Tuesday 7-2-96
The relevant section in the textbook is 15.1
Main concepts:
The more detailed picture:
Probably everyone is familiar with the first three concepts, but what does it mean to say that charge is quantized? It means that charge comes in multiples of an indivisible unit of charge, known as the electronic charge, e. In other words, charge comes in multiples of the charge on the electron or the proton. These things have the same size charge, but the sign is different. A proton has a charge of +e, while an electron has a charge of -e.
Electrons and protons are not the only things that carry charge. Other fundamental particles, positrons, for example, also carry charge in multiples of the electronic charge. Those are not going to be discussed, for the most part, in this course, however.
Putting "charge is quantized" in terms of an equation, q = ne. q is the symbol used to represent charge, while n is a positive or negative integer, and e is the electronic charge, 1.602 x 10^-19 Coulombs.
The Law of Conservation of Charge : the net charge of an isolated system remains constant. In other words, if a system starts out with an equal number of positive and negative charges, there's nothing that can create an excess of one kind of charge in that system unless we bring in charge from outside the system (or remove some charge from the system). Likewise, if something starts out with a certain net charge, say with 100 more protons than electrons, it will always have 100 more protons than electrons unless the system is allowed to interact with something external to it.
Charge can be created and destroyed, but only in positive-negative pairs.
Table of elementary particle masses and charges:
| particle | mass | charge |
|---|---|---|
| electron | 9.11 x 10^-31 kg | -1.602 x 10^-19 C |
| proton | 1.672 x 10^-27 kg | +1.602 x 10^-19 C |
| neutron | 1.674 x 10^-27 kg | 0 |