Electrostatics Concept Summary

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 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.

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.

Coulomb’s Law where k = 9.0 x 109 N m2/C2 Q q Felectric = k 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 Bring a charged object near (but not touching) a neutral object. Ground the neutral object. Remove the ground. Remove the charged object The neutral object now has a charge opposite to the charged object.

The End