Monday, January 9, 2012

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, 2012

• Outline: Charge properties and interactions• Questions: How do we know there are two kinds of charge? What

kinds of interactions do we observe?

Later, it was discovered that many different bodies could be “electrified”by rubbing them against wool, silk, etc. – and that there were twodifferent kinds of “electricity”. ForInstance, between two pieces of amber“electrified” by rubbing against woolthere 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.

Ancients Greeks and Romans discovereda peculiar property of amber, a precioussubstance found at the coasts of Baltic Sea, andoffered to them by merchantscoming from thatarea, at a pricehigher that the price of gold!

Amber, rubbed against wool,attracted light things, such asfeathers or straws.

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?

• 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.

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”.

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’?

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 -1.610-19 Coulomb, and a protonhas a charge of +1.6010 -19 Coulomb. Normally, there is the same number of electrons and protons in each atom, so it is “neutral”.

It all starts with atoms….

Caution! You can oftensee atoms pictured asthe one here – but keepIn mind, it’s only “an artist’s impression”.

Pictures with ellipticalelectron orbits like theones here are WRONGand misleading.

Actually, the electrons form a “cloud” that does not consist simpleelectron orbits.

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 totheir “parent atoms”. Sometimes, it’s not too difficultto “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!

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 lessthan the number of protons in it, and the “net charge” ofBody 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 comesfrom the “migration” of some of the body’s electronsto another body!

Electroscopes – somewhat more advanced electric charge detectors

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

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.

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.

Charles Augustine de Coulomb’s torsion balance experiments(1785): they led to the formulation of a quantitative relationsdescribing the interaction between electric charges (knownas “The Coulomb Law”, or “Coulomb’s Inverse Square Law”.

Wimhurst

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