-analog in there?

5. Jul 9, 2012

### mikelepore

Re: Question about the existence of "Charge"

The phrase "negative electric field" has no meaning. An electric field is a mapping of all point in space that answers the question: If a positive test charge were to be inserted at each point, what magnitude of force of would it experience, and in what direction? We can refer to, for example, a direction parallel to or perpendicular to the electric field, or a direction in which the magnitude of the electric field increases or decreases, etc., but the field can't be negative.

6. Jul 9, 2012

### mikeph

Re: Question about the existence of "Charge"

I don't see how light can ever interact with matter in a charge-less universe. It is a very different place to our universe.

7. Jul 9, 2012

### Drakkith

Staff Emeritus
Re: Question about the existence of "Charge"

With or without charge any theory must accurately predict real experimental results. If two theories do this equally well, then go with the one that is the simplest and makes the fewest assumptions. I see no reason to do away with charge.

8. Jul 9, 2012

### K^2

Re: Question about the existence of "Charge"

You can't have light in charge-less universe to begin with, so it's a moot point.

9. Jul 9, 2012

### Simon Bridge

Re: Question about the existence of "Charge"

Ha yes - I missed that little bit of subtlety:
... the idea of a "negative electric field" would be that field that arises from a negative charge or one which has a negative potential for a positive charge ... so just saying "it has a negative electric field" is the same as saying "it has a negative charge".

In other words - the concept of an electric field is intimately tied with the concept of charge. One could remove the word, but we'd need something analogous.

Off Drakkith: come up with a theory of electromagnetism that does away with charge that also survives Occam's Razor and there is some sort of prize waiting for you. The concept of "charge" may, ultimately, be discarded ... but right now? Needed.

aside: back from a tour of OPs other threads...
Perhaps I should put the question down to the choice of wording? I should not expect people asking questions to always use words in a way I am familiar with right?

Electricity and Magnetism are taught starting with charges and then moving into fields and potentials ... in poorly taught courses the charges just pop up "out of nowhere". OP seems to be asking: "where did they come from?" "Do we have to use these magical thingies?" "Are they something in nature or just made up as a useful fiction?"

I suppose I could reformulate the question to ask if we can formulate a theory of electrical phenomena that does not make any a-priori assumptions about the existence of charge and, from there, discover a need to introduce the concept?

Well - I suspect that is what happened historically: there was a time when it was thought there were two different kinds of electricity. The concept of charge arose as a way of making sense of their mutual behavior in a way that makes the math easier - for who likes doing math?

10. Jul 10, 2012

### mikeph

Re: Question about the existence of "Charge"

I think a steady state universe could have light traversing it forever and never doing anything.

11. Jul 11, 2012

### arul_k

Re: Question about the existence of "Charge"

I don't quite understand why you can't have light in a charge less universe. Are you refering to the generation of EM waves by oscillating charges?

12. Jul 11, 2012

### arul_k

Re: Question about the existence of "Charge"

I do not agree, as per our present concept of charge and field the two are different from each other. Of course the presence of charge (assuming its existance) can only be detected through its field, but charge is consisdered to be localised, not so with the field.

It is only an assumption that a field must arise from charge. For example an EM wave once generated exists without the presence of charge.

I agree with you here, it appears that the concept of tiny particles of charge arose historically to explain the behaviour of static electricity and has continued to this day.

13. Jul 11, 2012

### Staff: Mentor

This is true, you do not need charge to describe two EM fields interacting with each other. However, you do need charge in order to describe a field interacting with matter. Since we want to describe that interaction we need charge.

Furthermore, even if we just started with the fields, as soon as we write down the Lagrangian we would notice that it has a gauge symmetry. From Noethers theorem we would immediately know that there is a conserved quantity. That conserved quantity is charge.

14. Jul 11, 2012

### Simon Bridge

Re: Question about the existence of "Charge"

It is all very well saying "I do not agree" ... Then how do you get a "negative field"? How do you know which has the positive field and which has the negative?

How do you write down the field equations ... or as dalespam suggested, the Lagrangian, without reference to charge?

I believe your original question has been answered ... it is up to you whether or not you believe the answer.

15. Jul 13, 2012

### arul_k

Re: Question about the existence of "Charge"

How do you get "negative Field"?. Well, could you tell me how you get "negative charge"? But the question is not about how you get charge or field, it is about the existance of charge. Is there experimental proof to validate the existance of charge?

I am not familiar with Lagrangian equations, but in my very first post I did concede that mathematically the concept of charge is required.

16. Jul 13, 2012

### Simon Bridge

Re: Question about the existence of "Charge"

Yes there is - insofar as an experiment can be said to "prove" anything - the existence of charge is as well determined experimentally as the presence of any other property of matter. Like I said earlier:
This is pretty much the way we know anything in physics.

