The Electric Field

THE ELECTRIC FIELDChapter 17.3

Field Force• Electric force, just like gravitational force is a field force.

• A field force is capable of acting through space, producing an effect even when there is no contact between two objects.

Electric Field• Electric Field – A field that permeates the space around a

charge object and in which another charged object experiences an electric force.

Electric Field of Point Charge

E = Strength of Electric Fieldkc = Coulomb’s Constantq = Charge producing the electric fieldr = Distance

Electric Field Units• Strength of Electric Field (E) is the force per charge.

• SI Units: • The electric Field is a vector quantity.• If q is positive (+), The field due to this charge is directed

outward radially.• If q is negative (-), the field due to this charge is directed

towards q.

Field Lines From a Point Charge• An electric field at a given point depends only on the

charge (q) of the object setting up the field and the distance to the specific point in space.

• All electric charges create electric fields, even when there is no test charge within the field.

Example Problem• A point charge of -5.1μC is placed into space. Calculate

the strength of the electric field 1.2cm away from the point charge. Which direction is the electric field pointing?

Example Answer

Strength of the field points towards the point charge.

Electric Field Lines• Electric Field Lines – Lines that represent both the

magnitude and direction of the electric field.

• Electric field lines do not really exist, but it allows useful means of analyzing electric fields by representing both strength and direction of the field at different points.

Rules for Drawing Field Lines1. The lines must begin on positive charges or at infinity

and must terminate on negative charges or at infinity.

2. The number of lines drawn leaving a positive charge or approaching a negative charge is proportional to the magnitude of the charge.

3. No two field lines can cross each other.

Electric Field Lines

Realistic Electric Fields

Conductors in Electrostatic Equilibrium

• The electric field is zero everywhere inside a conductor.

• Any excess charge on an isolated conductor resides entirely on the conductors outer surface.

• The electric field just outside a charged conductor is perpendicular to the conductor’s surface.

• On an irregularly shape conductor, charge tends to accumulate where the radius of curvature of the surface is smallest, that is, at its sharp points.

Van de Graff Generator• A common laboratory device for building up high voltages

is the Van de Graaff generator. • In a Van de Graaff generator, a moving rubber belt carries

electrons from the voltage source to a conducting sphere.

Van de Graaff Explained• A large hollow metal sphere is supported by a cylindrical

insulating stand.

• A rubber belt inside the support stand moves past metal needles that are maintained at a high electric potential.

• A continuous supply of electrons is deposited on the belt through electric discharge by the points of the needles.

• The electrons are carried up into the hollow metal sphere.

Van de Graaff Explained• The electrons leak onto metal points attached to the inner

surface of the sphere.

• Because of mutual repulsion, the electrons move to the outer surface of the conducting sphere.

• This leaves the inside surface uncharged and able to receive more electrons.

• The process is continuous, and the charge builds up to a very high electric potential—on the order of millions of volts.