1. From Last Time… Magnitude of the electric force+
Direction of the electric force

2. Review Group/Quiz (solutions on website). Review lab question sheets.
Exam 1
Mon. Sep. 29, 5:30-7 pm
Covers Chap , 22, , 23.7, , lecture, lab, discussion, HW
8 1/2 x 11 handwritten note sheet (both sides) allowed
Students with scheduled class conflicts: stay after lecture Tuesday Sep 23 to arrange time
Review Group/Quiz (solutions on website).
Review lab question sheets.
Review sample exams on website.

3. From Last Time… Magnitude of the electric force+
Direction of the electric force

4. Quick Quiz - - Left Right Up Down Zero
Equal but opposite charges connected by a rigid insulating rod, placed near a negative charge as shown. What direction is the net force on the two connected charges?
Left
Right Up
Down
Zero - + -

5. The electric dipole Can all be approximated by electric dipole.
Two opposite charges magnitude q separated by distance s
Dipole moment
Vector
Points from - charge to + charge
Has magnitude qs

6. Force on an electric dipole
What is the direction of the force on the electric dipole from the positive point charge? Up
Down
Left
Right
Force is zero +
How does the magnitude of the force depend on ?

7. Induced dipoles (charge redistribution)
charged rubber rod
Talk about approximating these as two different charges, separated by a small distance. An electric dipole. Say why charges should be equal and opposite in magnitude.
Bring negative charge close. Electrons on sphere move away from rod.

8. Unusual dipoles: Electrogenic fish
Dipole + nearby conducting object
Some fish generate charge separation - electric dipole.
Dipole is induced in nearby (conducting) fish
Small changes detected by fish.
Say that this gets quite complicated thinking only about charges, and forces between charges. Need to come up with a new idea.

9. The idea of electric fields
EM wave made up of oscillating electric and magnetic fields.
But what is an electric field?
Electric field is a way to describe the force on a charged particle due to other charges around it.
Force = charge  electric field
The direction of the force is the direction of the electric field.

10. Electric field of a point charge
Force on this charge… Q1 Q2 + + +
…due to this charge
After arrows appear, comment that this is a ‘force field’. But the force at each point depends on the magnitude of the charge at that point. So it is not just a property of the source charge Q1.
Electric field fixes this. + +
= Force/charge
Units?
N/C

11. Calculating the electric field
r = 10 cm +
Q1=1µC

12. Electric field Electric field vector defined at every point in space.
Gives magnitude and direction of force on test particle
e.g. wind velocity (speed and direction) in different parts of the country.
Comment that electric field is defined at every point in space.
But how can it be different than the arrows shown previously? Answer is superposition.

13. Superposition with Electric Fields
At any point P, the total electric field due to a group of source charges equals the vector sum of electric fields of all the charges
Find the electric field due to q1, E1
Find the electric field due to q2, E2
E = E1 + E2
Remember, the fields add as vectors

14. Question
Which vector best represents the electric field at the red dot? A B E C D - -

15. Electric Field Direction
a) q is positive, F is directed away from q
b) The direction of E is also away from the positive source charge
c) q is negative, F is directed toward q
d) E is also toward the negative source charge

16. Relationship Between F and E
Fe = qE
Valid for a test charge that does not disturb the source charge distribution
If q is positive, F and E are in the same direction +
r = 1x10-10 m
Qp=1.6x10-19 C
Electric field 1Å away from proton E
(to the right)

17. Quick Quiz - Which is the direction of the electric field at dot?
A. Left
B. Right
C. Up
D. Down
E. Zero
Away from positive charge (right) y + - x
Net E field is to right.

18. Quick Quiz
In this electric dipole, what is the direction of the electric field at point A? Up
Down
C) Left
D) Right
E) Zero A
x=-a
x=+a +Q -Q

19. Electric field: summary
Electric field -> will be a force on a charged particle.
This force ( and electric field) can arise from electric charges (via Coulomb’s law)
But once electric field is known, don’t need to know the charges that produce it.

20. The electric dipole Can all be approximated by electric dipole.
Two opposite charges magnitude q separated by distance s
Dipole moment
Vector
Points from - charge to + charge
Has magnitude qs

21. Calculating dipole electric field
On the y-axis y q s x
For -q
Since points from - charge to + charge
on y-axis of dipole only

22. Question: electric dipole
A and B are the same (large) distance from dipole. How do the magnitude of the electric fields at A and B compare? A B

23. Pictorial representation of E: Electric Field Lines
After field lines up ask to compare efield at two points of equal distance

24. Electric field lines Local electric field tangent to field line
Density of lines proportional to electric field strength
Fields lines can only start on + charge
Can only end on - charge.
Electric field lines can never cross
Ask why electric field lines can never cross.

25. Electric field of a dipole+ -
Do on board with selected points before showing results.

26. Electric field of two + charges

27. Question How are the charges A and B related? A+, B-, A-, B+, A B
Ask where the field is strongest.
Do example on the board where we try to fulfill electric-field line requirements (e.g. start with drawing field lines out of positive charge. Stronger charge -> more field lines.

28. Point particles