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## AP®︎/College Physics 1

### Course: AP®︎/College Physics 1>Unit 10

Lesson 1: Electric charge

# Electric charge review

Overview of the key terms and skills related to electric charge, including how to calculate net charge.

## Key terms

TermMeaning
Elementary charge ($e$)Smallest possible unit of charge. Equal to the charge on one proton or the magnitude of charge on one electron, which is $1.6×{10}^{-19}\phantom{\rule{0.167em}{0ex}}\text{C}$. Also called electron charge. Sometimes uses symbol ${q}_{e}$.
Net chargeSum of the charges on an object.
Coulomb (C)SI unit for electric charge. The amount of charge transferred in $1\phantom{\rule{0.167em}{0ex}}\text{s}$ by a current of $1\phantom{\rule{0.167em}{0ex}}\text{A}$.

## How to calculate net charge

There are two kinds of electric charge, positive and negative. On the atomic level, protons are positively charged and electrons are negatively charged.
Although the mass of a proton is much larger than that of an electron, the magnitudes of their charges are equal.
If an object has more protons than electrons, then the net charge on the object is positive. If there are more electrons than protons, then the net charge on the object is negative. If there are equal numbers of protons and electrons, then the object is electrically neutral.
When charge is transferred between objects, it’s only the electrons that move. An electron can’t be broken into smaller pieces, so charge always changes in multiples of the elementary charge. This property of electrons means that charge is quantized and the charge on any object must be an integer multiple of the elementary charge. An object’s charge can be $0\phantom{\rule{0.167em}{0ex}}e,1\phantom{\rule{0.167em}{0ex}}e,-1\phantom{\rule{0.167em}{0ex}}e,2\phantom{\rule{0.167em}{0ex}}e$, etc., but not $\frac{1}{4}\phantom{\rule{0.167em}{0ex}}e,\frac{1}{2}\phantom{\rule{0.167em}{0ex}}e$, etc.

For deeper explanations of electric charge, see our video triboelectric effect and charge.
To check your understanding and work toward mastering these concepts, check out the exercise on quantization of charge in experimental data.

## Want to join the conversation?

• How come quarks have fractional charges?
• The idea of electrons/protons having an "elementary" (lowest level) charge was developed and set in stone before physicists realized that there were sub-sub-atomic particles (quarks) that had charges smaller than the previous elementary charge.
• Quantization of charge does not work at the macroscopic level, Why?
• It works. But we ignore it. suppose a body has a charge of 250C then adding an electron would mean we are adding 0.0000000....016 C to it. thus we can ignore it and assume that charge is continuously added .
• what mean is by integer multiple of e
• e= 1.6 x 10 to the power of -19. So essentially multiples of e will be 1.6, 3.2, 4.8 and so on which are e, 2e, 3e,.. respectively. This are integer multiples of e.
• Bit of a dumb question, but how exactly do you read this: -2e or 2e? Is it something like, "2 negatively charged protons" and "2 positively charged protons" or some other way? Apologies if this has been asked somewhere already
• Nope, your question isn't dumb at all!
I believe you would call it "charge of negative two" for the first one and "charge of two". You can omit the "protons" portion as one unit of charge is already defined as the charge of one proton/electron.
Of course, you can say it slightly differently for each situation, like "the beryllium atom has a charge of two."

Cheers!
• "deficiency of electrons results in positive charge on body, while excess of electrons means body have negative charge. we do not use proton excess or deficiency terminologies because protons are way more heavier than electrons and do not move when we charge a body either negative or positive. it is electrons that just move and decides positive and negative charge on body".

just like when an neutral atom of Na(sodium) become positive charge ion because it has deficiency of 1e- but its proton remains fix on place. a case of deficiency of electrons.
Na ----------> Na+ +1e-

if you imaginize it other way around neutral Cl(chlorine) atom become negative charge ion for its accept a electron but its proton do not go anywhere. a case of excess of electrons.
Cl + e- ------------> Cl-

and i think the case is same when we say something is negative charge( have predominant electrons) or positive charge( have fewer electrons).

am i right? all about the charge.
• You're partially correct, let me explain why so, only electron can transferred and not proton not because of electron is light and proton is heavy in weight, it is because energy required to pluck an electron from a neutral atom is way more less that that of proton. Energy required to pluck the Proton is very high because Protons reside inside the nucleus. And that of electrons they revolve around the nucleus in orbits(path), hence electrons are easy to be removed from atom. We need to give very very high energy to remove a proton from nucleus. Therefore electrons are used to deal the transfer of charge
• what is quantization of charge
• Charge only comes in countable numbers. The smallest amount of charge that has ever been observed is the charge on the electron and/or the charge on the proton (both the same amount, but opposite type). Hence the term "quantization of charge." Quantum or "quantized" means that it is numbered or integer-countable (can come in -1, 0, 1, 2, 3, etc. but not 1.5 or pi, etc), and a quantum is essentially one of those quantized objects. The total charge on a charged object is therefore the number of individual charges, all of which are either electrons or protons, multiplied by the charge on one of them. This explains the existence of the following equation:

Q=n*e
• what is the role of prtotons if only electrons move
• The protons are just...there in atoms.
Their role is to generate an electric force to bind the electrons to an atom, simply put. Every-Single-Atom have them.
About their origin, I think we would have to focus upon the theory of conservation of charge, which states that the net charge of a closed system remains constant overtime. So, from and after the formation of the universe, we can reasonably assume that whatever caused the formation of electron must also have simultaneously (at the same time) produced a positive charged particle (let it be a sub-atomic particle or something, I don't know the exact name, because I don't know much, if any, about quantum physics.) , thus neutralizing the effect the generation of the electron might have had upon the total charge of the universe. This mystery particle (might not be a mystery today anymore) might have been the proton itself, or something which gave the positive charge for the proton.
(1 vote)