Electric Forces and Fields

The Electric Field

Electric Field Strength

• Electric field – the region in space around a charged object in which a stationary charged object experiences an electric force because of its electric charge– Any charged particle in that electric field will

experience an electric force

Electric Field Strength

• Electric field strength is a vector• Measured in Newtons/Coulomb (N/C)• The direction of the electric field depends on the

sign of the charge producing the field• The strength of an electric field depends on

charge and distance– Positive charges produce an outward electric field– Negative charges produce an inward electric field

Electric Field Strength

Electric Field Strength

• Electric field strength is measured by placing a very small test charge near a larger charge– The test charge (q0) must be very small compared to

the main charge (Q)• If not, is will cause a redistribution of charge on Q, which will

effect electric field strength

• Electric field strength = coulomb constant * charge producing the field/distance2

• E = kCq/r2

• Electric fields exist near any charged body, even in the absence of a test charge

Electric Field Strength

Electric Field Strength

Electric Field Lines

• Show electric fields with electric field lines– Lines that represent both the magnitude and

the direction of the electric field– The stronger the field, the more lines are

present– Point out from a positive charge– Point in toward a negative charge

Electric Field Lines

Electric Field Lines

• Rules for drawing electric field lines– The lines must begin on positive charges or at

infinity and terminate on negative charges or at infinity

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

– Electric field lines from the same field never cross

Electric Field Lines

• Electric dipole – the electric field produced between to objects of equal and opposite charge– The number of electric field lines leaving the

positive charge must be equal to the number of electric field lines entering the negative charge

• If one charge or field is twice as strong as another, it should have twice as many field lines

Electric Field Lines

Electric Field Lines

Electric Field Lines

Conductors in Electrostatic Equilibrium

• Good conductors contain charges (electrons) that are free to move about within the material

• When there is no net motion of charge within a conductor, it is said to be at electrostatic equilibrium

Conductors in Electrostatic Equilibrium

• Electrostatic equilibrium has the following conditions– The electric field is zero everywhere inside the

conductor– Any excess charge on an isolated conductor resides

entirely on the conductor’s outer surface– The electric field just outside a charged conductor is

perpendicular to the conductor’s surface– On an irregularly shaped conductor, charge tends to

accumulate where the radius of curvature of the surface is smallest, that is, at sharp points

Conductors in Electrostatic Equilibrium

• The air surrounding a very strong electric field can become ionized– A bluish glow can be seen as charges leap

from the surface to the air• Called a corona or St. Elmo’s fire

• Van de Graaff generators collect electric charge– Uses static electricity to build up negative

charge in the belt and ball and positive charge on the pulleys moving the belt

Conductors in Electrostatic Equilibrium

Conductors in Electrostatic Equilibrium