Lightning

Hail

Ice Crystals

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

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

Path of lightning is about 1 cm in diameter

Thunder: shock wave created by rapid heating of air

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

Light due to e-’s recombining w/ions

General Physics 2 Electric Field 2

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

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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

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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 5

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.

Electric Field: The electric field just before a lightning strike points:

1. toward the ground 2. toward the cloud 3. along the ground

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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.

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.

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.

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 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 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 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.

QUESTIONS:

The torque on a dipole in an electric field is

where θ is the angle the dipole makes with the electric field.

Water Demo

•  Rub a rod with rabbit fur and bring near a stream of water

•  Group problem - Explain what’s happening

The electric field exerts a force

In a uniform field, the acceleration is constant:

on a charged particle. If this is the only force acting on q, it causes the charged particle to accelerate with

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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

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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

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