note: This post relies heavily on the basics explained in Part 1.

As was mentioned in the previous post, mankind domesticated electricity. We now have batteries, which are electrically charged objects with two terminals and a known and measured voltage between them:

There’s potential electricity in sockets which can be found in most of the walls around the house – electric tension just waiting to get released through an electric appliance. Mankind also became master of the numbers representing voltage, resistance and current. We can calculate the voltage and resistance to allow a the right amount of current through circuits that drive electromagnetic motors without burning the wires.

At the base of the design of an electrical appliance, such as a computer for example, there’s always a circuit through which electrons flow. They provide the energy to make things happen in the circuit (light up a light bulb for example).

A circuit contains the following elements:

1. A Source, an electrically charged object with two terminals with voltage between them (can be a battery for example). These two terminals are usually dubbed positive and negative, and it agreed upon that current flows (at least should flow by design) from the positive to the negative terminal, or from the source to the “ground”.
2. A Load, something that will harness the energy of electron flow and thus induce resistance to the circuit. Having a load in a circuit brings actual meaning to a circuit. Connecting the positive and negative terminals with a conductive wire without a load would make turn the low resistance wire into a load, causing a massive amount of current to flow through a wire. Depending on the amount of electrons or voltage the source can supply, the massive flow of current can create explosive results:

Remember that when there’s tension and resistance involved, current will tend to flow through the path of least resistance. When suddenly introducing a low resistance path to a circuit with a load, you create a “short circuit” for the current which will probably look like the above picture (depending on the amount of current the source can provide). Notice that the current flows directly through the metal screw but does not flow towards the pliers, because the metal screw provides the path of least resistance to alleviate the electric tension. If you’re wondering why the screw burns up while the copper wires don’t, it’s because the screw provides much more resistance, which is analogous to friction which generates heat.

Now lets look at a diagram of a basic DC (direct current, the same we’ve been talking about since the previous post) circuit:

The source and load are labeled, and the symbol at the bottom and top left represents the point of lowest possible electrical tension, or “ground”. The source generates tension measured by 9 volts, which will drive the current through the load towards the ground.  Using the number representing the resistance of the load, you can calculate the number representing the current running from the source to the ground (using Ohms law described in the previous post). Should be simple to understand based on what we know so far.

Another important component that can be found in circuits is a capacitor. The idea behind the capacitor is simple – take two metal plates, put them very close to each other (without them touching). Now saturate one of them with electrons, and drain electrons from the other. What you’ll get is on one plate being pulled towards the overall positive charge on the other plate. Since there’s no medium through which these electrons can flow, they are just stuck on the edge of their plate:

The term “holes” is used above to describe the lack of electrons in atoms, meaning a positive electrical charge.

Now the interesting thing happens when you introduce a capacitor into a DC circuit.  When you connect the capacitor to a source, current will flow as one side of the plate saturates with electrons while the other side has its electrons drain to ground (or the “negative” terminal of the source, which usually represent the exact same thing), and so the capacitor gets “charged”. Eventually, the amount of electrons on the saturated plate will completely counter the flow of new electrons and the current will drop to 0. Everything stops, as if the wire was cut. Now it is possible to disconnect the source, and connect the capacitor to another circuit as a source by itself!

In the above picture, we have the source (a 9 volt battery) , a load (a resistor and a light emitting diode) and a capacitor (in the center of the circuit). If the switch is set to point A, current will flow to the capacitor saturating one side with electrons, and draining the electrons from the other side. Eventually, the capacitor is at its full capacitance, at which point the switch is set to point C, where the electron under electric tension will finally receive a path to reach the other low tension plate. This way goes through a resistor, limiting the current to a controlled and predictable level.

By this point, we’ve covered the basics needed to understand the function of semi-conductors (which are the most basic block of a computer as we know it), which will be discussed in the next post.

Hope you found this post informative. Feel free to leave comments, and ask questions.

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