1. Phys102 Lecture 2 The Electric Field
Key Points
Coulomb’s Law
The electric field (E is a vector!)
References
Textbook: 16-1,2,3,4,5,6,7,8,9,+.

2. The Electric Field
The electric field is defined as the force on a small charge, divided by the magnitude of the charge:
Figure Force exerted by charge Q on a small test charge, q, placed at points A, B, and C.

3. The Electric Field An electric field surrounds every charge.
Figure An electric field surrounds every charge. P is an arbitrary point.

4. The Electric Field
For a point charge:

5. The Electric Field Force on a point charge in an electric field:
Figure (a) Electric field at a given point in space. (b) Force on a positive charge at that point. (c) Force on a negative charge at that point.

6. Example: Electric field above two point charges.
Calculate the total electric field (a) at point A and (b) at point B in the figure due to both charges, Q1 and Q2.
Solution: The geometry is shown in the figure. For each point, the process is: calculate the magnitude of the electric field due to each charge; calculate the x and y components of each field; add the components; recombine to give the total field.
a. E = 4.5 x 106 N/C, 76° above the x axis.
b. E = 3.6 x 106 N/C, along the x axis.

7. Example: Electric field above two point charges.
Calculate the total electric field (a) at point A and (b) at point B in the figure due to both charges, Q1 and Q2.
Solution: The geometry is shown in the figure. For each point, the process is: calculate the magnitude of the electric field due to each charge; calculate the x and y components of each field; add the components; recombine to give the total field.
a. E = 4.5 x 106 N/C, 76° above the x axis.
b. E = 3.6 x 106 N/C, along the x axis.

8. 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.
Check your answer!

9. Field Lines
The electric field can be represented by field lines. These lines start on a positive charge and end on a negative charge.
Figure Electric field lines (a) near a single positive point charge, (b) near a single negative point charge.

10. 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.

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

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

13. 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.

14. 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 resides on its outer surface.
Figure A charge inside a neutral spherical metal shell induces charge on its surfaces. The electric field exists even beyond the shell, but not within the conductor itself.

15. Electric Fields and Conductors
The electric field is perpendicular to the surface of a conductor – again, if it were not, charges would move.
Figure If the electric field at the surface of a conductor had a component parallel to the surface E||, the latter would accelerate electrons into motion. In the static case, E|| must be zero, and the electric field must be perpendicular to the conductor’s surface: E = E┴.

16. Electric Fields and Conductors
Conceptual Example: Shielding, and safety in a storm.
A neutral hollow metal box is placed between two parallel charged plates as shown. What is the field like inside the box?
The field inside the box is zero. This is why it can be relatively safe to be inside an automobile during an electrical storm.

17. i-clicker quiz 2-1 A) positive B) negative C) neutral
D) positive or neutral
E) negative or neutral
A metal ball hangs from the ceiling by an insulating thread. The ball is attracted to a positive-charged rod held near the ball. The charge of the ball must be:

18. Van de Graaff Generator
The electric field is defined as the force on a small charge, divided by the magnitude of the charge:
Figure Force exerted by charge Q on a small test charge, q, placed at points A, B, and C.

19. i-clicker 2-2
Two neutral conductors are connected by a wire and a charged rod is brought near, but does not touch. The wire is taken away, and then the charged rod is removed. What are the charges on the conductors?
A) 0 0
B) + –
C) – +
D) + +
E) – – ?

20. i-clicker 2-3
A) 3/4 N
B) N
C) 12 N
D) 16 N
E) 48 N Q
F1 = 3 N
F2 = ? 4Q Q
F1 = ?
F2 = ?
If we increase one charge to 4Q, what is the magnitude of F1?

21. i-clicker 2-4
The force between two charges separated by a distance d is F. If the charges are pulled apart to a distance 3d, what is the force on each charge?
A) 9F
B) 3F
C) F
D) 1/3F
E) 1/9F Q F d ? 3d

22. i-clicker 2-5
You are sitting a certain distance from a point charge, and you measure an electric field of E0. If the charge is doubled and your distance from the charge is also doubled, what is the electric field strength now?
A) 4E0
B) 2E0
C) E0
D) 1/2E0
E) 1/4E0

23. i-clicker 2-6 4 3 2 1 +Q -Q 5
What is the direction of the electric field at the position of the X ?
A) 1
B) 2
C) 3
D) 4
E) 5