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.
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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