PCB Currents: Fundamentals of Electrons and Charges
1.1. The Flow of Electrons
Current is the flow of electrons. This is the most important definition in this book! If there is a current, electrons are moving, and if there is no current, electrons are not moving. It’s that simple. It is no accident that the world of voltages and currents is called electronics.
When we measure current, we are determining how many electrons are moving. The definition of 1 amp of current is 1 coulomb of charge (6.25 × 1018 electrons) passing by a point (or across a surface) in 1 second. So one way to measure current is to actually count the number of electrons passing by a point in one unit of time. Of course, electrons are small and hard to see, so it is not practical to count each individual one. But if we could, that would be one way to measure current. Actual current measurement methods are outlined in Chapter 2.
Why is it important that electrons can move? The answer is because we can do useful things with that information if we have it under control. Suppose we move electrons back and forth along a conductor at a frequency that is the same as the pitch (frequency) of some music we are listening to. Suppose we move more electrons when the music is louder and fewer electrons when the music is quieter. That is, we let the magnitude of the electron flow (the current) be proportional to the volume of the music. The frequency and magnitude of the current flow therefore becomes an analog of the music. There are several ways we can transmit music across a room, but current flow is a very convenient way to transmit (an analog of) music across long distances.
Or suppose we can place a large number of electrons at one point in a circuit at one point of time and a smaller number of electrons at another point in time. These two different levels of electrons can represent one bit of information.2 If we have thousands or millions of such points available to us (as in a microprocessor), we can assemble these bits of information into meaningful communication or information processing systems.