Today 2/24 Read 18.6 about E-Fields

note we put E-field first and Force second E-Field Lines Chapter 18.7 Today Lab: “Electrostatics”

Read 18.4 in text for background HW: None assigned

Electrostatics Lab: This should be Fun!

Tools:

•electroscope

•“proof plane”

•metal cup

•ground wire

•charging rod and glass

delicate gold foil

proof plane

Proof Plane: What happens…(Start with the proof plane uncharged)

...when the metal touches a charged insulator?

proof plane

...when the metal touches an uncharged plate?

...when the metal touches a charged plate?

some rubs off

nothing

charge moves through the conductors

…when the metal touches the center of the plate???

Two identical positive charges, A and B, are arranged as shown. The distance from point C to A is twice the distance from point C to B. Which of the following best represents the electric field at point C?

e

d

c

b

aA

B C

e

d

c

b

aA

B C

Two identical positive charges, A and B, are arranged as shown. The distance from point C to A is twice the distance from point C to B. Which of the following best represents the electric field at point C?

Field Lines

If we want to draw a picture of the E-Field everywhere at once we must fill space with X’s and arrows.

Pepe le Pew:

Where is Pepe?

Where is Pepe?

Where is Pepe?

The closer we get to the source, the closer together the lines of STINK and the bigger the PU.

PU!

PU!

What if it’s a HOT DAY?

More STINK means more lines.

Field Lines

If we want to draw a picture of the E-Field everywhere at once we must fill space with X’s and arrows.

Red “E-Field lines” display the information more clearly.

Field Lines

If we want to draw a picture of the E-Field everywhere at once we must fill space with X’s and arrows.

Red “E-Field lines” display the information more clearly.

Electric Field Lines:

direction of E-field vector at any point is parallel to the lines near that point

magnitude of E-field vector at any point is proportional to line density (line spacing)

number of lines leaving or entering a charge is proportional to the size of the charge (more charge, more lines)

away from positive charge and toward negative charge

What direction would the force point on a small positive charge at 1,2,3?

Compare the size of the force exerted on a small charge at

a. 1 and 2b. 2 and 3

The test charge is doubled when at 2

a. how does the force change?b. how does the electric field change?

What direction would the force point on a small positive charge at 1,2,3?

Compare the size of the force exerted on a small charge at

a. 1 and 2b. 2 and 3

The test charge is doubled when at 2

a. how does the force change?b. how does the electric field change?

1 larger

What direction would the force point on a small positive charge at 1,2,3?

Compare the size of the force exerted on a small charge at

a. 1 and 2b. 2 and 3

The test charge is doubled when at 2

a. how does the force change?b. how does the electric field change?

2x

Stays the same

3 larger1 larger