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# Mostly Ch 21 – getting into 22…. Three pithballs are suspended from thin threads. Various objects are then rubbed against other objects (nylon against.

## Presentation on theme: "Mostly Ch 21 – getting into 22…. Three pithballs are suspended from thin threads. Various objects are then rubbed against other objects (nylon against."— Presentation transcript:

Mostly Ch 21 – getting into 22…

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.

A metallic object can be charged without ever touching it with a charged rod. An insulator can become polarized when placed near a charged object, causing it to be attracted to the object.

++ + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ++++++++++++++ Conductor

++ + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ________________ Conductor

Wire to Ground ++ + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ________________ Conductor Grounded

++ + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ++++++++++++++ Insulator

++ + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ++++++++++++++ Insulator

Two uniformly charged spheres are firmly fastened to, and electrically insulated from, frictionless pucks on an air table. The charge on sphere 2 is three times the charge on sphere 1. Which force diagram correctly shows the magnitude and direction of the electrostatic forces: A. C. B. D. E. none of the above

Which of the following is not true? The electric force 1)decreases with the inverse of the square of the distance between two charged particles. 2)between an electron and a proton is much stronger than the gravitational force between them. 3)between two protons separated by a distance d is larger than that between two electrons separated by the same distance d. 4)may be either attractive or repulsive.

F e =k e q1q2q1q2 r2r2 k e =9.0 x 10 9 Nm 2 /C 2 q is the amount of charge on each object Charge of an electron e = 1.6 * 10 -19 C C= Coulomb - a unit of charge r q2q2 q1q1 Charles-Augustin de Coulomb 1736-1806

 Vectors!  Scale (compare to gravity) – ◦ 36 orders of magnitude stronger!!  Superposition  Symmetry ◦ We will say this again and again and again

Human Hands Fur (rabbit) Glass Human Hair Nylon Wool Quartz Fur (cat) Silk aluminum Paper, Cotton Wood Copper, brass Rubber Polyester, Styrene (Styrofoam) Saran Wrap Polyethylene (like scotch tape) Polypropylene, Vinyl (PVC) Teflon (very negative) India rubber On contact between any two substances shown in the column, the one appearing above becomes positively charged, the one listed anywhere below it becomes negatively charged Triboelectric Sequence

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