Grounding ® Providing a path from a charged object

to the Earth is called grounding it. ® Charges will be attracted from (or

repelled to) the Earth by the charged object.

® Since the Earth is so large, both the charged object and the Earth are neutralized.

Concept Summary Adapted from Batesville High School Physics

Forces ® By the early 19th century, physicists

had classified the apparent myriad of forces in nature to just 3 kinds: ® Gravitational force ® Electric force ® Magnetic force

Forces ® By the end of the 19th century, they had

narrowed the list to just 2 forces:

® Gravitational force ® Electromagnetic force

Forces ® The 20th century first added two new

forces to this list that are observed only inside the atomic nucleus: ® Gravitational force ® Electromagnetic force ® Weak force ® Strong force

Forces ® And then found theoretical links that

narrowed the list back to 2 kinds of forces: ® Gravitational force ® Strong/Electroweak force

The Electric Force ® We will turn our attention to the electric

force, which is a force between objects with charge, just as the gravitational force is a force between objects with mass.

Electrostatics ® Electrostatics is the study of electric

charge at rest. ® (Or more or less at rest, in contrast with

current electricity.)

Electrical Charges ® Electric charge is a fundamental

property of matter. ® Two types of electric charges

® Positive charge - every proton has a single positive charge.

® Negative charge - every electron has a single negative charge.

Electrical Charge ® An object with an excess of electrons

is negatively charged. ® An object with too few electrons (too

many protons) is positively charged. ® An object with the same number of

electrons and protons is neutral.

Electrical Forces

® Like charges repel. ® Opposite charges attract.

Elementary Charges ® Protons carry the smallest positive

charge. ® Protons and uncharged neutrons

generally reside in an atom’s nucleus. ® Protons are held in the nucleus by the

strong force.

Elementary Charges ® The smallest negative charge is the

charge on the electron. ® In normal atoms, electrons orbit the

nucleus. ® The electric force between electrons

and protons supplies the centripetal force to keep electrons in the atom.

Elementary Charges ® The charges carried by the proton and

electron are equal in size. ® The mass of the proton is about 2000

times the mass of the electron.

Units of Charge ® The SI unit of charge is the Coulomb.

1 Coulomb = the charge of 6.24 x 1018

electrons

Charge is Conserved ® Electric charge is conserved -

® Electric charge moves from one place to another - no case of the net creation or destruction of electric charge has ever been observed. ®  In solids, only electrons can move. ®  In liquids, gasses, and plasmas, both positive

and negative ions are free to move.

Conductors & Insulators ® Materials in which charges are free to

move about are called conductors.

® Materials in which charges are not free to move about are called insulators.

Semiconductors ® Semiconductors are materials which

are good insulators in pure form, but their conducting properties can be adjusted over a wide range by introducing very small amounts of impurities. ® Silicon, germanium, etc. ® Transistors, computer chips, etc.

Superconductors ® Superconductors are materials that

lose all resistance to charge movement at temperatures near absolute zero (0 K or about -273oC).

® Recently, “high temperature” (above 100 K) superconductors have been discovered.

“Creating” an Electric Charge ® When you “create” an electric charge

(by rubbing your feet on a carpet) you are actually separating existing charges - not creating charges.

® One object ends up with an excess of electrons (- charge), and the other a deficit of electrons (+ charge).

Charging by Friction ® If one neutral material has more affinity

for electrons than another (neutral) material, it will attract electrons from the other.

® One material becomes negatively charged, the other positively charged.

Charging by Contact ® If a charged object is brought in contact

with a neutral object, charges will be repelled from (or attracted to) the charged object.

® The neutral object will gain a charge of the same sign as the charged object.

Electrical Forces ® The electrical force between 2 charges

depends on: ® The size of each charge

® More charge means more force. ® The distance between the charges

® More distance means less force.

Electrical Forces ® The electrical force between 2 charges

is: ® Directly proportional to each charge.

® Inversely proportional to the square of the distance between the charges.

•  where k = 9.0 x 109 N m2/C2

Coulomb’s Law

Felectric = k Q q

r 2

Polarization ® Bringing a charged object near (but not

touching) a neutral object polarizes (temporarily separates) the charge of the neutral object. ® Like charges in the neutral object are

repelled by the charged object. ® Unlike charges in the neutral object are

attracted by the neutral object. ® The neutral object returns to normal

when the charged object is removed.

Electric Dipoles ® An object that is electrically neutral

overall, but permanently polarized, is called an electric dipole. ® Example: H20 molecule

Charging by Induction 1.  Bring a charged object near (but not

touching) a neutral object. 2.  Ground the neutral object. 3.  Remove the ground. 4.  Remove the charged object 5.  The neutral object now has a charge

opposite to the charged object.

The End