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Monday, January 9 , 201 2. Outline, Monday January 9 , 201 2. Announcements: THERE IS NO LAB TUESDAY BUT THERE IS LAB THURSDAY THIS WEEK!!! Recitation starts Thursday as well. Get a copy of the syllabus, and problem solving guide and rubrics

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outline monday january 9 201 2
Outline, Monday January 9, 2012
  • Announcements:
    • THERE IS NO LAB TUESDAY BUT THERE IS LAB THURSDAY THIS WEEK!!! Recitation starts Thursday as well.
    • Get a copy of the syllabus, and problem solving guide and rubrics
    • IF YOU ARE REPEATING THIS CLASS and PASSED It within the last two years, you may be exempt from lab – EMAIL ME YOUR NAME, Student ID #, when you last took the course and your grade, AND what day and time you are signed up for in lab
    • Mastering Physics and Blackboard will be set up by the end of Wednesday – your first homework is due next Monday
    • Class notes and other relevant course documents will be posted on on http://science.oregonstate.edu/~tgiebult/COURSES/ph213 Web site, and, perhaps with some delay, on blackboard!
    • Qwizdom quizes will start next week.
outline wednesday jan 11 201 2
Outline, Wednesday, Jan. 11, 2012
  • Outline: Charge properties and interactions
  • Questions: How do we know there are two kinds of charge? What kinds of interactions do we observe?
slide4

Ancients Greeks and Romans discovered

a peculiar property of amber, a precious

substance found

at the coasts of

Baltic Sea, and

offered to them

by merchants

coming from that

area, at a price

higher that the

price of gold!

Amber, rubbed against wool,

attracted light things, such as

feathers or straws.

Later, it was discovered that many

different bodies could be “electrified”

by rubbing them against wool, silk,

etc. – and that there were two

different kinds of “electricity”. For

Instance, between two pieces of amber

“electrified” by rubbing against wool

there was a weak repulsive force – but

“electrified” amber attracted glass

“electrified” by rubbing against silk! It

was Benjamin Franklin, who started calling the “glass electricity” as positive,

and the “amber electricity” as negative.

time for some simple experiments
Time for some simple experiments:
  • We need a ‘sensor’ capable of detecting electrification.
  • One such device used in the early days of research on electricity was a “pithball” suspended from a piece of thread.
  • “Pith” is a very light natural substance found in some plants – it’s very much like styrofoam (today, we usually make “pithballs” of styrofoam).
why should pithballs be light
Why should “pithballs” be light?
  • Well, because the forces one can observe in simple “amber-rubbing” experiments are not too strong…. The lighter the ball is, the more sensitive it is to “electrification” effects.
slide7

Suppose that our “pith ball detector” is “negatively electrified”:

Then, of course, we can figure out whether the item “electrified”

by rubbing is “negatively electrified”, or “positively electrified”.

slide8

Learned disputes, speculations, etc., concerning the

mechanism of ‘electrification’, what’s the effect of

rubbing, what changes in the ‘electrified’ body, and

so on, continued for many centuries….

We will skip the ‘historical part’ – it’s definitely very

interesting, but we don’t have enough time!

So, what’s the modern understanding of ‘electrification’?

slide9

It all starts with atoms….

Atoms, the “building blocks” of matter, consist of negative electrons which form

a “cloud” around the nucleus; the nucleus consists of neutral neutrons and positive

protons. An electron has an “electric charge” of -1.610-19 Coulomb, and a proton

has a charge of +1.6010 -19 Coulomb. Normally, there is the same number of

electrons and protons in each atom, so it is “neutral”.

Caution! You can often

see atoms pictured as

the one here – but keep

In mind, it’s only “an

artist’s impression”.

Pictures with elliptical

electron orbits like the

ones here are WRONG

and misleading.

Actually, the electrons

form a “cloud” that

does not consist simple

electron orbits.

slide10

A body in its “normal state” consists of a large number

of atoms. The atoms in solids do not like to “travel”.

They “stay where they are”.

However, the electrons are not too strongly bound to

their “parent atoms”. Sometimes, it’s not too difficult

to “detach” a few of them.

This is exactly what happens in “electrification by

rubbing”. Some electrons from Body A are “relocated”

to Body B. But the atomic nuclei with all their protons

do not move!

slide11

As the result, in Body B there are more electrons than

protons, and the body has a “net negative charge”.

Conversely, the number of electrons left in Body A is less

than the number of protons in it, and the “net charge” of

Body A is now positive.

Remember – positive “elementary charges”, the protons,

do not move! The only “mobile guys” are the electrons!!!

Bodies do not acquire positive charge because some

“extra” positive particles moved in! Positive charge comes

from the “migration” of some of the body’s electrons

to another body!

links to some web sites worth watching
Links to some Web sites worth watching:
  • John Travoltage: http://phet.colorado.edu/simulations/sims.php?sim=John_Travoltage
  • Scotch tape experiments: http://paer.rutgers.edu/pt3/experimentindex.php?topicid=10&cycleid=21
  • Static electricity simulation: http://phet.colorado.edu/simulations/sims.php?sim=Balloons_and_Static_Electricity
  • Electroscope: http://paer.rutgers.edu/pt3/experimentindex.php?topicid=10&cycleid=50
slide14

Three pithballs are suspended from thin threads. Various objects are then rubbed against other objects (nylon against silk, glass against polyester, etc.) and each of the pithballs is charged by touching them with one of these objects. It is found that pithballs 1 and 2 repel each other and that pithballs 2 and 3 repel each other. From this we can conclude that

1. 1 and 3 carry charges of opposite sign.

2. 1 and 3 carry charges of equal sign.

3. all three carry the charges of the same sign.

4. one of the objects carries no charge.

5. we need to do more experiments to determine the sign of the charges.

slide15

Three pithballs are suspended from thin threads. Various objects are then rubbed against other objects (nylon against silk, glass against polyester, etc.) and each of the pithballs is charged by touching them with one of these objects. It is found that pithballs 1 and 2 attract each other and that pithballs 2 and 3 repel each other. From this we can conclude that

1. 1 and 3 carry charges of opposite sign.

2. 1 and 3 carry charges of equal sign.

3. all three carry the charges of the same sign.

4. one of the objects carries no charge.

5. we need to do more experiments to determine the sign of the charges.

slide16

Charles Augustine de Coulomb’s torsion balance experiments

(1785): they led to the formulation of a quantitative relations

describing the interaction between electric charges (known

as “The Coulomb Law”, or “Coulomb’s Inverse Square Law”.

wimhurst
Wimhurst

http://www.youtube.com/watch?v=Zilvl9tS0Og