Plan for Today (AP Physics I)
• Lecture on Electric Fields and Field Lines
• Work on Homework (Problem Set 2) – due
Tomorrow
Picture of 2 Charges with Electric Field
Lines
• Which has more charge?
Picture
• Are they positive or negative?
Picture
• If the first is 20 microC, what is the second?
1 line = 4
Rules for Drawing Electric Field Lines
• 1. Begin at the +, end at – or infinity
• 2. Set a scale
• 3. Never cross lines
Example
• Scale: 1 line = 2
C
Scale: 1 line = 2 microcoulombs
What to look for
•
•
•
•
At least 4 or 5 lines
Uniform distribution – if uniform
Correct number of lines
Arrows on lines
Phet App
• Electric field charges and fields
• Line show general direction a positive charge
will go, but not completely momentum
• Phet Electric Field Hockey
Demo
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•
•
•
String Stand on Vandergraph
I can use my magic wand to kill the string
Why does this happen?
Sharp points concentrate electric field lines
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
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?
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
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
• Electric field inside a conductor is zero
• On a conductor field lines are perpendicular to
the surface
Assignment
• Work on 15.2 HW (due tomorrow)