By the end of this section, you will be able to do the following:
- List the three properties of a conductor in electrostatic equilibrium
- Explain the effect of an electric field on free charges in a conductor
- Explain why no electric field may exist inside a conductor
- Describe the electric field surrounding Earth
- Explain what happens to an electric field applied to an irregular conductor
- Describe how a lightning rod works
- Explain how a metal car may protect passengers inside from the dangerous electric fields caused by a downed line touching the car
The information presented in this section supports the following AP learning objectives:
- 2.C.3.1 The student is able to explain the inverse square dependence of the electric field surrounding a spherically symmetric electrically charged object.
- 2.C.5.1 The student is able to create representations of the magnitude and direction of the electric field at various distances—small compared to plate size—from two electrically charged plates of equal magnitude and opposite signs and is able to recognize that the assumption of uniform field is not appropriate near edges of plates.
Conductors contain free charges that move easily. When excess charge is placed on a conductor or the conductor is put into a static electric field, charges in the conductor quickly respond to reach a steady state called electrostatic equilibrium.
Figure 1.28 shows the effect of an electric field on free charges in a conductor. The free charges move until the field is perpendicular to the conductor's surface. There can be no component of the field parallel to the surface in electrostatic equilibrium, since, if there were, it would produce further movement of charge. A positive free charge is shown, but free charges can be either positive or negative and are, in fact, negative in metals. The motion of a positive charge is equivalent to the motion of a negative charge in the opposite direction.
A conductor placed in an electric field will be polarized. Figure 1.29 shows the result of placing a neutral conductor in an originally uniform electric field. The field becomes stronger near the conductor but entirely disappears inside it.
Misconception Alert: Electric Field Inside a Conductor
Excess charges placed on a spherical conductor repel and move until they are evenly distributed, as shown in Figure 1.30. Excess charge is forced to the surface until the field inside the conductor is zero. Outside the conductor, the field is exactly the same as if the conductor were replaced by a point charge at its center equal to the excess charge.
Properties of a Conductor in Electrostatic Equilibrium
- The electric field is zero inside a conductor.
- Just outside a conductor, the electric field lines are perpendicular to its surface, ending or beginning on charges on the surface.
- Any excess charge resides entirely on the surface or surfaces of a conductor.
The properties of a conductor are consistent with the situations already discussed and can be used to analyze any conductor in electrostatic equilibrium. This can lead to some interesting new insights, such as described below.
How can a very uniform electric field be created? Consider a system of two metal plates with opposite charges on them, as shown in Figure 1.31. The properties of conductors in electrostatic equilibrium indicate that the electric field between the plates will be uniform in strength and direction. Except near the edges, the excess charges distribute themselves uniformly, producing field lines that are uniformly spaced—hence uniform in strength—and perpendicular to the surfaces—hence uniform in direction, since the plates are flat. The edge effects are less important when the plates are close together.