Monday, January 9, 201 2

Outline, Monday January 9, 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 – 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 Web site, and, perhaps with some delay, on blackboard! – Qwizdom quizes will start next week.

Outline, Wednesday, Jan. 11, 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 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. 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.

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 -1.6  Coulomb, and a proton has a charge of  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 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.

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!

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!

Electroscopes – somewhat more advanced electric charge detectors

Links to some Web sites worth watching: John Travoltage: John_Travoltage John_Travoltage Scotch tape experiments: picid=10&cycleid=21 picid=10&cycleid=21 Static electricity simulation: Balloons_and_Static_Electricity Balloons_and_Static_Electricity Electroscope: picid=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 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 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 relations describing the interaction between electric charges (known as “The Coulomb Law”, or “Coulomb’s Inverse Square Law”.

Wimhurst