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18.1 Static electricity and charge: conservation of charge  (Page 2/8)

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(a) Negatively charged cloth is attracted to the positively charged glass rod which is hanging by the thread. (b) A positively charged glass rod is hanging with a thread. When another positively charged glass rod brought close to the first rod it deflects due to the repulsive force. (c) Two negatively charged silk cloth brought close to each other repel each other.
A glass rod becomes positively charged when rubbed with silk, while the silk becomes negatively charged. (a) The glass rod is attracted to the silk because their charges are opposite. (b) Two similarly charged glass rods repel. (c) Two similarly charged silk cloths repel.

More sophisticated questions arise. Where do these charges come from? Can you create or destroy charge? Is there a smallest unit of charge? Exactly how does the force depend on the amount of charge and the distance between charges? Such questions obviously occurred to Benjamin Franklin and other early researchers, and they interest us even today.

Charge carried by electrons and protons

Franklin wrote in his letters and books that he could see the effects of electric charge but did not understand what caused the phenomenon. Today we have the advantage of knowing that normal matter is made of atoms, and that atoms contain positive and negative charges, usually in equal amounts.

[link] shows a simple model of an atom with negative electrons orbiting its positive nucleus. The nucleus is positive due to the presence of positively charged protons . Nearly all charge in nature is due to electrons and protons, which are two of the three building blocks of most matter. (The third is the neutron, which is neutral, carrying no charge.) Other charge-carrying particles are observed in cosmic rays and nuclear decay, and are created in particle accelerators. All but the electron and proton survive only a short time and are quite rare by comparison.

Three electrons are shown moving in different direction around the nucleus and their motion is similar to planetary motion.
This simplified (and not to scale) view of an atom is called the planetary model of the atom. Negative electrons orbit a much heavier positive nucleus, as the planets orbit the much heavier sun. There the similarity ends, because forces in the atom are electromagnetic, whereas those in the planetary system are gravitational. Normal macroscopic amounts of matter contain immense numbers of atoms and molecules and, hence, even greater numbers of individual negative and positive charges.

The charges of electrons and protons are identical in magnitude but opposite in sign. Furthermore, all charged objects in nature are integral multiples of this basic quantity of charge, meaning that all charges are made of combinations of a basic unit of charge. Usually, charges are formed by combinations of electrons and protons. The magnitude of this basic charge is

∣ q e ∣ = 1 . 60 × 10 − 19 C . size 12{ lline q rSub { size 8{e} } rline =1 "." "60" times "10" rSup { size 8{ - "19"} } C} {}

The symbol q size 12{q} {} is commonly used for charge and the subscript e size 12{e} {} indicates the charge of a single electron (or proton).

The SI unit of charge is the coulomb (C). The number of protons needed to make a charge of 1.00 C is

1. 00 C × 1 proton 1 . 60 × 10 − 19 C = 6 . 25 × 10 18 protons .

Similarly, 6 . 25 × 10 18 size 12{6 "." "25" times "10" rSup { size 8{"18"} } } {} electrons have a combined charge of −1.00 coulomb. Just as there is a smallest bit of an element (an atom), there is a smallest bit of charge. There is no directly observed charge smaller than ∣ q e ∣ size 12{ lline q rSub { size 8{e} } rline } {} (see Things Great and Small: The Submicroscopic Origin of Charge ), and all observed charges are integral multiples of ∣ q e ∣ size 12{ lline q rSub { size 8{e} } rline } {} .

Things great and small: the submicroscopic origin of charge

With the exception of exotic, short-lived particles, all charge in nature is carried by electrons and protons. Electrons carry the charge we have named negative. Protons carry an equal-magnitude charge that we call positive. (See [link] .) Electron and proton charges are considered fundamental building blocks, since all other charges are integral multiples of those carried by electrons and protons. Electrons and protons are also two of the three fundamental building blocks of ordinary matter. The neutron is the third and has zero total charge.

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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