Electric Field

Physics 102Professor Lee

CarknerLecture 11

Force on Charges Fy = 8.99X109(4)

(2)/(22) = Fx = 8.99X109(2)

(5)/(32) = F2 = Fx

2 + Fy2

F = tan = (Fy/Fx) = arctan (Fy/Fx) =

3 m

2 m

q2 = 5 C

q3 = 4 C

q1 =-2 C

Electric Field at Origin Ey = 8.99X109(4)/(22)

=

Ex = 8.99X109(5)/(32) =

E2 = Ex2 + Ey

2

E = Field indicates

direction positive test charge will move

3 m

2 m

q2 = 5 C

q3 = 4 C

To charged objects attract each other with force F. If the amount of charge on one object is doubled and the distance between the objects is also doubled, what is the new force in terms of the old?

A) ¼ FB) ½ FC) 1F (force is unchanged)D) 2 FE) 4F

A charge +Q is placed in the center of a square. When a charge –Q is placed on one corner of the square the net force between them is 2 N. What is the net force on the center charge if a charge –Q is placed on all corners?

A) 0 NB) 2 NC) 4 ND) 8 NE) You can’t tell from the information

given

What information does the electric field give us about a point in space?

A) The sign of the charges that produced the field

B) The distance to the nearest chargeC) The magnitude of the electric force at

that pointD) The direction a positive charge would

move from that point E) The velocity of a charge at that point

Field Lines

We defined the electric field as:

The field fills the entire region around a charge

We would like to draw the field to give us a “map” of how the charge will effect other charges

Field lines indicate how a charge will move

Field Lines and the Field

Direction:

At any point the F vector is tangential to the field lines Strength:

Density of lines proportional to field strength

Examples of Fields

Dipole

Called a dipole

Magnetic fields are always dipolar

Dipolar Field

How to Draw Field Lines Lines determine the force experienced

by a positive charge

Number of lines touching a charge proportional to the magnitude of the charge

Lines must begin and end at a charge or infinity

Today’s PAL

Draw below a point charge of +3q, and to the right, a point charge of –1q. Draw the electric field lines between them. Draw at least 10 lines evenly spread out.

At a very large distance away, would a small positive test charge be attracted or repelled?

If q is one electron’s worth of charge, pick a point on a line connecting the charges and compute E there.

Van De Graaff Generator

The positively charged belt attracts the electrons in the dome making it positively charged

Conveyer belt to remove electrons Charge builds up until it is discharged through

the air

Van De Graaff

Why does your hair stand up when you touch the Van De Graaff generator?

Why do you need to stand on the box?

Conductors and Fields

The charges in the conductor are free to move and so will react to the field

Charge distributes itself uniformly over the surface of a conductor

Inside the Conductor

If we consider both positive and negative charge in an electric field,

A charge placed in the middle would feel an equal force from both sides and not move, thus, The field inside the conductor is zero

A conductor shields the region inside of it

Faraday Cage

If we make the conductor hollow we can sit inside it an be unaffected by external fields

Your car is a Faraday cage and is thus a good place to be in a thunderstorm

Charge Distribution

How does charge distribute itself over a surface?

e.g., a sphere

No component parallel to surface, or else the charges would move

Excess charge there may spark into the air

Conducting Ring

Charges

Pushed

To

Surface

No E Field

InsideField Lines

Perpendicular

to Surface

Next Time

Monday, January 8 Read 17.1-17.6 Homework: Ch. 16, P 28, 31, 32,

Ch. 17, P 2, 14