Charge and Charge Motion

All electrical principles rely on the concept of electrical charge, or simply charge. The concept of charge is based on the observation that some bodies exert non-gravitational forces on one another when they are placed close together. Like gravity, this force acts at a distance; but unlike gravity, the bodies can either attract or repel each other (gravity only attracts masses to one another). This force is also much stronger than gravitational forces. We explain this force as a result of these bodies having electrical charges. The ability of electrical force to either attract or repel bodies is explained by theorizing that both positive and negative charges exist—two bodies with the same charge (both positive or both negative) will repel one another, while two bodies with opposite charges will attract one another

Note: We can easily observe the effects of these charges. A negative charge can be produced by rubbing a balloon on a nearby cat (if no cat is available, you can rub it in your own hair). The balloon can then be stuck to the wall, which will be uncharged. The balloon and the wall's charges are opposite relative to one another; so they have an attractive force.

The actual mechanism for creating this force between charges is called an electric field. It is not necessarily crucial to fully understand electric fields in order to work with electricity, but the link to the right provides a brief description of the overall idea. You may want to read it at some point, since we will occasionally mention electrical fields in later exercises.

The above observations and theory leads to a useful electrical viewpoint of materials. We consider objects to be composed of particles which have either positive or negative charges, as shown in Fig. 1. The overall charge associated with the object is due to an imbalance of positive and negatively charged particles in the body. If there are more positive than negative particles, the object will be “positively charged”. If there are more negative than positive charges, the body will be “negatively charged”.

Over time, our knowledge and understanding of the behavior of particles became more refined. We now consider materials to be composed of atoms, which are in turn comprised of more fundamental particles. The most important of these particles (important for us, anyway) are protons and electrons. Protons are positively charged while electrons are negatively charged1).

Protons (and neutrons) are bound together in the nucleus of the atom. They are not easily dislodged from one another2). Electrons, however, are more loosely organized in a sort of “cloud” around the nucleus. Fig. 2 gives the general idea.

Figure 1. Electrical model of matter.

Figure 2. Our model of an atom.

Charge Motion

As it turns out, we can use the force that charges exert to move electrons from one atom to another (the protons are too tightly bound to the nucleus to make them moveable). If we put another electron very close to the atom of Fig. 2, as shown in Fig. 3; the negative charge of this electron will repel the other electrons. If we are insistent about it, we can force our new electron into the electron “cloud” around the nucleus, and force one of the atom's previous electrons out of the cloud. The displaced electron will typically join an adjacent atom, causing one of that atom's electrons to move away. By moving electrons from atom to atom in this way, we can cause a “charge” to move through the material.

Figure 3. Moving charge by moving electrons.

Figure 4 illustrates how this process works when charge is transferred down a wire. We put electrons into one end of the wire (using an electron source— such as a battery). These electrons displace other electrons. The effect propagates down the wire and electrons come out the other side. Notice that the electrons we put into one end of the wire are not the same electrons that are leaving the other end; however, The number of electrons entering must be balanced by the number of electrons leaving, so that we don't “accumulate” any charge in the wire. The situation is analogous to running water through a pipe —water goes in one end, and out the other, but the amount of water in the pipe stays the same.

Figure 4. Charge transfer in a wire.

In electrical circuits, our main goal is to move electrons (or charges) around —as in Fig. 4—in order to perform some task. Since electrons are fundamental to the movement of charge, amounts of charge are defined in terms of electrons. The units of charge are coulombs (abbreviated as C), and one coulomb corresponds to the magnitude of electrical charge in 6.24 x 1018 protons or electrons3).

Insulators, Conductors, and Resistance

In Fig. 4, the electrons which are being transferred from atom to atom will encounter some opposition to their movement. It takes energy4) to cause an electron to leave its atom. The amount of opposition to the movement of electrons is a property of the material. Some materials allow their electrons to move easily from atom to atom, these materials are called conductors. Other materials have atoms which hold on tightly to their electrons, these are called insulators. In conductors, it takes very little energy to move charges through the material. Insulators, on the other hand, require a lot of energy to cause charges to move through them.

The degree of difficulty in moving electrons in a material is characterized as resistance. Loosely speaking, resistance quantifies the amount of energy required to cause a given rate of electron motion through a material. Insulators have very high resistance (they require a lot of energy to get electrons to move through them), while conductors have a low resistance (very little energy is required to move electrons). Materials such as copper, silver, and gold have very low resistances, while materials such as ceramics and rubber have very high resistances.

Important Points

  • Charges placed next to one another cause a force between them. If the charges are the same, the force repels them from one another. If the charges are different, the force attracts them together.
  • Charges in materials are carried by protons and electrons. Protons have positive charge, and electrons have negative charge. Protons don't move around easily, so we transfer charges in electric circuits by moving electrons.
  • When charge is transferred through a material, electrons move from atom to atom within the material. If we transfer charge through a wire, the electrons we put into one end of the wire are not necessarily the same electrons we get out of the other end.
  • Resistance is a property which tells us how easy it is to move charge through a material. A low resistance means that little energy is required to move charge through the material, while high resistance means that a lot of energy is necessary to force charges to move through the material. Materials are often classified according to their resistance; insulators have high resistance, while conductors have low resistance.
  • As electrical engineers, we are mostly concerned with moving charges around, or energy transfer—we generally move electrical energy around in order to perform useful work, like spinning a motor, or lighting a light bulb. It is awkward to deal with energy transfer directly in terms of charge, so we introduce a couple of parameters that directly relate to electrical energy: voltage and current. These are discussed in the next project.
, , , ,
1)
Neutrons are also important, if we are to explain the mass of atoms. But they carry no electrical charge, so we don't need to include them in this discussion.
2)
There are four types of forces. Protons and neutrons are held together by nuclear force—it is the strongest of the four. The force which repels or attracts charges is the electromagnetic force; it is the second-strongest type— electromagnetic forces are on the order of 100 times weaker than nuclear force. Gravitational force, which is probably the most apparent in our day-to-day life, is the weakest of the four.
3)
The negative sign is because we think in terms of movement of positive charge. When these concepts were originally formulated, the above model of the atom didn't exist, and we didn't know that charge motion was due to the movement of negative charges. Ben Franklin hypothesized the movement of positive charges, and his convention stuck. He also provided us with many useful sayings such as “early to bed and early to rise make a man healthy, wealthy, and wise” and “A penny saved is two pence clear, a pin a day's a groat a year”. Thanks, Ben!
4)
We will define what we mean by energy later