Before we can understand how the computing components work, we need to review a few facts about the flow of electrical current. Electrical current is the result of electrons ( ) flowing from a point with a more negative potential to a point with a more positive potential. Current (denoted by the letter i) flows, by definition, from the higher voltage point to the lower point in a circuit. (Note that the conventional direction of current is opposite that of the electron flow.) In analyzing a circuit, we generally ascribe an arbitrary direction to the current and denote it with an arrow. If the actual current flows in the same direction as we've labeled, then it's magnitude is positive, otherwise, it is negative. Current is given in units of Amperes, but we'll nearly always be dealing in much smaller units of milliamps (mA).
In many circuits, there are points where several branches of the circuit come together. Because current is based on electron flow and because of the conservation of mass principle, the current flowing into such a point must equal that flowing out of it. In other words, the sum of all currents flowing into any point in a circuit must equal zero.
The next thing we need to understand is a device called a resistor. As its name implies, a resistor resists the flow of current. In particular, the current through a resistor is proportional to the voltage across it and inversly proportional to its resistance value. The value of a resistor is given in units called Ohms (denoted ). We will often be dealing with relatively large resistance values measured in kilohms ( ) or megohms ( ). Given the current direction relative to the voltage polarity shown in Figure 1, the current is given by
Figure 1: Voltage and Current in a Resistor
The next important component we will be using is the capacitor. Capacitance refers to the ability to store electrical charge in such a device. The storage value of a capacitor is measured in Farads, but real capacitors are nearly always a tiny fraction of a full Farad. We will usually be dealing in micro-farads ( ). Current through a capacitor is proportional to the capacitance and to the first derivitive of the voltage across it. In Figure 2, the current is given by
Figure 2: Voltage and Current in a Capacitor