1. Plan for Today (AP Physics I)
Lecture on Electric Fields and Field Lines
Work on Homework (Problem Set 2) – due Tomorrow

2. Picture of 2 Charges with Electric Field Lines
Which has more charge?

3. Picture
Are they positive or negative?

4. Picture
If the first is 20 microC, what is the second?

5. 1 line = 4

6. Rules for Drawing Electric Field Lines
1. Begin at the +, end at – or infinity
2. Set a scale
3. Never cross lines

7. Example
Scale: 1 line = C

8. Scale: 1 line = 2 microcoulombs

9. What to look for At least 4 or 5 lines
Uniform distribution – if uniform
Correct number of lines
Arrows on lines

10. Phet App Electric field charges and fields
Line show general direction a positive charge will go, but not completely momentum
Phet Electric Field Hockey

11. Demo String Stand on Vandergraph
I can use my magic wand to kill the string
Why does this happen?
Sharp points concentrate electric field lines

12. Sharp points concentrate electric field lines
So if we concentrate the electric field, we ionize the air
Once the air is ionized, the air turns from an insulator to a conductor
We get a spark
One reason not to have an umbrella in a thunderstorm

13. Demo Packing peanuts in a metal container on a van de graaf generator
Wait for something to happen
Nothing will
Repeat with plastic container
Should fly out
But why?

14. Demo explained Charge stays on the outside of the container
Electrons work their way up the conductor
When electrons get to the top and into the cup, they push back through the cup

15. Demo Explained Continued
In an insulator, charge can work its way inside
Because it can’t get pushed back out
This is why airplanes have been struck by lightning and people are ok
Same thing if you hit a power line pole and the line hits your car
Stay inside

16. Electric field inside a conductor is zero

17. On a conductor field lines are perpendicular to the surface

18. Assignment
Work on 15.2 HW (due tomorrow)