That's not what we are saying with the Langrangians ... we are saying that you cannot come up with a description of electromagnetism that does not include some concept that amounts to the same thing as having charge. See the second sentence in my self-quote (above) ... using Lagrangian mechanics, we can set up a theory of electrodynamics which does not pre-suppose the existence of charge, or anything beyond the directly observed constraints on the system. When we do that, charges pop out anyway. Ergo - we didn't put them there, they are part of the implications of what we are observing.

If something persists in being needed despite all attempts to disbelieve it - it's real.

Conversely, is there any experimental evidence to suggest that we are mistaken to conclude the existence of charge? Where is all this coming from anyway? What is motivating the question? Perhaps if I understood the context I'd be able to come up with a more satisfactory answer?

Meantime, you'll probably benefit from having an early intro to Lagrangian and Hamiltonian mechanics. Especially since you are interested in the underlying reality of stuff.

Last edited: Jul 13, 2012
17. Jul 13, 2012

### Khashishi

Re: Question about the existence of "Charge"

The charge is the divergence of the electric field. So it is redundant information if you know the electric field. But the charge is a scalar property and is fixed for a given particle, whereas the electric field of the particle depends on how it is moving.

18. Jul 13, 2012

### Staff: Mentor

Re: Question about the existence of "Charge"

In science we make mathematical models called theories. We then use those mathematical models to make a hypothesis, which is a predicted outcome of a specific proposed experiment. We then run the experiment and see if the outcome matches the hypothesis. If it does, then we call the mathematical model "verified" or "confirmed" and we have evidence to believe that the various mathematical objects in the model represent an accurate description of nature within the domain of the experiment.

The concept of charge was originally developed for a theory called Maxwell's equations, and has been since carried over as a central concept of a theory called Quantum Electro Dynamics. QED is the complete modern description of all electromagnetic phenomena. It explains all EM phenomena observed to date. It has been experimentally verified to greater precision than any other theory ever conceived or tested.

Charge is not only a mathematical concept, it is a mathematical concept with more exquisite experimental validation than any other concept ever developed. If you want something other than that then you are looking beyond the boundaries of science and the discussion doesn't belong here.

19. Jul 13, 2012

### Neandethal00

Re: Question about the existence of "Charge"

You'll be taking a lot of beatings for this thread in this forums.
I have never found the answer to my question, 'what is charge'?
It has no mass, it can not exist by itself, it always need a carrier. What is it?
I do not know if 'field' is the answer to my question. But it is a proposal worth looking into.
We can say 'field' is quantized for multiple charges, we can use imaginary lines of force in the fields for positive and negative fields as we do now.

I understand a 'field' must have a 'source', according to our current understanding of nature. Why don't we associate a 'charge like' concept with gravitational field also instead of only mass? We can call it 'Garge' or 'Gharge' etc, embedded in the mass which is causing gravitational field.

20. Jul 14, 2012

### Drakkith

Staff Emeritus
Re: Question about the existence of "Charge"

Charge is a property, like mass and spin. It is something that particles simply have that causes them to react in a certain way to another charged particle that has charge. We don't have a charge in gravitation because we have already accounted for gravitational interactions through mass and energy. More mass means a greater force exerted, whereas in electromagnetism more charges means a greater force.

21. Jul 14, 2012

### DennisN

Re: Question about the existence of "Charge"

I just wanted to pop in and second what Simon Bridge said before (my bolding):
I did not want to second that because of the OP's question or because of the topic, I wanted to second it because there is very much truth in that in general, in my opinion. And it's one of the best statements I've seen on this forum, thumbs up!

Last edited: Jul 14, 2012
22. Jul 14, 2012

### Neandethal00

Re: Question about the existence of "Charge"

I also agree with this statement.

Aren't we following a double standard here? For electric field we use a strange 'property' calling it charge not mass, but for gravitational field we do not use a similar 'property' but use mass. All this because mathematical treatments satisfy observations.

Simon's statement above hits the bull's eye. Anyway, I guess we have to live with this.

23. Jul 14, 2012

### Staff: Mentor

Re: Question about the existence of "Charge"

What double standard do you think exists?

Roughly speaking, there is an EM interaction governed by a property called charge and a gravitational interaction governed by a property called mass. There seems to be no double standard to me.

24. Jul 14, 2012

### Dead Boss

Re: Question about the existence of "Charge"

I think this is an excellent observation. Indeed there are (at least) two definitions of mass. One is so called inertial mass and is defined by Newton's second law, F = ma. The other is called gravitational mass and is defined by Newton's law of universal gravitation. These two are equal. This equality is one of the things that lead Einstein to his theory of general relativity.

25. Jul 16, 2012

### arul_k

Re: Question about the existence of "Charge"

That happens to be the case for most of my questions!!!!!!!!
Maybe the asumption that field must have a surce is false. As I stated earlier an EM wave once generated can exist independent of any "source"

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