1. Electric Field

2. Objectives
Properly explain/predict the behavior of objects, before and after being electrically charged/discharged.
Use equations and constants to solve word problems involving electric field, forces and motions of charged particles.

3. Definition Electric Field is defined as Force per charge
Units are Newtons per Coulomb (N/C)
So, what’s an “Electric Field?”
You have probably heard or even spoken of a “FORCE FIELD”
Esp in science fiction
You have been using equations to describe Gravity Fields already.
Let’s see how such “Force Fields” compare.

4. Gravity vs. Electricity
Gravity Field Electric Field

5. Example 1
A metal covered Styrofoam ball of mass kg and charged to 4.0 micro C is placed in an electric field and suspended motionless in midair. What is the strength of the electric field?

6. Solution Start with F=ma! Free body diagram. F = qE – mg = 0;
E=mg/q = x 9.8 / 4.0 x 10-6 =12000 N/C
Fe = qE
Fg=mg
(12250 if you don’t like sig figs or units)

7. Forces are vectors, so too must be the Electric Field.
Sign - Arrows on field lines point away from positive charge. They show the direction of force by the field on a positively charged particle.

8. Drawing!
Point charge +q

9. Dipole Electric dipole
Equal number of lines originate from pos charge and terminate at neg charge.
Note symmetry.
When charges are not equal…only the smaller amount of lines begin/end at the larger charge. Some will not terminate!

10. Example 2 r
q, m Q
What is the acceleration of the small sphere?

11. Conductors and Insulators
Conductors – charge (electrons) moves freely – (metals)
Insulators, …they don’t! (rubber, plastic, glass)
Field lines can help us determine where the charge on a conductor will reside.
Four rules…

12. Rules for electrostatic equilibrium
Electric Field is zero everywhere INSIDE a conducting material.
Excess charge on a conductor resides entirely on its surface.
The electric field caused by a charged conductor is always perpendicular to its surface.
Charge is more concentrated at the smaller radius of irregularly shaped objects.

13. Rules for electrostatic equilibrium
Rule #1: Electric Field is zero everywhere INSIDE a conducting material.
Reasoning: If there were an electric field inside a conductor, what would happen to the charge there?

14. Rules for electrostatic equilibrium
Rule #2: Excess charge on a conductor resides entirely on its surface.
Reasoning: It’s complicated, but has to do with inverse square nature Coulomb’s Law.
Think back to rule #1

15. Rules for electrostatic equilibrium
Rule #3. The electric field caused by a charged conductor is always perpendicular to its surface.
Reasoning: If it weren’t perpendicular, it would shove the charges sideways, not equilibrium.

16. Rules for electrostatic equilibrium
Rule #4. Charge is more concentrated at the smaller radius of irregularly shaped objects.
Reasoning: Less sideways component of force.

17. Van De Graaff
A number of the properties we’ve discussed can be demonstrated.
Like charges repel
Net Charge migrates to the outside
Field Lines perpendicular
“Dipole”

18. How it works!

19. Example 3
A Van de Graaff dome of radius 0.20 meters is charged to 3.00 x 10-4 C. What is the strength and direction of the electric field at the following distances from the center?
5.0 cm
20.0 cm
1.0 meter

20. Picture 1.0 m 20.0 cm 5.0 cm Zero inside!
E=keQ/r2 and radialy outward on surface and outside.
E=kq/r2
3.00E-04
8.99E+09
E+07 N/C
1 8.08E+19 N/C

21. Electric Field For point charges
The electric field may also be uniform, or approximated that way in many cases. This will simplify your math, just like little “g” simplifies things near the Earth’s surface.

22. Millikan Oil drop experiment
Shows quantized nature of charge. (He got the Nobel Prize for this one.) + -

23. Free Body Oil Drop -q mg falling -q qE mg rising
After thousands of trials, Millikan observed discrete velocities (very small) and concluded charge came in discrete amounts

24. Example 4 q m E
What is the charge, Q, on the sphere?

25. Faraday Ice Pail Experiment
Charge will migrate outward in conductors.
Touch a charged conductor to another just like it, ½ the charge will move to it.
But if you touch the inside of the neutral container, all of it will transfer!
Let’s draw to explain.

26. AP note Your equation sheet shows Coulomb’s Law as:
It also shows the value of e0 and ke. You can see they must be related.

27. Wrap up
Properly explain/predict the behavior of objects, before and after being electrically charged/discharged.
Use equations and constants to solve word problems involving electric field, forces and motions of charged particles.
Like charges repel, opposites attract
Net charge resides on surface
Concentrates at small radius
No field inside
Field lines perpendicular
Point in direction a positive charge will be accelerated.
Charge from one object will be shared with a neutral object if touched on the outside
Charge will be completed given away when charged object is touched inside another
Charge is conserved, stripping one electron leaves a net charge of +e one beind and -e with the electron.
Charge is quantized at 1.6e-19 C per elementary charge.
F=kqq/r^2
E=F/q=kq/r^2

28. Summary Like charges repel, opposites attract
Charge is conserved, stripping one electron leaves a net charge of +e one beind and -e with the electron.
Net charge resides on surface
Concentrates at small radius
No field inside
Field lines perpendicular to surface
Point in direction a positive charge will be accelerated.
Charge is quantized at 1.6 x C per elementary charge.
Charge from one object will be shared with a neutral object if touched on the outside
Charge will be completed given away when charged object is touched inside another

29. Lightning Rods
What property of charged objects explains how they work?
page 493 #17, 18, 19, 20, 28, 34