3.1 Charge

To answer this question, we need to answer another question first: What is electricity? Electricity is the flow of charge. So what is charge?

Charge is a fundamental quantity in physics—it is not a combination (that we know of) of any other quantity. A particle can be charged in one of three ways—it can be positively charged (represented by a + sign), negatively charged (represented by a – sign), or neutrally charged (i.e., has no charge). Figure 3-1 shows what an atom looks like. In the center of the atom are larger, heavier particles called protons and neutrons. Protons are positively charged particles, and neutrons are neutrally charged particles. Together, these form the nucleus of the atom and determine which atom we are talking about. If you look on a periodic table, the large printed number associated with an element is known as its atomic number. This number refers to how many protons it has in its nucleus. Sometimes, there is a number in smaller print as well. This is the total number of protons and neutrons combined.¹ Note that the number of neutrons in an element can vary, so this number is often a decimal representing the average number of combined protons and neutrons in any particular element.

Circling around the nucleus are electrons. Electrons are negatively charged particles. Even though electrons are much smaller and lighter than protons, the amount of negative charge of one electron is equal to the amount of positive charge of one proton. Positive and negative charges attract each other, which is what keeps electrons contained within the atom. Electrons are arranged in shells surrounding the nucleus. The outermost shell, however, is the most important one when thinking about how atoms work.

When we think about individual atoms, we think about them when they are isolated and alone. In these situations, the number of electrons and the number of protons are equal, making the atom as a whole electrically neutral. However, especially when atoms interact with other atoms, the configuration of their electrons can change. If the atoms gain electrons, then they are negatively charged. If the atoms lose electrons, then they are positively charged. Free electrons are all negatively charged.

If there are both positively and negatively charged particles moving around, their opposite charges attract one another. If there is a great imbalance of positive and negative charges, usually you will have a movement of some of the charged particles toward the particles of the opposite charge. This is a flow of charge, and this flow is what is referred to when we speak of electricity.

The movement of charge can be either positively charged particles moving toward negatively charged ones, negatively charged particles moving toward positively charged ones, or both. Usually, in electronics, it is the electrons which are moving through a wire, but just know that this is not the only way in which charge can move.

Electricity can be generated by a variety of means. The way that electricity is generated in a battery is that a chemical reaction takes place, but the reactants (the substances that react together) are separated from each other by some sort of medium. The positive charges for the reaction move easiest through the medium, but the negative charges for the reaction move easiest through the wire. Therefore, when the wire is connected, electricity moves through the wire to help the chemical reaction complete on the other side of the battery.

This flow of electric charge through the wire is what we normally think of as electricity.

3.2 Measuring Charge and Current

Atoms are very, very tiny. Only in the last few years have scientists even developed microscopes that can see atoms directly. Electrons are even tinier. Additionally, it takes a lot of electrons moving to have a worthwhile flow of charge. Individual electrons do not do much on their own—it is only when there are a very large number of them moving that they can power our electronics projects.

Therefore, scientists and engineers usually measure charge on a much larger scale. The coulomb is the standard measure of electric charge. One coulomb is equivalent to the electric charge of about 6,242,000,000,000,000,000 protons. If you have that many electrons, you would have a charge of −1 coulomb. That’s a lot of particles, and it takes particles on that scale to do very much electrical work. Thankfully, protons and electrons are very, very small. A typical 9-volt battery can provide about 2,000 coulombs of charge, which is over 10,000,000,000,000,000,000,000 charged particles (ten thousand billion billion particles).

However, electricity and electronics are not about electric charge sitting around doing nothing. Electricity deals with the flow of charge. Therefore, when dealing with electricity, we rarely deal with coulombs. Instead, we talk about how fast the electric charge is flowing. For that, we use amperes , often called amps and abbreviated as A. One ampere is equal to the movement of 1 coulomb of charge out of the battery each second.

For the type of electronics we will be doing, an ampere is actually a lot of current. In fact, a full ampere of current can do a lot of physical harm to you, but we don’t usually deal with full amperes when creating electronic devices. Power-hungry devices like lamps, washers, dryers, printers, stereos, and battery chargers need a lot of current—that’s why we plug them into the wall. Small electronic devices don’t usually need so much current. Therefore, for electronic devices, we usually measure current in milliamperes, usually called just milliamps and abbreviated as mA. Remember that the prefix milli- means one-thousandth of (i.e., or 0.001). Therefore, a milliamp is one-thousandth of an amp. If someone says that there is 20 milliamps of current, that means that there is 0.020 amp of current. This is important, because the equations that we use for electricity are based on amps, but since we are dealing with low-current devices, most of our measurements will be in milliamps.

