Although it may be easier to visualize the behavior of objects under the laws of mechanics that were discussed in the first eight chapters of the text, many of the phenomena of electrostatics should be quite familiar to you in this age of electronics. You simply may not have realized that it was electrostatics that you were observing. The "static cling" that seems to be the bane of the laundry room in television commercials is one contemporary example of electrostatic phenomena.

Experiments have shown that there are two types of electrical charge that are labeled positive and negative as the result of an arbitrary assignment made by Benjamin Franklin. Additional research has led to an identification of the negative charge as being that of an electron. The positive charge is that carried by a proton. The source of the charges need not be known for us to investigate the behavior of charged objects. The most fundamental result is the observation that like charges repel each other, and unlike charges attract each other.

Some materials, notably metals, have the property of allowing charge to move freely on them. They are called conductors. Other materials, such as wood and plastic, do not allow charges to move freely on them. They are called insulators. You may note an analogy between these terms as used here in electrostatics and the same terms as used in thermodynamics; that is no coincidence. In general, good conductors of heat, such as metals, are also good conductors of electricity while poor conductors of heat, such as plastic, are also poor conductors of electricity.

Charge may be applied to an object by placing it in direct contact with another charged object whereupon the two objects share the charge, so that the charge on each is of the same sign. Charge may also be applied by an indirect method called charging by induction. In the case of charging by induction, a charged object is brought near to the object we wish to charge, but it is not allowed to touch the second object. Instead we connect the second object to a third object that can serve as a large source of charge. A person's body may serve as this large source of charge. The charge on the first object attracts charges of the opposite sign to the end of the second object, because unlike charges attract. If we now remove our contact with the second object before moving the original charged object, the second object will have a charge with a sign opposite to that of the original object.

There are two differences between charging by contact and charging by induction. In charging by contact the original charged object actually touches the second object while in charging by induction the objects do not touch each other. Also, in charging by contact both objects end up with the charges of the same sign, while in charging by induction the objects have opposite signs for their charges at the end of the process.

The fact that like charges repel and unlike charges attract means that there is a force involved in electrostatics. We describe that force in terms of Coulomb's Law, but it turns out the same effect on a given charge can be produced by different collections of charges if they are properly arranged. We introduce the concept of electric field to provide a more general description of the behavior of electric charges. Basically the electric field strength is obtained by dividing the electric force by the charge that we use to probe the field in a given area. In that way we are able to express the effect that a given collection of charges has on any charge regardless of the size of that test charge.

The basic unit of charge that we employ in the study of electrostatics is called the Coulomb. The Coulomb can be expressed in terms of a certain number of electron charges. One electron has a charge of magnitude 1.6 x 10^-19 Coulombs.

In equation form, Coulomb's Law is F = k q₁ q₂ / r² where q₁ and q₂ are the respective charges, r is the distance between them, and k is a constant that plays the same role for Coulomb's Law as the universal gravitational constant, G, played in Newton's Law of Universal Gravitation. In our system of units k = 9.0 x 10⁹ N m² / C².

The electric field is defined as the electric force per unit positive test charge. The concept of an electric field enables us to describe the behavior of a charge in a region without dependence upon the size of that test charge. Actually this goal is accomplished quite easily by simply dividing the electric force vector, F, by the size of the charge we are using, q_o. Thus electric field is defined as E = F / q_o and it has units Newton/Coulomb.

Work was defined in Chapter 6 as the product of force times the distance moved, W = F d, so if we express the force in terms of the electric field as F = q E we obtain W = q E d. This equation states that the work done by an electric field upon a charge is equal to the product of the strength of that charge, the strength of the electric field, and the distance the charge moves through that electric field. You may recall that in Chapter 6 we identified the work done by a conservative force with a change in potential energy. The electrostatic force is a conservative force, so it is natural for us to do the same here by defining the electrical potential energy as DPE = q E d. We carry the process one additional step in defining a new quantity, the electric potential, by dividing the electrical potential energy by the charge to obtain DV = DPE / q.

As with the definition of electric field we use a positive charge as our standard of reference. With potential energy expressed in Joules and charge in Coulombs the unit for electric potential is 1 Volt = 1 Joule / Coulomb. We use the electric potential expressed in Volts extensively in electric circuit analysis, because it is much easier to use the scalar quantity than to use the vector electric field.

We can aid the process of visualizing the behavior of electrical charges even more if we use electric field lines to express the strength of the electric field in a given region. The lines are drawn so that they begin on a positive charge and end on a negative charge. The direction of the lines is the direction in which a positive test charge would move if it were to be placed at a given point in space. Figures 12.14, 12.15, and 12.16 on page 243 in the text show diagrams of such field lines in two dimensions. In regions where the field lines are closer together the electric field (and hence the electric force on a charge) is stronger. In regions where the lines are farther apart the electric field is weaker. You can check this for yourself using Figure 12.14 on page 243 in the text. According to Coulomb's Law the electric field should be stronger closer to the positive charge and the electric field lines are indeed closer together near to the charge. At greater distances from the charge the lines are farther apart just as we expect from the inverse square relationship in Coulomb's Law.

Electric fields are vectors. This means proper consideration of their effects upon charges requires the consideration of magnitude and direction. Physicists have introduced the concept of the electrical potential as a scalar quantity to provide an easier way to describe electrostatic effects. The concept of electrical potential is defined in terms of the work done in moving an electric charge through an electric field. The electric potential is simply the work done per unit charge in moving that charge through an electric field. Its units are Volts, and it is always expressed as a potential difference. There must be some reference point for measuring electrical potential difference just as we always had to have a reference point for the gravitational potential energy. The electrical potential is the same quantity and the Volt is the same unit that you use when you talk about purchasing a 9-volt battery for your portable radio or plugging your television set into a 110-volt outlet in your house.