Electrostatics Study Guide
Analogy is a powerful tool, and we use it often in physics. Remember, we started this journey with the static of mechanical objects. Now we move further from macroscopic to microscopic and analyze interaction between charges and the effect on the surrounding medium.
Electric Charge and Charge Conservation
A basic description of substances leads us to the concepts of electrical charges and the fundamental particle—the electron. Empirically, it has been found that a piece of amber gemstone, rubbed with a piece of animal fur, will create a new type of interaction; it will be able to attract to its surface small pieces of material and dust particles. The material is said to be electrically charged, and the smallest particle of charge is called the electron. The electron was assigned a negative charge of 1.6 · 10–19 Coulombs. These electrons are considered one of the elementary subatomic particles. Electrons (negatively charged) are constituents of atoms, which are electrically neutral (zero net charge). The atom also contains positive charges, or protons, which are positioned in the central part of the atom, the nucleus. The nucleus is also composed of neutral particles, called neutrons.
Contrary to the previous example where the amber was charged up with electrons, other materials tend to charge positively (with protons). This has been summarized in the electrostatic series. Materials such as amber, rubber, and polyethylene charge negatively, whereas paper, cat's fur, and nylon charge positively. The difference between the types of charge can be easily shown by letting two of these materials interact. Materials from the same group repel each other, and materials from the opposite group attract each other. This defines the electrical interaction.
Materials are considered to be conductors, semiconductors, or insulators. The nomenclature is revealing. Conductors will let charge spread in the volume. Semiconductors will vary their electrical properties and sometimes behave as conductors while at other times behaving as insulators. Insulators will localize the charge acquired; their electrons bond strongly to the atoms and make it hard for electrical charge to flow through the material.
A last observation. Charging an object can be achieved by different means: friction, conduction, or induction. Charging by friction involves rubbing two materials against each other and in the process electrons are transferred from one object to the other. Conduction involves a conductive material in which electrons are free to move in the volume. Induction is established between two materials that do not touch, but where the material 'that is charged interacts with particles in the uncharged material, creating a distribution of charge on the surface close to it.
In the neutral state, the atom has an equal number of electrons and protons, and the net charge is zero.
n = p
where n is the number of electrons and p is the number of protons. If an electron is given sufficient energy to be able to break the bond with the atom and leave it, then the atom becomes charged, as it now has one more proton than electrons. The atom becomes positively charged, and we call this new particle a positive ion (n < p). If by friction between two objects, one of the objects is charging positively (losing electrons), we typically retrieve the electrons on the second object. Then the second object, initially neutral, becomes negatively charged. This ion is called a negative ion (n > p). If the two objects are isolated from the exterior (that is, isolated from other objects), then the charges are moving from one to the other, and the total number of electrons and protons will stay the same; the ions simply rearrange between the two objects. We can define this as a new law of conservation, similar to the conservation of energy and conservation of momentum in mechanics and to the conservation of thermal energy in thermodynamics. This new law is the conservation of charge.
After the charge is rearranged between the objects in contact, the state remains unchanged as long as there is no other interaction. This defines a situation of equilibrium similar to previous cases of mechanical and thermal equilibrium where the charge, whether positive or negative, is measured in Coulombs (1 C), and its usual symbol is q. As mentioned previously, an electron has negative charge:
qe = –1.6 · 10–19 C
The proton is equal in charge but has more mass:
qp = +1.6 · 10–19 C
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