Chapter 17-3b 1

A convenient aid for visualizing electric field patterns is to draw lines pointing in the direction of the electric field called electric field lines. 2

Although these lines do not really exist, they are useful for analyzing fields by representing both the strength and the direction of the field at different points in space. 3

This beneficial because the field of each point is often the result of more than one charge. 4

Usually, electric field lines are drawn so that the electric field vector, E, is tangent to the lines at each point. 5

Further the number of lines per unit area through a surface perpendicular to the lines is proportional to the strength of an electrical field of a given region. 6

Thus, E, is stronger where the field lines are close together and weaker where they are further apart. 7

A positive point charge is represented by a circle in the middle with rays pointing outward radially, somewhat like quills radiate from the body of a porcupine. 8

Because a positive test charge placed in this field would be repelled by the positive charge, extending to infinity. Similarly, the electric field lines for a negative point charge are directed inward towards the charge. 9

10

Note that the lines are closer together as they get near a charge, indicating that the strength of the field is increasing. 11

The figure shows the electric field lines for two point charges of equal magnitudes but opposite signs - this is an electric dipole. 12

In this case, the number of lines that begin at the positive charge must equal the number of lines terminating on the negative charge. 13

The figure shows the electric field lines in the vicinity of two equal positive point charges which result in the same number of lines emerging from each charge as they are equal in magnitude. 14

A good electric conductor (Cu) contains charges that are not bound to any atom and are free to move about within the material. When no net motion of charge is occurring within a conductor, the conductor is said to be in electrostatic equilibrium. 15

A conductor that is isolated has four properties: (1) The electric field is zero everywhere inside the conductor (2) Any excess charge on an isolated conductor resides entirely on the conductor’s outer surface. 16

(3) The electric field just outside a charged conductor is perpendicular to the conductor’s surface (4) On an irregularly shaped conductor, charge tends to accumulate where the radius of curvature of the surface is smallest (at sharp points) 17

In nuclear physics research, a Van de Graaff generator is often used to generate electric charge. 18

If protons are introduced into a tube attached to the dome, the large electric field of the dome exerts a repulsive force on the protons causing them to accelerate to energies high enough initiate nuclear reactions between the protons and various target nuclei. 19

A simple Van de Graaff generator is shown below. It is designed to avoid ionizing the air, which would allow charge to leak off into the atmosphere. 20

In a Van de Graaff generator, charge is transferred to the dome by a rotating belt that is kept in motion by a pulley. The charge is deposited on the belt and then transferred to the dome at the top. 21