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9. Electrostatics
What makes plastic wrap cling? Static electricity. Not only are applications of static electricity common these days, its existence has been known since ancient times. The first record of its effects dates to ancient Greeks who noted more than 500 years B.C. that polishing amber temporarily enabled it to attract bits of straw. The very word electric derives from the Greek word for amber (electron).
Some of the most basic characteristics of static electricity include:
- The effects of static electricity are explained by a physical quantity not previously introduced, called electric charge.
- There are only two types of charge, one called positive and the other called negative.
- Like charges repel, whereas unlike charges attract.
- The force between charges decreases with distance.
When various materials are rubbed together in controlled ways, certain combinations of materials always produce one type of charge on one material and the opposite type on the other. By convention, we call one type of charge “positive”, and the other type “negative.” For example, when glass is rubbed with silk, the glass becomes positively charged and the silk negatively charged. Since the glass and silk have opposite charges, they attract one another like clothes that have rubbed together in a dryer. Two glass rods rubbed with silk in this manner will repel one another, since each rod has positive charge on it. Similarly, two silk cloths so rubbed will repel, since both cloths have negative charge.
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 |qc| = 1.6 ×10-19C.
Examples:
- A glass rod becomes positively charged when rubbed with silk, while the silk becomes negatively charged.
- The glass rod is attracted to the silk because their charges are opposite.
- Two similarly charged glass rods repel.
- Two similarly charged silk cloths repel.
Simple model of an atom prestent as central positive nucleus (combined by protons and neutrons) with negative electrons orbiting its. 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.
Charges in atoms and molecules can be separated—for example, by rubbing materials together. Some atoms and molecules have a greater affinity for electrons than others and will become negatively charged by close contact in rubbing, leaving the other material positively charged. Positive charge can similarly be induced by rubbing. Methods other than rubbing can also separate charges. Batteries, for example, use combinations of substances that interact in such a way as to separate charges. Chemical interactions may transfer negative charge from one substance to the other, making one battery terminal negative and leaving the first one positive.
No charge is actually created or destroyed when charges are separated as we have been discussing. Rather, existing charges are moved about. In fact, in all situations the total amount of charge is always constant. This universally obeyed law of nature is called the law of conservation of charge.
- When enough energy is present, it can be converted into matter. Here the matter created is an electron–antielectron pair. me is the electron’s mass. The total charge before and after this event is zero.
- When matter and antimatter collide, they annihilate each other; the total charge is conserved at zero before and after the annihilation.
Some substances, such as metals and salty water, allow charges to move through them with relative ease. Some of the electrons in metals and similar conductors are not bound to individual atoms or sites in the material. These free electrons can move through the material much as air moves through loose sand. Any substance that has free electrons and allows charge to move relatively freely through it is called a conductor. The moving electrons may collide with fixed atoms and molecules, losing some energy, but they can move in a conductor.
Other substances, such as glass, do not allow charges to move through them. These are called insulators. Electrons and ions in insulators are bound in the structure and cannot move easily - as much as 1023 times more slowly than in conductors. Pure water and dry table salt are insulators, for example, whereas molten salt and salty water are conductors.
Clectroscope can be charged by touching it with a positively charged glass rod. Because the glass rod is an insulator, it must actually touch the electroscope to transfer charge to or from it. Since only electrons move in metals, we see that they are attracted to the top of the electroscope. There, some are transferred to the positive rod by touch, leaving the electroscope with a net positive charge.
Electrostatic repulsion in the leaves of the charged electroscope separates them. The electrostatic force has a horizontal component that results in the leaves moving apart as well as a vertical component that is balanced by the gravitational force. Similarly, the electroscope can be negatively charged by contact with a negatively charged object.
It is not necessary to transfer excess charge directly to an object in order to charge it. Induction is a method of 'transfer' charge without direct contact (charge is created in a nearby object). For example, it could be done by two neutral metal spheres in contact with one another but insulated from the rest of the world. A positively charged rod is brought near one of them, attracting negative charge to that side, leaving the other sphere positively charged.
Coulomb’s law calculates the magnitude of the force F between two point charges, q1 and q2, separated by a distance r. In SI units, the constant k is equal to k = 8.988 × 109Nm2/C2, the equation is F = k * |q1*q2|/r2. The electrostatic force is a vector quantity and is expressed in units of newtons. The force is understood to be along the line joining the two charges.
To simplify things, we would prefer to have a field that depends only on Q and not on the test charge q . The electric field is defined in such a manner that it represents only the charge creating it and is unique at every point in space. Specifically, the electric field E is defined to be the ratio of the Coulomb force to the test charge E=F/q, where F is the electrostatic force (or Coulomb force) exerted on a positive test charge q. Here we can write, E = k|Q|/r2.
Electric field is a vector quantity. And it decreases with the increasing distance.
- Electric field cannot be seen, but you can observe the effects of it on charged particles inside electric field.
- To find the electric field vector of a charge at one point, we assume that as if there is a +1 unit of charge there.
- If you want to find the total electric field of the charges more than one, you should find them one by one and add them using vector quantities.
If the electric field lines are parallel to each other, we call this regular electric field and it can be possible between two oppositely charged plates. E is constant within this plates and zero outside the plates.