This chapter is from the book
This chapter is from the book
This chapter is from the book
1.4. Charge Field
Most of us learned in school that “like charges repel” and “unlike charges attract” each other. Every electron has a negative charge, and that charge is equal to every other electron’s charge. Electrons, being of like charge, tend to repel each other. So, as shown in Figure 1-2, if one electron jumps into a valence band that is already occupied by another electron, it will tend to repel the other electron onward.
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We might picture a charged particle as being a sphere, as shown in Figure 1-3. A charged particle radiates an electric field away from it that extends out in all directions. The strength of this field is inversely proportional to the square of the distance.
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Figure 1-3. A charged particle has an electric field that radiates away from it in all directions.
Suppose we have a surface (maybe a trace) with some “extra” electrons at a point along it. By “extra” I mean more electrons than would normally be contained in the atomic elements making up the trace. (We’ll talk later about how we might make that happen.5) Suppose we have another surface (maybe another trace) nearby with the same number of extra electrons on it. The extra electrons, being the same charge, will tend to repel one another.
Furthermore, they will do so with some force. The force might be very small, or it might be quite large, depending on the number of extra electrons. If the extra electrons are free to move, they may move away from each other, and in doing so, they would become moving electrons, or current. The force with which they repel one another is inversely proportional to the square of the distance between them, so by moving away from one another, the force is reduced.
If the electrons are not free to move, the force will still exist between them. If nothing moves, we call this a static force. If there is a force between two objects (or sets of objects), we can imagine that a force field exists between them. That force field is strongest along the shortest (most direct) line between them. The field weakens as we move farther away from this line. This field is called a charge field or an electrical field. Any time we have two charges separated by some distance, there will be a force field between them.
Now suppose we have the same situation as before, and the two sets of charges are not equal. If there are dissimilar charges, there will be a force between them that is attractive. That is, the two sets of charges will want to move toward each other.
It is very important to notice something here. The two sets of charges do not have to be opposite each other. That is, one set does not have to be negative and the other positive. The charge difference between two or more objects is a relative concept. We can have two negative sets of charges with one more negative than the other or two positive sets of charges with one more positive than the other. There will be an attractive force in either case that will be proportional to the net difference in their total charge.