However, our question asked about how much has moved after 1 minute. Since there are 60 seconds in each minute, we can multiply 0.037 by 60 for our answer. 0.037 ∗ 60 = 2.22. So, after 1 minute, 37 milliamps of current moves 2.22 coulombs of charge.

3.3 AC vs. DC

You may have heard the terms AC and DC when people talk about electricity. What do those terms mean? In short, DC stands for direct current, and AC stands for alternating current . So far, our descriptions of electricity have dealt mostly with DC. With DC, electricity makes a route from the positive terminal to the negative. It is the way most people envision electricity. It is “direct.”

However, DC, while great for electronics projects, very quickly loses power over long distances. If we were to transmit current that simply flows from the positive to the negative throughout the city, we would have to have power stations every mile or so.

So, instead of sending current in through one terminal and out through another, your home is powered with alternating current. In alternating current, the positive and negative sides continually reverse, switching back and forth 50–60 times per second. So the current direction (and thus the direction the electrons are moving) continually switches back and forth, over and over again. Instead of moving in a continuous flow, it is more like someone is pushing and pulling the current back and forth. In fact, at the generator station, that is almost exactly what is going on! This may seem strange, but this push and pull action allows for much easier power generation and also allows much more power to be delivered over much longer distances.

AC, such as the current that comes out of a wall socket, is much more powerful than we require for our projects here. In fact, converting high-power AC to low-power DC voltage used in electronic devices is an art in itself. This is why companies charge so much money for battery chargers—it takes a lot of work to get one right!

Now, not all AC is like this. We call this current AC “mains,” because it comes from the power mains from the power stations. It is supposed to operate at about 120 volts, and the circuits are usually rated for about 15–30 amps (that’s 15,000–30,000 milliamps). That’s a lot of electricity!

In addition to AC mains, there are also ACs which we will call AC “signal.” These currents come from devices like microphones. They are AC because the direction of current does in fact alternate. When you speak, your voice vibrates the air back and forth. A microphone converts these air vibrations into small vibrations of electricity—pushing and pulling a small electric current back and forth. However, these ACs are so low powered as to be almost undetectable. They are so small we have to actually amplify these currents (see Chapter 25) just to work with them using our DC power!

So, in short, while we will do some work with AC signal voltages later in the book, all of our projects will be safe, low-power projects. We will often touch wires with our projects active or use multimeters to measure currents and voltages in active circuits. This is perfectly safe for battery-operated projects. But do not attempt these same maneuvers for anything connected to your wall outlet unless you are properly trained.

3.4 Which Way Does Current Flow?

One issue that really bungles people up when they start working with electronics is figuring out which way electric current flows. You hear first that electric current is the movement of electrons, and then you hear that electrons move from negative to positive. So one would naturally assume that current flows from negative to positive, right?

Good guess, but no—or, at least, not quite.

Current is not the flow of physical stuff like electrons, but the flow of charge. So, when the chemical reaction happens in the battery, the positive side gets positively charged, and the influence of that charge moves down the wires. The electrons are a negative charge that moves toward that positive charge. The positive charge is just as real as the negative charge, even though physical stuff isn’t moving with the positive charge.

Think about it this way. Have you ever used a vacuum cleaner? Let’s say we are tracing the action of a vacuum cleaner. Where do you start? Usually, you start at the inside where the suction happens and then trace the flow of suction through the tube. Then, at the end of the tube, the dust comes into the tube.

Engineers don’t trace their systems from the dust to the inside, they trace their systems from the suction on the inside out to the dust particles on the outside. Even though it is the dust that moves, it is the suction that is interesting.

Likewise, for electricity, we usually trace current from positive to negative even though the electrons are moving the other way. The positive charge is like the suction of a vacuum, pulling the electrons in. Therefore, we want to trace the flow of the vacuum from positive to negative, even though the dust is moving the other way.

The idea that we trace current from positive to negative is often called conventional current flow . It is called that way because we conventionally think about circuits as going from the positive to the negative, and it is the common convention to draw them that way (any arrow in an electronics diagram is pointing toward the movement of positive charge).

If you are tracing charge the other way, that is called electron current flow , but it is rarely used.

3.5 Review

3.6 Apply What You Have Learned