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Electric Charges

The basic sub atomic particles are the Proton, Electron, and Neutron. These particles are the constituents of the atoms of all the chemical elements. A simple pictorial representation of an atom is presented in the drawing at the left. These three particles represent the kinds of reactions that matter can have with respect to electric interactions. In fact, two electrons repel each other, likewise, two protons repel each other, while neutrons do not interact electrically among themselves nor with electrons and protons. On the other side, between electrons and protons, there is electric attraction. The property that particles possess that accounts for their electric interactions is called charge. Based on the previous interaction among charges, it is concluded that there are two types of charges; positive (+) assigned to the proton, and negative (-) for the electron. The neutrons are uncharged or neutral.

Experimentally, it has been established that the magnitude of the charge of an electron is the same as the magnitude of the charge of a proton.  Therefore, the atom on the drawing above is a neutral atom.

The first experiments in electricity were in what is called electrostatic. Electrostatic studies the interaction among charges after the charges can be considered at rest, i.e., without motion. Usually, when a charge is placed in the vicinity of other charges, a re-distribution of charge may occur. Electrostatic studies the interaction among the charges after the re-distribution of charges has occurred.

One such experiment is to rub a rubber rod with a piece of fur (cat or rabbit). During the rubbing, charges are exchanged between the rod and the fur. In fact, electrons are taken from the fur and transferred to the rubber rod leaving the fur with an excess of positive charge and the rubber rod with an excess of negative charge. Similarly, when rubbing a glass rod with a silk rug results in electrons being removed from the glass rod and transferred to the silk rug. In this case, the glass rod ends with an excess of positive charge while the silk rug ends up with an excess of negative charge.

With the help of the two rods, rubber and glass, different electrostatic experiments can be set. Repulsion and attraction of charges can be visualized with the following drawings:

The drawings of the sequence represent two small metallic spheres (conductors) hanging from insulating strings, such as fishing line (insulator).

The first drawing represents the spheres without charge. There is not electric interaction between the spheres and they just hang vertically due to their weight.

The second drawing represents the same spheres after both of them have been touched by a negatively charged rubber rod. Because of the electric repulsion between the charges, the two spheres move away from each other. The third drawing also represents electric repulsion but, in this case, between positive charges. Here, the spheres have been charged by touching them with a positively charged glass rod.

The last drawing shows electric attraction between the spheres. In this case, one of the spheres (left) has been touched with the positively charged glass rod while the other (right) has been touched with the negatively charged rubber rod.

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Unit of Charge

The first unit of charge was defined as a hundred rubs on cat fur with an amber rod (Classical Electrodynamics 1st Ed., John D. Jackson, section 1.2). Later, the unit of charge was called the Coulomb, C. Modern definitions of the Coulomb are based on the current (see electric current).

Using this unit, the elementary charge is . This charge corresponds to the magnitude of the charge of an electron and also to the charge of a proton. As indicated above, the magnitude of the charge of an electron is the same as the magnitude of the charge of a proton; consequently, atoms with the same number of protons as electrons are neutral. Atoms with different number of electrons than protons are charged and called ionized.

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Conservation of the Charge

Experimentally, it is observed that in all charging and discharging of objects, there is only exchange of charges between objects. Thus, the combination of charge of the involved objects (total charge) is unchanged. Based on these experimental results, it is said that the electric charge is conserved and, in nature, the electric charge can not be created nor destroyed.

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Quantization of the Charge

Also experimentally, it is observed that the smallest amount of free charge that can be found is the elementary charge, . Since no fraction of the elementary charge can be found, the electric charge is quantized. Thus the charge of an object can be

 

where n represent an integer.

 

1)

J

 

Q 25

PTF573

 

Which of the following statements are true about charges?

         I.      The electric charge is conserved.

       II.      The minimal amount of charge that can be found free is the charge of an electron.

     III.      In addition to the charges having opposite sign, the charge of a proton is slightly larger in magnitude than the charge of an electron.

 

a)

Only I and III

 

b)

Only II and III

N

c)

Only I and II

 

d)

All of them I, II, and III.

 

e)

None of the above.

 

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Conductors and Insulators

 

 

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Static Charging

 

The picture at the left shows two pieces of plastic tape brought closer together at the top. However, the tapes repel each other everywhere.

 

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The Electroscope

 

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Charging by Induction

 

 

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by Luis F. Sez, Ph. D.    Comments and Suggestions: LSaez@dallaswinwin.com