The Electric Field

The electric field E at a point in space is defined as an electric force F, acting on a positive test charge q divided by the magnitude of the test charge

Units of Electric Field 1 N/C

Superposition Principle for Electric Fields

Field Lines

1.At every point of electric field line, electric field E is tangent to this line. No two field lines can cross! 2.The line must begin at positive charge and terminate on the negative one unless go to infinity. 3.The number of line per unit area is proportional to the magnitude of electric field.

Electric Fields and Conductors

Important: electric field inside of good conductor is zero!!!!

Important: any net charge on a good conductor distributes itself on the surface!!!!

Electric field is always perpendicular to the surface of conductor:

Conceptual Example A hollow metal box is placed between two parallel charged plates. What’s the field like inside the box?

q Q P d d Is it possible to place a charge at point Q such that the electric field produced at point P by the two charges will be add to zero?

Electric Potential and Electric Energy

Electric Potential and Potential Difference

We define the change in electric potential energy, PE a -PE b when a charge moves from some point b to some point a, as the negative of the work done by the electric force to move the charge from b to a.

It’s useful to define the electric potential (or simply the potential) as a potential energy per unit charge:

Units of Electric Potential 1 V = 1 J/C

Relation between Electric Potential and Electric Field

Important: this simple formula for E may be used only for uniform field!!!!

Equipotential Lines

The equipotential surface must be perpendicular to the surface.

Important: the surface of a good conductor is always an equipotential surface!!!!

1.Even though electric forces are very much stronger than gravitational forces, gravitational forces determine the motions in the solar system. Why? 2. We compared the electric and gravitational forces between an electron and a proton. Why is the result valid for all separations? 3. When we approach another person, we are not aware of the gravitational and electric forces between us. What are the reason in each case?