Read E2.ppt text version

E2 - Electric Field

·

Electrostatic & gravitational force

·

act at a distance, no direct push or pull 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

·

Electric Field

· · · ·

General Physics 2

Electric Field

1

E2 - Electric Field

·

Coulomb's Law

· ·

Action-at-a-distance model Charges act directly on one each other

charge " charge

·

Electric Field

· ·

!

Field model Field acts on charge

charge " field " charge

General Physics 2 Electric Field 2

Electric Field

·

Electric field is a vector expressed in terms of the electric force and the charge

r r Fe E" q

r r Fe = qE

r E=

! · If Q > 0, E points away from Q

·

1 q ^ rPC 2 4 "#0 rPC !

Electric Field

If Q > 0, E points away from Q

3

!

General Physics 2

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

General Physics 2 Electric Field 4

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?

1. 2. 3. 4.

North South East West

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?

1. 2. 3. 4.

North South East West

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?

1. 2. 3. 4.

3.2x10-16, east 3.2x10-16, west 8.0x10-23, east 8.0x10-23, west

Think-Pair-Share

·

Calculate the electric field 0.1 m from a 10-6 C charge.

General Physics 2

Electric Field

8

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?

General Physics 2

Electric Field

9

Electric Field Lines

·

Field lines show the magnitude (in a relative way) and the direction of the force felt by a positive test charge

General Physics 2

Electric Field

10

Electric Field Lines (L)

·

Draw field lines for the following charge distributions

-

+

General Physics 2

Electric Field

11

Electric Field Lines (L)

·

Draw field lines for the following charge distributions

-

-

General Physics 2

Electric Field

12

Superposition

·

How do you calculate the electric field due to multiple charges? · Calculate the field due to each charge and add them up!

r r r r E = E1 + E 2 + E 3 + ...

General Physics 2 Electric Field 13

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?

General Physics 2

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. Where on the line joining Q1 and Q2 is the electric field equal to zero?

General Physics 2

Electric Field

15

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?

General Physics 2

Electric Field

16

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

General Physics 2

Electric Field

17

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

General Physics 2

Electric Field

18

Field of the dipole

General Physics 2

Electric Field

19

Torque of a dipole

·

The torque of a dipole in an external electric field

·

r r v r v v v " dp = d # qE = qd # E = pe # E r r pe $ qd

General Physics 2 Electric Field

d is the distance displacement vector from "-" to "+"

20

Water is a dipole

·

If water is put in an electric field, the water molecules will experience a torque that attempts to twist them

General Physics 2

Electric Field

21

Lightning

Interactions b/w hail and smaller ice crystals transfer e- to hail. They are heavier and sink to base of cloud. Ice Crystals Hail

Thunder: shock wave created by rapid heating of air

Light due to e-'s recombining w/ions Path of lightning is about 1 cm in diameter

Upward streamers of + ions travels up from ground. Lightning occurs if it meets stepped leader from cloud.

e- run away from cloud base, leaving net + charge on ground under cloud

Lethal Strike

·

Physiological effects of a direct strike:

· · ·

stops heart paralyzes muscles internal burns need mouth-to-mouth Side flash or touching struck object Ground current Dead-end upward streamer

General Physics 2 Electric Field 23

·

Heart may restart, but breathing won't

·

·

Other types of strikes:

· · ·

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 24

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 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 25

·

If outside,

· · · · ·

Lightning: Physical Characteristics

·

Voltage

·

several hundred million volts 1020 electrons in a fraction of a second ~10 kiloamps Good question. We'll get to that soon.

·

Current

· ·

·

So what is a Volt? what is an amp?

·

General Physics 2

Electric Field

26

Electric Field:

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

Group Problems

·

E2B.4, E2B.5, E2S.2

General Physics 2

Electric Field

29

Information

E2.ppt

29 pages

Report File (DMCA)

Our content is added by our users. We aim to remove reported files within 1 working day. Please use this link to notify us:

Report this file as copyright or inappropriate

156485