March 5March 5

AP PhysicsAP Physics

IN:If the distance between two positive point

charges is tripled, then the strength of the electrostatic repulsion between them will decrease by a factor of

a) 3

b) 6

c) 8

d) 9

e) 12

Objective: To understand:

– Electric field

Success Criteria:

Be able to calculate electric forces and fields

Electric Field

Electric Field

E = Fe

qo

Fe = kqq

r2

E = kq

r2

Electric Field

Direction: The direction of the force a positive charge would feel.

Strength:

E=kq

r2

Conducting spheres

Electric field is zero everywhere inside the conducting material

All excess charge is found on the surface of the material.

The electric field just outside the conductor s perpendicular to the surface.

Charge accumulates at points

Electric field between two plates

Between two plates (except at the edges) the electric field is uniform.

An airplane is flying through a thundercloud at a height of 2 000 m. If there are charge concentrations of +40.0 C at a height of 3 000 m within the cloud and −40.0 C at a height of 1000 m, what is the electric field at the aircraft? at the aircraft?

Electric field lines

1. Lines begin on positive charges and end on negative.

2. The number of lines leaving a positive charge or ending on a negative is proportional the the magnitude of the charge.

3. No lines can cross eachother4. Lines are perpendicular to the surface of

the conductor

Draw the field lines around two charges:

• positive charge of 1 Coulomb and negative charge of .5 Coulombs.

• Two negative charges 1 Coulomb and 2 Coulombs

Robert Millikan

Oil Drop experiment

• Measured

charge to mass

ratio of electron

• Published in 1913

Nobel prize 1932

Millikan oil-drop experiment

qE = mg

q/m=g/E

qe= -1.6x10-19

Electrostatic problem set

Out:  +Q P -Q

 The figure above shows two point charges, +Q and –Q. If the negative charge were absent, the electric field at point P due to +Q would have strength E. With –Q in place, what is the strength of the total electric field at P, which lies at the midpoint of the line segment joining the charges?a) 0

b) E/4

c) E/2

d) E

e) 2E