1. Electric Field Lines Drawing electric field lines
Motion of charged particles in an electric field
Electric flux

2. Electric Field Lines
Electric field lines are a way of visualizing the field.
Rules for Drawing field lines:
Lines start on (+) charges, end on (-) charges, or go to infinity
(# of lines)  charge
Lines never cross
Strength of field is proportional to the density
of field lines
Interpreting the picture:
is parallel to the field line at each point.

3. + Electric Field lines for an isolated Charge +Q E
E – magnitude of field E + Q

4. Field lines +
Area S,
Where is the density of field
lines greatest?

5. 2 Point Charges Note: number of lines on -2Q is twice as many as on +Q

6. Quiz: Which way will the dipole start to move in the electric field?up
down
left
right
nowhere – there is no net force.

7. Quiz: Does this dipole feel a torque ? Yes - clockwise
Yes – counter clockwise No
Depends on the strength of E

8. Parallel Charged Plates+ + + + + + + + - - - - - - -
E approx. uniform, between the plates, except
near the edges.

9. Electric Force Therefore we can solve for motion as easily
as projectile motion!

10. Example: Uniform E
An electron enters a uniform field of E = 200N/C j with an
initial velocity of vo = 3x106 m/s i. Find:
a) The acceleration of the electron b) The time it takes to travel through the region of the field c) The vertical displacement of the electron while in the field
0.1m -

11. Solution:

12. Electric Flux
Electric flux is the measure of the “number of field lines passing through a surface S ”
For uniform :
Define: Electric Flux
Units: N•m2/C S
A is the surface area perpendicular to , so Φ=EAcos(θ)

13. Notes: is a scalar called electric flux Units: N•m2/C
represents the “number of field lines through surface S.”
For a closed surface, the area vector points
in the outward direction.
5) Flux is zero for a surface parallel to the field (normal is at 90o to E)

14. Example: (rectangle, 1m x 2m) (hemisphere, radius 1m) S2 S1 S3
30°
(rectangle,
1m x 2m)
(hemisphere,
radius 1m) S2 S1 S3
Find: flux through S1, S2, S3.

15. solution

16. If E is not uniform, or S is not flat, then:
For a small surface
For the whole surface,

17. Summary Electric field lines help show the direction of E
Electric flux is defined as the magnitude of the field times the area (maybe negative if the angle between the vectors is more than 90 degrees)
Electric flux is a quantitative equivalent to “the number of field lines through a surface”.