Lecture 1-2 Charge - Continued

What’s Happ’nin? By now you should have registered on WebAssign and tried the initial assignment. Watch for the appearance of a REAL assignment. You should be reading the first chapter. You should have read the syllabus (web).

Yesterday We looked at charge. and Magic

Summary - Rubbings

TRIBOELECTRIC SERIES your hand glass your hair nylon wool fur silk paper cotton hard rubber polyester polyvinylchloride plastic TRIBOELECTRIC SERIES When we rub two different materials together, which becomes positively charged and which becomes negative? Scientists have ranked materials in order of their ability to hold or give up electrons. This ranking is called the triboelectric series. A list of some common materials is shown here. Under ideal conditions, if two materials are rubbed together, the one higher on the list should give up electrons and become positively charged. You can experiment with things on this list for yourself. GIVES UP MORE ELECTRONS +-+-

Forces Between Charges Q1Q1 Q2Q2 AttractRepel ++X +-X -+X --X

Pith Main Entry: 1 pith Pronunciation: 'pith Function: noun Etymology: Middle English, from Old English pitha; akin to Middle Dutch & Middle Low German pit pith, pit 1 a : a usually continuous central strand of spongy tissue in the stems of most vascular plants that probably functions chiefly in storage b : any of various loose spongy plant tissues that resemble true pith c : the soft or spongy interior of a part of the body

Pith Ball Genuine Pith !!!

A PITHY DEMO

Pithy Happenings Charge Transfer by Contact

Metal Rod Interaction

Induction

More Induction

Magic?

How many ways can we transfer charge? Let us count the ways. ……………..Willie the Shake Direct Contact Separation of bonds (rubbing, etc,) Conduction Induction Magic.

Coulomb’s Law F is a vector.  magnitude  direction 1/4  0 =k=8.99 x 10 9 Nm 2 /C 2 DEFINITION – Unit of Charge is the coulomb

Very Important Concepts in Physics SS S uperposition S ymmetry

Let’s Do Some Problems Strategy 1. Think about how you will proceed. 2. What information is given, what is missing? 3. Develop the equations in an algebraic format. 4. Solve for an algebraic solution. 5. Plug in the numbers at the end. 6. Look at some special cases to check on consistancy. 7. Think about what you have learned that is worth retaining.

In the figure, a central particle of charge -2q is surrounded by a square array of charged particles, separated by either distance d or d/2 along the perimeter of the square.

In the figure, particle 1 of charge +1.0 µC and particle 2 of charge -4.0 µC, are held at separation L = 11.0 cm on an x axis. If particle 3 of unknown charge q 3 is to be located such that the net electrostatic force on it from particles 1 and 2 is zero, what must be the coordinates of particle 3? x=[-11] cm y=[0] cm

A mechanics problem from the past! 

Free Body Forces?

Find force on lower left particle.

What is this “charge” thing? There are two charged particles we will be concerned with:  Electrons  Protons Electrons have a charge of –e Protons have a charge of +e ALL charges are an integral multiple of this fundamental charge “e”

What is e?? |e|=1.6 x Coulombs Exact value in textbook

How Do We Know??? Millikan Oil Drop Experiment A Critical Balance

What have we found? There are TWO types of charge.  Positive  Negative Like Charges Attract Un-Like charges repel The force between charges increases as they are brought closer together. This charge separation results from chemical bonds which are severed in the process.

Forces Between Charges Q1Q1 Q2Q2 AttractRepel ++X +-X -+X --X

The Electroscope