1. E2 - Electric Field Electrostatic & gravitational force Electric Field
act at a distance, no direct push or pull
Electric Field
conceived by Michael Faraday
extends out from every charged object
other charges interact with field
direction of field determined by motion induced on a positive test charge
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Electric Field

2. E2 - Electric Field Coulomb’s Law Electric Field
Action-at-a-distance model
Charges act directly on one each other
Electric Field
Field model
Field acts on charge
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Electric Field

3. Electric Field
Electric field is a vector expressed in terms of the electric force and the charge
If Q > 0, E points away from Q
Insert fig. E2.1
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Electric Field

4. Magnitude of Test Charge, q0
The test charge is a conceptual tool to help understand the magnitude and direction of the electric field.
The magnitude of the test charge is VERY small.
The test charge does not affect existing charge distribution
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Electric Field

5. Electric Field: A proton is released in a uniform electric field, and it experiences an electric force toward the south. What is the direction of the electric field?
North
South
East
West
E = F/q, since q is positive, E has the same direction as F.
South

6. Electric Field: An electron is released in a uniform electric field, and it experiences an electric force toward the west. What is the direction of the electric field?
North
South
East
West
E = F/q
If force is west, E must be East

7. Electric Field: What are the magnitude and direction of the electric force on an electron (e=1.6x10-19 C) in a uniform electric field of strength 2000 N/C that points due east?
3.2x10-16, east
3.2x10-16, west
8.0x10-23, east
8.0x10-23, west
F=qE
So F is west because q is negative
F=1.6E-19*2000 = 3.2E-16 west

8. Think-Pair-Share
Calculate the electric field 0.1 m from a 10-6 C charge.
E = kq/r^2 = 9E9*1E-6/.1^2 = 9E5
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Electric Field

9. Analogy
Electric field is electric force per unit charge. What is the analogous quantity for gravitational force? Derive a mathematical expression for gravitational field. Have you seen this quantity before?
E = F/q, g = F/m = gravity!
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Electric Field

10. Electric Field Lines
Field lines show the magnitude (in a relative way) and the direction of the force felt by a positive test charge
Insert E2.2
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Electric Field

11. Electric Field Lines (L)
Draw field lines for the following charge distributions - +
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Electric Field

12. Electric Field Lines (L)
Draw field lines for the following charge distributions - -
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Electric Field

13. Superposition
How do you calculate the electric field due to multiple charges?
Calculate the field due to each charge and add them up!
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Electric Field

14. Think-Pair-Share (L)
Q1 = -1x10-6 C
Q2 = 10 x 10-6 C
Two charges, Q1 and Q2, are separated by 100 nm. What is the electric field midway between the charges?
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Electric Field

15. Think-Pair-Share (L)
Q2 = 10 x 10-6 C
Q1 = 1x10-6 C
Two charges, Q1 and Q2, are separated by 100 nm. Where on the line joining Q1 and Q2 is the electric field equal to zero?
Somewhere in between. The field on Q1 point in the positive x, the field on Q2 points in the negative x. Set x = 0 at Q1 and the distance between as d.
E = k(Q1/r^2 -Q2/(d-r)^2 = 0 solve for r
E = 1/r^2-10/(d-r)^2=0
(d-r)^2 = 10r^2
Quadratic equation
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Electric Field

16. Electric Field is a Vector
What is the net Electric field at point A?
What is the net force on a charge Q that is placed at A?
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Electric Field

17. Electric Field of a dipole
A dipole is a pair of particles separated by a small distance, equal magnitude and opposite signs
The field of a dipole decreases with distance more rapidly than a single charge
Insert E2.4
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Electric Field

18. A dipole has an electric field
A dipole is electrically neutral, so why does it have a field?
The electric field of the near charge is longer than that of the far charge
Insert E2.5
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Electric Field

19. Field of the dipole Insert E2.6 and E2.7, E2.8 E2.9, E2.10
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Electric Field

20. Torque of a dipole
The torque of a dipole in an external electric field
d is the distance displacement vector from “-” to “+”
Insert E2.9
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Electric Field

21. Water is a dipole
If water is put in an electric field, the water molecules will experience a torque that attempts to twist them
Insert E2.8 and E2.10
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Electric Field

22. 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

23. 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
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Electric Field

24. 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
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Electric Field

25. 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!)
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Electric Field

26. 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.
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Electric Field

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

29. Group Problems
E2B.4, E2B.5, E2S.2
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Electric Field