1. Electric Charge and Electric Field
Chapter 16
Electric Charge and Electric Field

2. Objectives: The students will be able to:
Explain the concept of electric field and determine the resultant electric field at a point some distance from two or more point charges.
Determine the magnitude and direction of the electric force on a charged particle placed in an electric field.

3. 16.7 The Electric Field
Region around a charge which its effect is experienced or felt.
The electric field is the force on a small charge, divided by the charge:
(16-3)
Only need one charge
to have an electric field
unlike Coulomb’s Law.
Notice direction of the force and E
for a proton and an electron.

4. Electric Field Test charge q is always positive. Therefore the
direction on the force of the test charge would
be away from the main charge which shows
direction of the electric field.

5. Electric Field Test charge q is always positive. Therefore the
direction on the force of the test charge would
be towards the main charge which shows
direction of the electric field.

6. Sample Problem 1 (similar to #23 page 466)
What are the magnitude and direction of the electric force on an electron in a uniform electric field of strength 2460N/C that points due East?

7. Sample Problem 1 (similar to #23 page 466)
What are the magnitude and direction of the electric force on an electron in a uniform electric field of strength 2460N/C that points due East?
Since the E electric field points east, the force on an electron would point in the opposite direction, west. (The F force on a proton would point in the same direction as the E field).

8. Sample Problem 2 (similar to #24 page 466)
A proton is released in a uniform electric field, and it experiences an electric force of 1.86 x 10-14N toward the south. What are the magnitude and direction( north, east, south, west) of the electric field?

9. Sample Problem 2 (similar to #24 page 466)
A proton is released in a uniform electric field, and it experiences an electric force of 1.86 x 1014N toward the south. What are the magnitude and direction( north, east, south, west) of the electric field?
Proton, positive charge, will always be attracted toward a negatively charged plate/point. Field lines always point from positive to negative. If it's being attracted south, negative charge must be somewhere south of it, so field must be in a southward direction.

10. Sample Problem 3 (similar to #27 page 466)
What is the magnitude and direction of the acceleration experienced by an electron in and electric field of 600 N/C?  How does the direction of the acceleration depend on the direction of the field at that point? 

11. Sample Problem 3 (similar to #27 page 466)
What is the magnitude and direction of the acceleration experienced by an electron in and electric field of 600 N/C? How does the direction of the acceleration depend on the direction of the field at that point? First, looking at the given information, we know we can find Force using E = F/q. From there, you can easily find the acceleration using F = ma. Given info: E = 600 N/C q = 1.602e-19 E = F/q 600 N/C = F/(1.602e-19) F = 9.612e-17 N F = ma 9.612e-17 N = (9.11e-31)a a = 1.055e14 m/s/s Because it's an electron, it has a negative charge. This means that the electron should always accelerate in the opposite direction of the E. Field.

12. Homework
Page 466 Problems 25, 27, and 30

13. Objectives: The students will be able to:
Explain the concept of electric field and determine the resultant electric field at a point some distance from two or more point charges. Determine the magnitude and direction of the electric force on a charged particle placed in an electric field. Sketch the electric field pattern in the region between charged objects.

14. 16.7 The Electric Field
For a point charge:
(16-4a)
(16-4b)

15. How to derive the equation
Source: at 8:15

16. 16.7 The Electric Field Force on a point charge in an electric field:
(16-5)
Superposition principle for electric fields:

17. 16.7 The Electric Field
Problem solving in electrostatics: electric forces and electric fields
Draw a diagram; show all charges, with signs, and electric fields and forces with directions
Calculate forces using Coulomb’s law
Add forces vectorially to get result

18. Sample Problem 1
What is the magnitude and direction of the electric field 30.0 cm directly above a 33.0 x 10-6 C charge?

19. Sample Problem 1
What is the magnitude and direction of the electric field 30.0 cm directly above a 33.0 x 10-6 C charge?
Now, we're into point charges. In the info given, we have a radius (distance between charge and point charge), as well as 1 Charge (33e-6 C).
E = kq/r2
E = (8.988e9)(33e-6)/(.3)2
E = 3.29e6 Up
Because this charge is positive, the electric field is always away from the charge. If the point is directly above the charge, then the electric field will travel out to the point, and past it... up.

