Real World Instrumentation with Python

Basic Circuit Theory

Electricity flows when there is a closed circuit that allows for the electrons to move from a high potential to a lower potential. Stated another way, in order to have current flow we need a source of electrons, and there must be a return point for the electrons. Electric current (a physical phenomenon) is characterized by four fundamental quantities: voltage, current, resistance, and power. We’ll use the simple circuit shown in Figure 2-4 as our baseline for the following discussion.

Figure 2-4. Simple electrical circuit

Current that flows in only one direction, as in Figure 2-4, is called direct current (DC). This is what is produced by a common battery, and by the DC power supply in a typical computer system. Current that changes direction repeatedly is called alternating current (AC). This is what comes out of a household wall socket (in the US, for example). It is also the type of current that drives the loudspeakers in a stereo system. The rate at which the current changes direction is called the frequency, and is measured in cycles per second in units of Hertz (abbreviated Hz). So, a 60 Hz signal is composed of a current changing direction at a rate of 60 times per second. We’ll stick to DC circuits for now, and save AC for later.

By convention, current is described as flowing from positive to ground (negative), whereas in reality electrons flow from the negative ...