8 julio, 2024

Ways to electrify a body (with examples)

There are three ways to electrify a body, or what is the same, cause a small imbalance in its electrical charges so that the object acquires a net charge. These forms of electrification are rubbing, induction and contact.

The ancient Greeks had observed that amber, the fossil sap of a tree, was capable of attracting hair or bits of fiber when rubbed with a fur cloth. Due to this, the material was electrified for a short time.

Other materials also have this interesting property, such as glass, plastic, and some precious stones.

For example, when separating pieces of clothing just taken out of the dryer, sparks fly, indicating that the clothes were somehow electrified after tumbling in the drum. And if we vigorously comb our hair with a plastic comb, it will attract bits of paper.

It also happens that when sliding down the seat of a car, an unpleasant jolt is felt when touching the handle or the bodywork.

These phenomena have their origin in subatomic particles: protons -positively charged-, neutrons -without charge- and electrons -negatively charged-.

Normally substances are in a neutral state, since atoms have the same number of protons as electrons. But by rubbing some with wool, silk or fur, they are able to attract or repel other materials.

And if an electrically charged body comes into contact with or approaches a second object, it is capable of giving up or picking up charges, leaving the other equally electrified. Let’s see the ways in which it happens.

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rubbing electrification

The electrification by rubbing consists of rubbing one material with another, so one of them captures or gives up electrons, leaving both with a certain net charge.

The electrons, although attached to the atomic nucleus formed by protons and neutrons, have good mobility and the outermost ones can even detach in certain cases. Of course, for this you have to do a lot of work, which will depend on the nature of the material.

Combing the hair with a plastic comb causes the electrons in the hair to be released and end up in the plastic, leaving the latter with an excess.

We can also try rubbing glass or ebonite bars with silk cloths. The electrons come off the glass and pass to the silk, which accepts them easily.

Now, when two glass rods rubbed with a silk cloth are brought closer together, it is observed that they repel. Instead, rubbing the ebonite or plastic bar with rabbit fur and bringing the glass one closer with silk, we observe that they attract.

The same happens when experimenting with other materials: some attract after being rubbed, and others repel. In any case, it is due to an excess or deficiency of electrons.

This means that there are two kinds of electric charge. When two bodies have different types of charge, they attract each other. But if they have the same type, they repel each other.

Benjamin Franklin (1706-1790) carried out many such experiments and suggested the name of positive electricity to that acquired by glass rubbed with silk, and the other type of load came to be called negative electricity.

Conservation and quantization of electric charge

It is important to note that during the loading processes it is not created or destroyed. What we observe is that the charge passes from one material to another, therefore it is possible to establish the principle of conservation of electric chargeas a fundamental principle of physics.

It is analogous to when we say that energy is neither created nor destroyed, but rather transformed. In the same way, it is established that electric charge is neither created nor destroyed, it is only transferred from one body to another.

Another important fact is that when there is a transfer of electrons from one material to another, it always occurs in whole amounts, because the electrons are not fractionated.

It is therefore concluded that the electric charge is quantized, being the quantum of charge -the smallest possible charge- that of the electron, denoted by the symbol and and the negative sign:

e = -1.6 x 10 -19 coulombs.

The coulomb, abbreviated C, is the SI unit for electric charge.

An object charged, let’s say, by an excess of electrons, has n times this value in negative charge. On the other hand, one with a lack of electrons, has a charge ne with positive sign.

induction electrification

No matter how much they are rubbed, metallic objects do not acquire a net charge through rubbing.

But a metal sphere becomes electrified when a charged plastic or rigid rubber rod is approached from one side without touching it, while the finger is touched from the opposite side.

In this way the negative charge will pass from the sphere to the body of the person. The finger is then withdrawn and the rod moved away, leaving the sphere with a net positive charge.

The experiment works regardless of whether the bar has a positive or negative charge, but the sphere has to be made of metal, because if it is made of glass it cannot be charged in this way.

This is due to a very interesting property: the electrons in metal have greater mobility than those in glass or plastic.

conductors and insulators

As we have seen, materials respond differently to electrification. Amber, plastic, glass, and rigid rubber belong to the group known as insulatorswhile metals and salt solutions are drivers.

In conductors, at least one of the outermost electrons of the atom has an easy way to break off and move within the material.

Therefore, if an agent does the necessary work, the electrons can move in an orderly fashion on metal wires or in salt water and thus create an electric current.

It should be noted that there is also a great variety of materials with an intermediate behavior, which are called semiconductorsvery important in the manufacture of electronic devices.

contact electrification

Electric charge flows between two objects placed in direct contact. If there are excess electrons in one, a part will go to the other object. And if, on the contrary, there is a defect, one of the objects could give up electrons to the other, leaving the bodies with charges of the same sign.

For example, by touching a metal sphere with a previously charged plastic rod, some of the excess electrons from the rod pass directly into the sphere.

In this way, the metallic sphere has been charged by direct contact, distributing the excess charge between them, always respecting the principle of conservation of charge.

We can also put two charged metal spheres in contact, placed on insulating supports. If the spheres are identical, the charge will be divided between them equally.

References

Bauer, W. 2011. Physics for Engineering and Science. Volume 2. Mc Graw Hill.
Figueroa, D. Physics Series for Science and Engineering. Vol. 5 Electrostatics. Edited by D. Figueroa. USB.
Giambattista, A. 2010. Physics. 2nd. Ed. McGraw Hill.
Giancoli, D. 2006. Physics: Principles with Applications. 6th. Ed. Prentice Hall.
Thomas, W. 2007. Conceptual Physics. McGraw Hill.

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