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 General Physics 2 Electric Field

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 General Physics 2 Electric Field

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 General Physics 2 Electric Field

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

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

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

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

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 General Physics 2 Electric Field

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! General Physics 2 Electric Field

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 General Physics 2 Electric Field

Electric Field Lines (L) Draw field lines for the following charge distributions - + General Physics 2 Electric Field

Electric Field Lines (L) Draw field lines for the following charge distributions - - General Physics 2 Electric Field

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

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

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 General Physics 2 Electric Field

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

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 General Physics 2 Electric Field

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 General Physics 2 Electric Field

Field of the dipole Insert E2.6 and E2.7, E2.8 E2.9, E2.10 General Physics 2 Electric Field

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 General Physics 2 Electric Field

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 General Physics 2 Electric Field

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

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

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

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

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

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

Group Problems E2B.4, E2B.5, E2S.2 General Physics 2 Electric Field