1. Lightning
Interactions b/w hail and smaller ice crystals transfer e- to hail. They are heavier and sink to base of cloud.
Ice Crystals
Light
due to e-’s
recombining
w/ions
Hail
Upward streamers of + ions travels up from ground. Lightning occurs if it meets stepped leader from cloud.
Thunder:
shock wave
created by rapid
heating of air
Path of
lightning
is about
1 cm in
diameter
e- run away from cloud base, leaving net + charge on ground under cloud

2. Lethal Strike Physiological effects of a direct strike:
stops heart
paralyzes muscles
internal burns
Heart may restart, but breathing won’t
need mouth-to-mouth
Other types of strikes:
Side flash or touching struck object
Ground current
Dead-end upward streamer
General Physics 2
Electric Field

3. Lightning Rods
Provide path for current to pass directly to ground without passing through building
path of least resistance
Diffuse charge build-up
Photo from lightning research center in FL
lightning rods may not provide enough protection to homes built on sandy soil
General Physics 2
Electric Field

4. Lightning Safety Lightning kills ~100 people per year in US
Stay inside if possible. Avoid:
showers
much lower resistance when wet
phones (cell phones are ok)
any electrical appliances
If outside,
avoid tall objects
seek low-lying area
duck head, crouch
pull feet together, run, hop on one foot (ground current)
stay in car - Faraday cage (Note: Saturns are not safe!)
General Physics 2
Electric Field

5. Lightning: Physical Characteristics
Voltage
several hundred million volts
Current
1020 electrons in a fraction of a second
~10 kiloamps
So what is a Volt? what is an amp?
Good question. We’ll get to that soon.
General Physics 2
Electric Field

6. Electric Field: The electric field just before a lightning strike points:
1. toward the ground
2. toward the cloud
3. along the ground

8. Electric Field of a Line of Charges
What is the electric field, E, at point P?
For continuous charge distributions, we need to use calculus.
General Physics 2
Electric Field

9. The Electric Field of a Continuous Charge Distribution
The surface charge density of a two-dimensional distribution of charge across a surface of area A is defined as
Surface charge density, with units C/m2, is the amount of charge per square meter.

10. A Plane of Charge
The electric field of an infinite plane of charge with surface charge density η is:
For a positively charged plane, with η > 0, the electric field points away from the plane on both sides of the plane.
For a negatively charged plane, with η < 0, the electric field points towards the plane on both sides of the plane.

11. A Disk of Charge
The on-axis electric field of a charged disk of radius R, centered on the origin with axis parallel to z, and surface charge density η = Q/πR2 is
NOTE: This expression is only valid for z > 0. The field for z < 0 has the same magnitude but points in the opposite direction.

12. A Sphere of Charge
A sphere of charge Q and radius R, be it a uniformly charged sphere or just a spherical shell, has an electric field outside the sphere that is exactly the same as that of a point charge Q located at the center of the sphere:

13. The Parallel-Plate Capacitor
The figure shows two     electrodes, one with charge     +Q and the other with –Q     placed face-to-face a     distance d apart.
This arrangement of two     electrodes, charged equally     but oppositely, is called a     parallel-plate capacitor.
Capacitors play important     roles in many electric     circuits.

14. The Parallel-Plate Capacitor
The electric field inside a capacitor is
where A is the surface area of each electrode. Outside the capacitor plates, where E+ and E– have equal magnitudes but opposite directions, the electric field is zero.

15. EXAMPLE 27.7 The electric field inside a capacitor
QUESTIONS:
Q = 3.5 nC
N = 2.2 x 10^10 electrons

16. Dipoles in an Electric Field
The torque on a dipole in an electric field is
where θ is the angle the dipole makes with the electric field.

17. Rub a rod with rabbit fur and bring near a stream of water
Water Demo
Rub a rod with rabbit fur and bring near a stream of water
Group problem -
Explain what’s happening

18. Motion of a Charged Particle in an Electric Field
The electric field exerts a force
on a charged particle. If this is the only force acting on q, it causes the charged particle to accelerate with
In a uniform field, the acceleration is constant:

19. Motion of q in a uniform E field
The figure shows an electron entering a uniform electric field of N/C. What is the acceleration of the e- once inside the field?
me = 9.11 x kg qe = x C
General Physics 2
Electric Field

20. Motion of q in a uniform E field
If the electron is initially moving with a horizontal velocity of 3.0 x 106 m/s, and l = 20 cm, what is the magnitude and direction of the electron’s velocity as it leaves the parallel plates?
me = 9.11 x kg qe = x C
General Physics 2
Electric Field

21. Matlab If you don’t have an account, go here to set up an account
sites.google.com/site/sienaphys140spring2012/matlab/creating-an-account
Go to the site below and follow the directions to plot the electric field from multiple charges.
sites.google.com/site/sienaphys140spring2012/matlab/matlab-activities/charge-in-a-box
Finish for Tuesday