Charging by Conduction Charged object touches the uncharged object The object will have the same charge Transfer of electrons 20.2 Electric Force

Separate Charges Bring a charged object close to an uncharged object

Charging By Induction The charged object and uncharged objects don’t touch The uncharged object gains the opposite charge

20.2 Electric Force Suspend a charged plastic rod What happens when a negative rod it brought near? When does the rod move? What happens when a positive glass is brought near?

Summarize the Experiment There are 2 kinds of charges: + & - Charges exert forces on other charges at a distance The force is stronger when the charges are closer together Like charges repel; opposite charges attract

Coulomb’s Law How does force depend upon distance? Indirectly Experiments show indirectly proportional to the distance squared, r 2

How does force depends on charge? Directly The product of the charge on the objects is directly proportional to the force F  q 1 q 2

Coulomb’s Law Coulomb summarized his results and the results are now known as Coulomb’s Law

The Unit of Charge The amount of charge is hard to directly measure Coulomb defined the standard quantity of charge in terms of the amount of force it produces SI unit of charge is the coulomb, C 1 C = 6.24 x e - or protons

Lightning can carry any where from 5 C to 25 C One electron has a charge of 1.60 x C The charge on one electron is called the elementary charge

To make Coulomb’s law equal and not proportional there needs to be a constant, k Where k = 9.0 x 10 9 N. m 2 /C 2 So

Like other forces this is a vector quantity They need both a magnitude and direction A resultant needs to be found if more than one force acts

Application of Electrostatic Forces Collect soot from smokestacks to reduce pollution Paint automobiles or lockers Photocopy machines