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

Electric Field of a Line of Charges What is the electric field, E, at point P? For continuous charge distributions, we need to use calculus. General Physics 2 Electric Field

The Electric Field of a Continuous Charge Distribution The surface charge density of a two-dimensional distribution of charge across a surface of area A is defined as Surface charge density, with units C/m2, is the amount of charge per square meter.

A Plane of Charge The electric field of an infinite plane of charge with surface charge density η is: For a positively charged plane, with η > 0, the electric field points away from the plane on both sides of the plane. For a negatively charged plane, with η < 0, the electric field points towards the plane on both sides of the plane.

A Disk of Charge The on-axis electric field of a charged disk of radius R, centered on the origin with axis parallel to z, and surface charge density η = Q/πR2 is NOTE: This expression is only valid for z > 0. The field for z < 0 has the same magnitude but points in the opposite direction.

A Sphere of Charge A sphere of charge Q and radius R, be it a uniformly charged sphere or just a spherical shell, has an electric field outside the sphere that is exactly the same as that of a point charge Q located at the center of the sphere:

The Parallel-Plate Capacitor The figure shows two     electrodes, one with charge     +Q and the other with –Q     placed face-to-face a     distance d apart. This arrangement of two     electrodes, charged equally     but oppositely, is called a     parallel-plate capacitor. Capacitors play important     roles in many electric     circuits.

The Parallel-Plate Capacitor The electric field inside a capacitor is where A is the surface area of each electrode. Outside the capacitor plates, where E+ and E– have equal magnitudes but opposite directions, the electric field is zero.

EXAMPLE 27.7 The electric field inside a capacitor QUESTIONS: Q = 3.5 nC N = 2.2 x 10^10 electrons

Dipoles in an Electric Field The torque on a dipole in an electric field is where θ is the angle the dipole makes with the electric field.

Rub a rod with rabbit fur and bring near a stream of water Water Demo Rub a rod with rabbit fur and bring near a stream of water Group problem - Explain what’s happening

Motion of a Charged Particle in an Electric Field The electric field exerts a force on a charged particle. If this is the only force acting on q, it causes the charged particle to accelerate with In a uniform field, the acceleration is constant:

Motion of q in a uniform E field The figure shows an electron entering a uniform electric field of 1500. N/C. What is the acceleration of the e- once inside the field? me = 9.11 x 10-31 kg qe = 1.602 x 10-19 C General Physics 2 Electric Field

Motion of q in a uniform E field If the electron is initially moving with a horizontal velocity of 3.0 x 106 m/s, and l = 20 cm, what is the magnitude and direction of the electron’s velocity as it leaves the parallel plates? me = 9.11 x 10-31 kg qe = 1.602 x 10-19 C General Physics 2 Electric Field

Matlab If you don’t have an account, go here to set up an account sites.google.com/site/sienaphys140spring2012/matlab/creating-an-account Go to the site below and follow the directions to plot the electric field from multiple charges. sites.google.com/site/sienaphys140spring2012/matlab/matlab-activities/charge-in-a-box-1-26-10 Finish for Tuesday