20. Sample Problem 2 example 16-8 page 452
Two point charges are separated by the distance of 10.0cm. One has a charge of -25μC (microCoulomb) and the other +50μC (microCoulomb). a.) Determine the direction and the magnitude of the electric field at point P between the two charges that is 2.0 cm from the negative charge. b.) If an electron (mass = 9.11 x 10 to -31 power kg) is placed at the rest at P and then released, what will be its initial acceleration (direction & magnitude) ?

21. Sample Problem 2 example 16-8 page 452
Two point charges are separated by the distance of 10.0cm. One has a charge of -25μC (micrometer) and the other +50μC (micrometer). a.) Determine the direction and the magnitude of the electric field at point P between the two charges that is 2.0 cm from the negative charge.

22. Sample Problem 2 example 16-8 page 452
b.) If an electron (mass = 9.11 x 10 to -11 power kg) is placed at the rest at P and then released, what will be its initial acceleration (direction & magnitude) ?
The force on the electron = qE
= 1.6x10-19 x 6.33x108
= 1.01x10-10 N
Because the electron is negative, the direction is opposite to the field's direction (away from the negative charge and towards the positive charge). It will feel a force to the right.
Since F=ma, a = F/m
a = 1.01x10-10 / (9.11x10-31)
= 1.11x1020 m/s²
The direction is the same as the force, away from the negative charge and towards the positive charge which will be to the right.

23. 16.8 Field Lines
The electric field can be represented by field lines. These lines start on a positive charge and end on a negative charge.

24. 16.8 Field Lines
The number of field lines starting (ending) on a positive (negative) charge is proportional to the magnitude of the charge.
The electric field is stronger where the field lines are closer together.

25. 16.8 Field Lines
Electric dipole: two equal charges, opposite in sign:

26. Electric Field Lines— Physical Meaning

27. Electric Field Lines—Examples
Beginning with positive charge and ending at negative or infinity

28. Drawing the electric field

29. Electric fields and electric force
On the Earth’s surface, the gravitational field creates 9.8 N of force on each kilogram of mass.
With gravity, the strength of the field is in newtons per kilogram (N/kg) because the field describes the amount of force per kilogram of mass.

30. Electric fields and electric force
GT stopped here.
With the electric field, the strength is in newtons per coulomb (N/C).
The electric field describes the amount of force per coulomb of charge.

32. 16.8 Field Lines
The electric field between two closely spaced, oppositely charged parallel plates is constant.

33. Capacitors A capacitor is a storage device for electric charge.
Capacitors can be connected in series or parallel in circuits, just like resistors.

34. Capacitors
A capacitor can be charged by connecting it to a battery or any other source of current.
A capacitor can be discharged by connecting it to any closed circuit that allows current to flow.

35. How a capacitor works inside
The simplest type of capacitor is called a parallel plate capacitor.
It is made of two conductive metal plates that are close together, with an insulating plate in between to keep the charges from coming together.
Wires conduct charges coming in and out of the capacitor.

36. How a capacitor works inside
The amount of charge a capacitor can store depends on several factors:
The voltage applied to the capacitor.
The insulating ability of the material between the positive and negative plates.
The area of the two plates (larger areas can hold more charge).
The separation distance between the plates.

37. Electric Field Lines—Examples -
How capacitors store electrical energy
Positive charge and and negative charge is no net charge
How Capacitors Store Electrical Energy

38. 16.8 Field Lines Summary of field lines:
Field lines indicate the direction of the field; the field is tangent to the line.
The magnitude of the field is proportional to the density of the lines.
Field lines start on positive charges and end on negative charges; the number is proportional to the magnitude of the charge.

39. 16.9 Electric Fields and Conductors
The static electric field inside a conductor is zero – if it were not, the charges would move.
The net charge on a conductor is on its surface.

40. 16.9 Electric Fields and Conductors
The electric field is perpendicular to the surface of a conductor – again, if it were not, charges would move.

41. phET Point charges in Electrostatic fields

42. Homework
Page 466
Problems 32 and 35
Electric Field lines hand-out