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E2 - 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 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 1 q ˆ E r PC 2 40 rPC • • Insert fig. E2.1 If Q > 0, E points away from Q If Q > 0, E points away from Q General Physics 2 Electric Field 3 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 • Insert E2.2 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! 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 Q2 = 10 x 10-6 C 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 Insert E2.4 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 E2.5 General Physics 2 Electric Field 18 Field of the dipole • • Insert E2.6 and E2.7, E2.8 E2.9, E2.10 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 “+” r r v v r v v dp d qE qd E pe E r r pe qd Insert E2.9 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 E2.8 and E2.10 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 • E2B.4, E2B.5, E2S.2 General Physics 2 Electric Field 29