Tutorial 1

Thinking about electricity

Introduction

Welcome to my new electronics tutorials! This section is dedicated to all the budding hobby-types out there who just can't seem to wrap their heads around this thing called electronics. I am going to attempt to be as thorough as I possibly can here, including the mathematics, but I will attempt to make this as easy to understand as possible. Also, since I am not about to write a textbook, this is just in plain conversation style. Sound ok?

These tutorials will probably be updated later with a whole pile of pictures, when I have a chance.

The Electron

Theory

As we've all guessed by now, the electron is the basis of electronics. Right?

Well, actually, it's not quite as easy as that. You were perhaps taught that an electron is a negatively charged particle, and that they flow from negative to positive, and that the flow of them is called a current. Right?

Now, here is where things get interesting. Electrons, by their very nature, are negatively charged, and will thus repel other electrons. So how do they carry a current?

In a non-ionized solid material (which accounts for most things you will see) a balance exists between the positive charged nuclei and the negatively charged electrons. Because of this, the electrons are shielded from their mutual repulsion by the nuclei, and hence they don't move by themselves.

Now, we have to remember something. Electric fields (what causes the electrons to be repelled from each other) increase according to inverse squares as two electrons get closer. In english, the closer two electrons are to each other, the more they want to get away from each other.

In an insulator, the electrons are very tightly held to the atoms, which are essentially fixed in position. As a result, other electrons can't normally move an electron from the insulator.

In a conductor, on the other hand, some electrons are only stabilized, but not actually bound, by the surrounding solid matter. In this case, when an electron comes near, the electrons in the matter are free to move away from it, and hence some current flows. Interestingly, the reverse is also true, when an electron moves away from the matter, it leaves a gap of more positive charge behind it, causing electrons to move towards the gap, and hence a current flows again.

This is a very important note. Electricity can be induced to flow in either direction, from either end. For this reason, we will adopt the "conventional current" model, which shows charge flowing from the positive to the negative. After all, it's all the same, right?

There are, of course, some cases where no material exists to cause this positive gap to form. The flow of charge in a vaccum, for instance, does this, but it is left for a later tutorial to show how charge flows in a vaccum.

Analogy

Electricity behaves a lot like water. The one difference is, we generally know more about water than electricity. Don't take this as a tutorial about fluids, though, since I'm going to pretend that water is EXACTLY like electricity, which it is not (they are close, but not identical).

A current flowing in a wire can be thought of the same as water in a pipe. Assume the water is in the pipe to begin with. When water is forced in one end, it is pushed out the other, and water flows through the pipe. When water is sucked from one end (assume the other end is in a vat of water), water is pulled through the pipe, and again there is a flow of water in the pipe. (We ignore the ability of water to have a maximum suction at the atmospheric pressure, since this effect does not exist in electricity).

Water flowing in either direction is able to do work. Of course, if we are trying to supply a garden hose, we can only use water in one direction. By the same token, if we are trying to spray electrons on something (say, a cathode ray tube in a TV or monitor), we need electrons in one direction. But for all other uses, it doesn't much matter which direction the flow is.

And hence, we use "conventional current" flow for charge, which is to say, from positive to negative. (Don't flame me for how stupid conventional current is, it's "conventional", and still works well in practice).

Current and Voltage

Theory

Current and voltage are two interrelated concepts that are governed by the following three laws:

Ohm's Law: V = I * R (Voltage = Current * Resistance)
Kirchoff's Voltage Law (KVL): Sum Voltage Rise = Sum Voltage Drops
Kirchoff's Current Law (KCL): Sum Current Enters = Sum Current Leaves

These laws define most of electronics. To explain what they actually mean, we will now jump to an analogy section.

Analogy

Current is defined as an amount of charge passing a point in a unit of time. We can think of this in the same way as, in a garden hose, a certain volume of water passes by a point in the hose in a certain time. The more open the hose nozzle, the more water comes gushing out in the same time.

Voltage was called, at one time, "Tension", and for a very good reason. Imagine for a second that a garden hose causes a certain amount of viscous resistance to the flow of water (there is that resistance word!). To get a certain amount of water to flow through the hose, you need a certain amount of pressure to push it through. Right?

Hey, wait... V=I*R. Voltage (Pressure?) = Current * Resistance. To get a certain current, I, flowing through a wire with resistance R, you need to push it through with a voltage of V. Ohm's Law. Right?

In reverse, you may have a pump that puts out a certain volume of water, but can push REALLY hard if it needs to. So we have a set current I, and a resistance determined by the resistance of hose R. And therefore, the pump is pushing it V hard. Right?

Kirchoff's Current Law (KCL) is even easier to explain. Say you have a hose going to a coupler. At this coupler, it goes into a few other hoses. Some push water into the coupler, some pull water from the coupler. Unless the coupler is leaking all over the place, the amount of water going in has to be the same as the amount of water coming out. Wow, simple!

Now, lets think for a second about what happens when you push water through a hose with a certain resistance. The water coming out the end will be flowing at the same rate as it is going in (again, assuming it's not making your floor really really wet), but it doesn't have as much push coming out as going in. This pressure drop because of the hose resistance is like a voltage. If the system were closed loop (which, in electronics, it always is), then the amount of pressure the pump adds would have to equal the amount of pressure used by the hose. If you had several hoses and several pumps, the pressure drop across each hose would have to be equal to the pressure increase across the pumps. Kirchoff's Voltage Law. Right? (Don't flame me if this paragraph isn't quite valid fluid mechanics, this is the way it works in electronics, and that's what matters here.)

This also points out that, at each coupler, there is only one magnitude of pressure on the coupler wall. By the same token, at a wire junction in electronics, the junction has only one value of voltage.

Conclusion

And thus ends the section, "Thinking About Electricity". Next tutorial, when it is ready, will be about sources and resistances.

All material on these pages is Copyright (c) Jennifer E. Elaan.