E 2 Electric Field Electrostatic gravitational force act

E 2 - Electric Field • Electrostatic & gravitational force • • 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 General Physics 2 Electric Field 1

E 2 - Electric Field • Coulomb’s Law • • • Action-at-a-distance model Charges act directly on one each other Electric Field • • Field model Field acts on charge General Physics 2 Electric Field 2

Electric Field • Electric field is a vector expressed in terms of the electric force and the charge Insert fig. E 2. 1 • • If Q > 0, E points away from Q General Physics 2 Electric Field 3

Magnitude of Test Charge, q 0 • 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. 6 x 10 -19 C) in a uniform electric field of strength 2000 N/C that points due east? 1. 2. 3. 4. 3. 2 x 10 -16, east 3. 2 x 10 -16, west 8. 0 x 10 -23, east 8. 0 x 10 -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 Insert E 2. 2 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! General Physics 2 Electric Field 13

Think-Pair-Share (L) Q 1 = -1 x 10 -6 C Q 2 = 10 x 10 -6 C • Two charges, Q 1 and Q 2, are separated by 100 nm. What is the electric field midway between the charges? General Physics 2 Electric Field 14

Think-Pair-Share (L) Q 1 = • 1 x 10 -6 Q 2 = 10 x 10 -6 C C Two charges, Q 1 and Q 2, are separated by 100 nm. Where on the line joining Q 1 and Q 2 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 Insert E 2. 4 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 Insert E 2. 5 General Physics 2 Electric Field 18

Field of the dipole • • Insert E 2. 6 and E 2. 7, E 2. 8 E 2. 9, E 2. 10 General Physics 2 Electric Field 19

Torque of a dipole • The torque of a dipole in an external electric field • d is the distance displacement vector from “ -” to “+” Insert E 2. 9 General Physics 2 Electric Field 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 Insert E 2. 8 and E 2. 10 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 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 Light due to e-’s recombining w/ions Path of lightning is about 1 cm in diameter

Lethal Strike • Physiological effects of a direct strike: • • Heart may restart, but breathing won’t • • stops heart paralyzes muscles internal burns 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 23

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 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 25

Lightning: Physical Characteristics • Voltage • • Current • • • several hundred million volts 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 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 • E 2 B. 4, E 2 B. 5, E 2 S. 2 General Physics 2 Electric Field 29