Electric Field Line Patterns

• Point charge• The lines radiate

equally in all directions

• For a positive source charge, the lines will radiate outward

Electric Field Line Patterns

• For a negative source charge, the lines will point inward

Electric Field Line Patterns

• An electric dipole consists of two equal and opposite charges

• The high density of lines between the charges indicates the strong electric field in this region

Electric Field Line Patterns

• Two equal but like point charges

• At a great distance from the charges, the field would be approximately that of a single charge of 2q

• The bulging out of the field lines between the charges indicates the repulsion between the charges

• The low field lines between the charges indicates a weak field in this region

Electric Field Patterns

• Unequal and unlike charges

• Note that two lines leave the +2q charge for each line that terminates on -q

Question

+ +A

B

C

If a positive test charge is placed at each of the three points, which point experiences the strongest electric field? And which point experience the weakest electric field?

Answer: Strongest at A, weaker at B and zero at C.

Two particles of the same mass carry charges +3Q and –2Q, respectively. They are shot into a region that contains a uniform electric field as shown. The particles have the same initial velocities in the positive x direction. The lines, numbered 1 through 5, indicate possible paths for the particles. If the electric field points in the negative y direction, what will be the resulting paths for these particles?

(a) path 1 for +3Q and path 4 for –2Q

(b) path 3 for +3Q and path 2 for –2Q

(c) path 4 for +3Q and path 3 for –2Q

(d) path 2 for +3Q and path 5 for –2Q

(e) path 5 for +3Q and path 2 for –2Q

Answer: e

Question:

An electron traveling horizontally enters a region where a uniform electric field is directed upward.What is the direction of the force exerted on the electron once it has entered the field?

(a) to the left

(b) to the right

(c) upward

(d) Downward

(e) out of the page, toward the reader

Answer: d

Question:

Conductors in Electric Field

The electric field inside a good conductor is zero in the static

situation Any excess charge resides on

the surface of the conductor

Why?

In a conductor electrons are free to move. If a conductor is placed into E, a force F = -eE acts on each free electron.  Soon electrons will pile up on the surface on one side of the conductor, while the surface on the other side will be depleted of electrons and have a net positive charge.  These separated negative and positive charges on opposing sides of the conductor produce their own electric field, which opposes the external field inside the conductor and modifies the field outside.

Electrons inside the conductor experience no force.

A cavity inside a conductor also is free of electric fieldsA conductor shields its interior from any outside electric fields.  Even if there are holes in the surface, the electric field does not penetrate very far.  A rule of thumb is that the electric field falls to zero over a distance approximately equal to the diameter of the hole.

Why are you safest inside your car during a thunderstorm?

Question

Answer: E=0 inside the car

In the static situation a conductor has the following properties:

  

Any excess charge resides on the surface of the conductor

  

The electric field is zero within the solid part of the conductor

  

The electric field at the surface of the conductor is perpendicular to the surface

  

Charge accumulates and the field is strongest on pointy parts of the conductor