Electric Fields

Force over a distance  Both gravity and electric force act over a distance without touching (unlike other forces)  Very difficult for early scientists to accept  Michael Faraday proposed the Electric Field to explain

Electric Field  Force that extends outward from any charged object and permeates through space  Any second charged object placed around the first charge will feel a force of attraction or repulsion due to this field  Tested with an imaginary positive test charge (q)

 Magnitude of the force acting on the test charge can be measured

Definition  Electric field (E) is defined as the amount of force (F) per unit of charge (q) E = F/ q units N/C

Another Equation  E = F/ q  F = k qQ/ r 2  E = k Q/ r 2

Example  Find the magnitude and direction of the electric field on a particle P which is located 30 cm right of a point charge of –3.0 x C.

Copying machines work by (+) charges on a drum attracting (-) charged ink particles. When a paper runs over the drum, the ink sticks to paper. Each ink particle has a mass of 9.0 x kg and carries 20 electrons. In order to stick, the force of attraction must be at least 2x greater than its weight. What is the field strength at the drum’s surface?

 If the field is due to more than one charge, the total field is the sum of the two individuals

Example  Find the total field acting on point P. If P is a proton initially at rest, what will its acceleration be? P Q 1 = -25μC Q 2 = +50μC

Field Lines  Lines of force can be drawn to visualize the force acting on a single point charge  Stronger fields have more lines  On point charges  On double points  Start on + and go to -  On plates

Fields and Conductors  The electric field inside any good conductor is zero - charge distributes itself evenly over the surface of a conductor making the net field inside zero  Electric field is always perpendicular to the surface of a conductor  Excess charge tends to accumulate on sharp points or areas of greatest curvature

What will happen? +