• Draw the distribution of charges of an electroscope when it is charged by bringing a glass rod that was rubbed against silk near its probe.

BW

BellWork

• If two charges of +1.6x10-19 C are separated by a distance of 2 meters, what is the electrostatic force between them? Is it attractive or repulsive?

+1.6x10-19 C +1.6x10-19C2 meters

BW

• What charge does the dome of a van de Graff generator have?

Electric Fields

• Electric Field – the region around a charged particle through which a force is exerted on another object.

Electric field lines• Imaginary line along which a positive test

charge would move

+Q

Positively Charged Sphere

In other words, the direction of an electric field is direction of the force on a stationary positive charge located any point on the field line (tangent to the line for a curved field line)

Negatively charged sphere

-Q

Between Opposite Charges

+ -

-          electric field lines go from positive to negative charges and never intersect

Between Positive Charges

+ +

Between Negative Charges

- -

Between Opposite Charged Plates

+ + + + + + + + + + + + + +

- - - - - - - - - - - - - -

+

+

+

Fnet

Fnet

Fnetnet force same

Uniform Field

The Field

+ + + + + + + + + + + + + +

- - - - - - - - - - - - - -

Identify the following particles, given the electric fields:

Draw the electric field lines between these particles:

+

-+

+

+

1.

2.

3.

BW

• Draw the electric field lines between these charged particles:

+ +

Electric Field Strength

• The force on a test charge per unit charge

• E = electric field strength (N/C)• Fe = electrostatic force (N)• q = electric charge (C)

qFE e

•The electric field strength is directly proportional to the electrostatic force.•If E increases, then Fe increases. If E decreases, then Fe decreases.

How does this relate to distance?

Example

• What is the magnitude of the electric field strength at a point in a field where an electron experiences a 1.0 N force?

If a proton is found in an electric field with a strength of 10 N/C, what is the electrostatic force that the proton experiences?

What are the magnitude and direction of the electric field 1.5 m away from a positive charge of 2.1*10-9 C?

From the diagram of A and B above, rA is 0.0005 m and rB is 0.0003 m. What is the force acting on q at A and B, if the charge of q is 1.2 * 10-11 C and the charge of Q is 1.5 * 10-11 C?

For situation A and B above, calculate the electric field for the charge Q passing into charge q’s electric field.

CONDUCTING SPHERE

• On the surface, charge is distributed uniformly (evenly).

• Electric field lines are normal (perpendicular) to the surface.

• Electric field strength varies with the square of the distance, just as a point charge.

Hollow Sphere

The electric field stength inside a hollow, charged conducting sphere is zero