# What is charge excatly?

#### benzun_1999

What is charge excatly?

-benzun
All for God

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#### chroot

Staff Emeritus
Gold Member
A characteristic of a particle (or object) that determines how it interacts via the electromagnetic force.

- Warren

#### pmb

Re: charge

Originally posted by benzun_1999

What is charge excatly?

-benzun
All for God
I wish I had a good answer for you. Things like this are difficult to define in a simple statement without being cirular. As chroot stated it has to do with the electric force. But the electric force is defined in terms of electric charge.

Back in the olden days (~600 B.C.)it was known to the Ancient Greeks that amber (Greek: 'elektron') was rubbed it would attract small bits of things like silk. That was all that was known about it for the next 2200 years. William Gilbert published a book in 1600 A.D. which explained that amber is not the only material to display this property. He listed others which displayed this property. The term "electified" was coined somewhere along the way. Another 133 years later when, in 1733, Charles DuFay showed that there electrification was possible which resulted in repulsion as well as attraction. Then Ben Franklin came on the scene and imagined that there was an invisible fluid which flowed from one place to another and of which there were two kinds. One he assigned sign of "positive" and the other he assigned a sign of "negative." However it was later shown that there was no fluid per se but there were particles which had these properties of attraction and repulsion. The force between two objects depends on the amount of these partilces on them. The particles were assigned a quantity called "charge" and is often labeled Q or q. Objects which have an imbalance of this "fluid" or f charges is said it be "charged."

The force between charged objects was later measured and the relation was found to be of the form

F = kQq/r^2

I hope that helps.

Pete

#### benzun_1999

Nucleus is said to be Positive and electrons are negative. i have heard many people tell that when there is equal number of +ive and
-ive that charge is neutral. but it doesnt make sense to me as still more electrons can attracted by the nucleus with its charge.[?]

#### chroot

Staff Emeritus
Gold Member

Originally posted by benzun_1999
Nucleus is said to be Positive and electrons are negative. i have heard many people tell that when there is equal number of +ive and
-ive that charge is neutral. but it doesnt make sense to me as still more electrons can attracted by the nucleus with its charge.[?]
When you're far enough away from the atom, you don't notice its individual positive and negative pieces. Think abou this way -- when you're reading this page here on physicsforums.com, you can see the individual letters. If you step back 20 feet, you probably will not be able to discern individual letters anymore. The same thing happens with charges -- when you're far enough away from an atom with equal positive and negative charge, you don't detect any charge at all.

- Warren

#### xAxis

chroot said:
When you're far enough away from the atom, you don't notice its individual positive and negative pieces. Think abou this way -- when you're reading this page here on physicsforums.com, you can see the individual letters. If you step back 20 feet, you probably will not be able to discern individual letters anymore. The same thing happens with charges -- when you're far enough away from an atom with equal positive and negative charge, you don't detect any charge at all.

- Warren
But what about that coulomb force then? F=Kq1q2/r^2
It's a function of r^2. You detect the charge by detecting the force, which is coulomb's force, which is function of distance.
Or am I completely wrong here?

#### kmarinas86

xAxis said:
But what about that coulomb force then? F=Kq1q2/r^2
It's a function of r^2. You detect the charge by detecting the force, which is coulomb's force, which is function of distance.
Or am I completely wrong here?
What you said is right.

The distance between charges is often very small and in such cases the force due to positive and negative charges approximately cancel each other. At the scale of atoms/molecules, the seperations between charges are more profound (since these seperations are similar in scale to the atomic/molecular scale) and leads to various forces that occur at the atomic/molecular scale. At far larger seperations, you can see the coulomb force at work during a lightning storm, where the seperation of charges is much greater (as big in scale as the seperation from the thunderclouds and the ground).

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#### xAxis

Ok. So I can see, coulomb force acts in accordance with Coulomb Law only on macroscopic scale? But it still doesnt mean that it doesn't belong to 4 basic forces?
And if it is, I suppose , EM force, then it behaves differently in different "worlds", but is the same force?
Id like to know if this is correct.

#### kmarinas86

xAxis said:
Ok. So I can see, coulomb force acts in accordance with Coulomb Law only on macroscopic scale?
No. It always works in accordance with Coulomb's law. If the charges are sufficently "blended in proximity" the charges are neutralized, reducing the force it has on distant charges.

But it still doesnt mean that it doesn't belong to 4 basic forces?
It belongs to the 4 basic forces, yes, but that's not what my post meant.

And if it is, I suppose , EM force, then it behaves differently in different "worlds", but is the same force?
Well, it is expressed in different ways (different strengths of electric fields), but that doesn't the mean the laws are any different. It's the same class of force.

#### xAxis

Is it then truth that EM force counts as much stronger than gravity only due to the fact that it is posible to reach small distances between the sources? But it is essentialy inverse square law, like gravitation?

#### russ_watters

Mentor
xAxis said:
Is it then truth that EM force counts as much stronger than gravity only due to the fact that it is posible to reach small distances between the sources? But it is essentialy inverse square law, like gravitation?
Yes, the laws are similar, but the proportionality constants are different by many orders of magnitude, and that is what determines the distances on which they have a noticeable effect.

#### sdemjanenko

the reason why EM doesn't affect things over large distances in a very noticable manner like gravity is that all charged particles would then need the same charge and they would repel each other. This case happens in the nucleus, mainly why nuclei don't get much bigger than uranium, the strong nuclear force then falls off, since its a -5 power force while EM is -2 and overpowers the strong force not letting the pile of charged particles to build up to the point where it can compete with gravity over distances. Due to the proportionality constants it would overpower gravity by far, but gravity is all attractive, at least we have not found any negative mass yet.

#### xAxis

Thank you for replies. I've clarified many things.
Is the nuclear force then always attractive, and does it depend on charges? And, yes, does EM force also keeps molecules together?

#### rbj

russ_watters said:
Yes, the laws are similar, but the proportionality constants are different by many orders of magnitude, and that is what determines the distances on which they have a noticeable effect.
Russ, those proportionality constants are a consequence of the units humans happened to cook up to measure things. physical units can (and have) certainly be judiciously defined so that for both gravity and E&M, their proportionality constants are both equal to 1 (in terms of those units). the common example is Planck units.

in Planck units, both the strength of the graviational constant and the strenght of the electrostatic contant (as well as the speed of light and Planck's constant) are set to unity. then the reason why (for fundamental particles that are charged) the gravitational force is so much weaker than the E&M force, is that the elementary charge is approximately the unit charge, but the mass of say, a proton, is much much much less than the unit mass. that is why gravity appears so ridiculously weak on that scale.

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