# Concept Summary Batesville High School Physics. Forces  By the early 19th century, physicists had classified the apparent myriad of forces in nature.

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Concept Summary 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 (We will see in Ch 36 how the electric and magnetic forces are really just 2 aspects of the same 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 10 18 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 -273 o C).  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.

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.

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 10 9 N m 2 /C 2 Coulomb’s Law F electric = 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: H 2 0 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

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