Lecture 05

Lecture 05 - Conductors and Insulators Different materials...

Info iconThis preview shows pages 1–9. Sign up to view the full content.

View Full Document Right Arrow Icon
Conductors and Insulators Different materials respond differently to electric field Conductor : contains mobile charges that can move through material Insulator : contains no mobile charges anywhere on or inside material only on surface Excess charges located in patches Spread over entire surface Distribution of excess charges E net nonzero inside E net = 0 inside Static equilibrium individual atoms/molecules polarize entire sea of mobile charges moves Polarization no yes Mobile charges Insulator Conductor
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Discharging by contact: On approach: body polarizes On contact: charge redistributes over larger surface Grounding: connection to earth (ground) – very large object Charging and Discharging
Background image of page 2
Charging by Induction
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Very complex phenomenon: charge is transferred from one object to another through air. Air: no movable charges, excellent insulator, all charges are bound to neutral atoms. During spark: electrons are ripped out of molecules (ionization) , leaving free electrons and positive N 2 + and O 2 + ions (plasma) . What is a physically reasonable mechanism for the creation of electric sparks in air? Sparks in Air
Background image of page 4
How can electric charge move through air? Why does a spark last only a short time? Why is light given off? How can air become ionized? To explain a spark we must explain these aspects of the phenomenon: A long chain of reasoning! Sparks are one of the most dramatic kinds of electric phenomena Sparks in Air
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
In order to construct a model for the motion of electric charge through ionized air, we will first look at a simpler case: Two charged metal spheres. Free electrons shift from left to right in a very short time. Are electrons from the left sphere transferred to the right one? Two Metal Balls If the air between them is not ionized , the air is an insulator: charges stay on the spheres . Simplify problem : Consider charge transfer through a wire.
Background image of page 6
1 meter long wire contains ~ 10 23 free electrons Typical amount of charge on sphere ~ 10 10 e (~10 -9 Coulomb) How far the free-electron sea must shift inside the wire in order to neutralize the positive sphere? In a fraction of 10 10 /10 23 = 10 -13 of the 1 m wire are enough electrons to neutralize the ball. The free-electron sea shifts about 10 -13 m . This is 1000 times less than one atomic size! Charge Transfer Through a Wire The electrons that go onto the positive ball are the ones that were initially very near the ball, not those on the other side of the wire!
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Guess: Electrons simply jump from the negative ball to the positive one How far, on average, an electron could travel in air before colliding with a gas molecule?
Background image of page 8
Image of page 9
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 02/10/2010 for the course PHYS 272h taught by Professor Staff during the Spring '08 term at Purdue.

Page1 / 30

Lecture 05 - Conductors and Insulators Different materials...

This preview shows document pages 1 - 9. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online