Electric charge, is a property of matter that causes objects to attract or repel each other. Electric charge comes in two varieties, which we call positive and negative. Like charges repel each other, and unlike charges attract each other. Thus, two positive charges repel each other, as do two negative charges. A positive charge and a negative charge attract each other.
How do we know there are two types of electric charge? When various materials are rubbed together in controlled ways, certain combinations of materials always result in a net charge of one type on one material and a net charge of the opposite type on the other material. 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. Because 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, because each rod has positive charge on it. Similarly, two silk cloths rubbed in this manner will repel each other, because both cloths have negative charge. Figure 18.2 shows how these simple materials can be used to explore the nature of the force between charges.
A glass rod becomes positively charged when rubbed with silk, whereas 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.
It took scientists a long time to discover what lay behind these two types of charges. The word electric itself comes from the Greek word elektron for amber, because the ancient Greeks noticed that amber, when rubbed by fur, attracts dry straw. Almost 2,000 years later, the English physicist William Gilbert proposed a model that explained the effect of electric charge as being due to a mysterious electrical fluid that would pass from one object to another. This model was debated for several hundred years, but it was finally put to rest in 1897 by the work of the English physicist J. J. Thomson and French physicist Jean Perrin. Along with many others, Thomson and Perrin were studying the mysterious cathode rays that were known at the time to consist of particles smaller than the smallest atom. Perrin showed that cathode rays actually carried negative electrical charge. Later, Thomson's work led him to declare, "I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter."
It took several years of further experiments to confirm Thomson's interpretation of the experiments, but science had in fact discovered the particle that carries the fundamental unit of negative electrical charge. We now know this particle as the electron.
Atoms, however, were known to be electrically neutral, which means that they carry the same amount of positive and negative charge, so their net charge is zero. Because electrons are negative, some other part of the atom must contain positive charge. Thomson put forth what is called the plum pudding model, in which he described atoms as being made of thousands of electrons swimming around in a nebulous mass of positive charge, as shown by the left-side image of Figure 18.3. His student, Ernest Rutherford, originally believed that this model was correct and used it (along with other models) to try to understand the results of his experiments bombarding gold foils with alpha particles (i.e., helium atoms stripped of their electrons). The results, however, did not confirm Thomson's model but rather destroyed it! Rutherford found that most of the space occupied by the gold atoms was actually empty and that almost all of the matter of each atom was concentrated into a tiny, extremely dense nucleus, as shown by the right-side of the image below. The atomic nucleus was later found to contain particles called protons, each of which carries a unit of positive electric charge.
The left drawing shows Thompson's plum-pudding model, in which the electrons swim around in a nebulous mass of positive
charge. The right drawing shows Rutherford's model, in which the electrons orbit around a tiny, massive nucleus. Note that the
size of the nucleus is vastly exaggerated in this drawing. Were it drawn to scale with respect to the size of the electron orbits,
the nucleus would not be visible to the naked eye in this drawing. Also, as far as science can currently detect, electrons are
point particles, which means that they have no size at all.
Protons and electrons are thus the fundamental particles that carry electric charge. Each proton carries one unit of positive charge, and each electron carries one unit of negative charge. To the best precision that modern technology can provide, the charge carried by a proton is exactly the opposite of that carried by an electron. The SI unit for electric charge is the coulomb (abbreviated as "C"), which is named after the French physicist Charles Augustin de Coulomb, who studied the force between charged objects. The proton carries +1.602x10-19C and the electron carries -1.602x10-19C. The number n of protons required to make +1.00 C is
n = 1.00 C x (1 proton)/(1.602x10-19C) = 6.26x1018 protons.
The same number of electrons is required to make -1.00 C of electric charge. The fundamental unit of charge is often represented as e. Thus, the charge on a proton is e, and the charge on an electron is -e. Mathematically,
e = 1.602x10-19C
Conservation of Charge
Because the fundamental positive and negative units of charge are carried on protons and electrons, we would expect that the total charge cannot change in any system that we define. In other words, although we might be able to move charge around, we cannot create or destroy it. This should be true provided that we do not create or destroy protons or electrons in our system. In the twentieth century, however, scientists learned how to create and destroy electrons and protons, but they found that charge is still conserved. Many experiments and solid theoretical arguments have elevated this idea to the status of a law. The law of conservation of charge says that electrical charge cannot be created or destroyed.
The law of conservation of charge is very useful. It tells us that the net charge in a system is the same before and after any interaction within the system. Of course, we must ensure that no external charge enters the system during the interaction and that no internal charge leaves the system. Mathematically, conservation of charge can be expressed as
qinitial = qfinal.
where qinitial is the net charge of the system before the interaction, and qfinal is the net charge after the interaction.
Article source: OpenStax is a nonprofit educational technology initiative based at Rice University. OpenStax's mission is to improve educational access and learning for everyone. Textbooks on OpenStax's site are licensed under a Creative Commons Attribution 4.0 International License.
