1. Benjamin Franklin Drawing Electricity from the Sky (1816)
The Electric Field
Vocabulary
Van de Graaff generator
test charge
electric field
electric field lines
Benjamin Franklin Drawing Electricity from the Sky (1816)
Benjamin West
This topic can be found in your textbook on pp

2. Think about the Van de Graaff Generator

3. How is charge distributed on the sphere?+ + + + + + + +

4. What will happen if I bring a test charge near the charged sphere?+ + + + +
Is contact necessary for the test charge to experience an electrical force? + + + +
Test Charge: a small positive charge placed near an object with a much larger charge.

5. What will happen if I bring a test charge near the charged sphere?+ + + + +
How will the force change if the test charge moves closer to the sphere? + + + +

6. What will happen if I bring a test charge near the charged sphere?+ + + + 2+
How will the force change if the charge on the test charge doubles? + + + +

7. Drawing Field Lines: What do you think?
Make a sketch of the “E” field for each charge or combination of charges.
Field lines have direction
What direction do you think the field lines go around a + or – charge? 1 2 3

8. Electric Field Line Rules
After giving students some time to draw the field, show them the second diagram. Show how the rules were applied to create this diagram.
Apply the above rules and sketch the E field around the charge shown.

9. Electric Field Line Rules
After giving students some time to draw the field, show them the second diagram. Show how the rules were applied to create this diagram.
Apply the above rules and sketch the E field around the charge shown.

10. Electric Field Line Rules
After giving students some time to draw the field, show them the second diagram. Show how the rules were applied to create this diagram.
Apply the above rules and sketch the E field around the charge shown.

11. Electric Field Line Rules
After giving students some time to draw the field, show them the second diagram. Show how the rules were applied to create this diagram.
Apply the above rules and sketch the E field around the charge combination shown.

12. Electric Field
The concept that a field exists around a charge object that will affect other charge objects is called an “Electric Field.”
where E = electric field in N/C, Felectric = force on the test charge in N q0 = charge on the test charge in C

13. Electric Field
Think about this: Is the strength of an electric field dependent on distance?
Where is distance in the equation then?
It’s in the Felectric term!

14. Electric Field
What was the equation for finding the force between two charges?
Substitute this into the electric field equation:

15. Electric Field Strength
Electric fields (E) have magnitude and direction.
The direction is defined as the direction of the force on a small, positive test charge (q0) placed in the field caused by Q.
The magnitude is calculated using or

16. Example Electric Field Strengths
The electric field in a fluorescent tube causes the electrons to move through the tube, and this causes the gas inside the tube to glow.
The electric field under a thundercloud just before the lightning bolt is sometimes strong enough to induce a charge in your hair, which will cause your hair to stand on end.

17. Practice Problem
Use your ninja physics skills!!!
An electric field around a charged object is  106 N/C at a distance of m. Find the charge on the object.
Answer: 6.62  10-6 C or 6.62 C
Suppose a small test charge of C was placed at the point that is m from the charged object. What force would be exerted on the test charge and on the object?
Answer: 0.04 N for both test charge and object
For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful.
Students do not need to use Coulomb’s law to calculate the answer to the second question. Felectric = Eq0 is much simpler.

18. Practice Problem
Jasmine pulls her wool sweater out of the dryer while some excess charge builds up on her. What is the electric field at a location where a 1.60 X C piece of lint experiences a force of 3.2 X 10-9 N as it floats near Jasmine?
 Answer: 2.0 X 1010 N/C
What is the net electric field halfway between a proton and an electron that are separated by a distance of 25 cm?
Answer: 1.84*10-7 N/C
For problems, it is a good idea to go through the steps on the overhead projector or board so students can see the process instead of just seeing the solution. Allow students some time to work on problems and then show them the proper solutions. Do not rush through the solutions. Discuss the importance of units at every step. Problem solving is a developed skill and good examples are very helpful.
Students do not need to use Coulomb’s law to calculate the answer to the second question. Felectric = Eq0 is much simpler.

19. Electrostatic Equilibrium
Electrostatic equilibrium occurs in conductors when no net motion of charges exists within the conductor.
Charges in a conductor are free to move, but are not moving when equilibrium exists. The rules below result from this fact
If the four rules stated were violated, the charges within the conductor would move.
This is because if there were an electric field inside a conductor, the free charges would move and a current would exist. This net movement of charge would mean it was not in electrostatic equilibrium.

20. Electrostatic Equilibrium
Electrostatic equilibrium occurs in conductors when no net motion of charges exists within the conductor.
Charges in a conductor are free to move, but are not moving when equilibrium exists. The rules below result from this fact
If the four rules stated were violated, the charges within the conductor would move.
This is because the charges repel each other and this forces them as far apart as possible, causing them to migrate to the surface

21. Electrostatic Equilibrium
Electrostatic equilibrium occurs in conductors when no net motion of charges exists within the conductor.
Charges in a conductor are free to move, but are not moving when equilibrium exists. The rules below result from this fact
If the four rules stated were violated, the charges within the conductor would move.
This is because if it were not perpendicular it would have a component along the surface of the object causing the free e- to move. Thus a current would exist and there would not be electrostatic equilibrium

22. Electrostatic Equilibrium
Electrostatic equilibrium occurs in conductors when no net motion of charges exists within the conductor.
Charges in a conductor are free to move, but are not moving when equilibrium exists. The rules below result from this fact
If the four rules stated were violated, the charges within the conductor would move.
This is because if there is less space for charge distribution (and they still seek equilibrium) – thus they get more concentrated in these sharp areas