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Electric Forces and Fields. AP PHYSICS B - Ch.18. Origins: The Ancient Greeks Found Amber.
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Electric Forces and Fields. AP PHYSICS B - Ch.18.
Origins: The Ancient Greeks Found Amber. Many myths surround the origin of amber. Ovid writes that when Phaeton, a son of Phoebus, the sun, convinced his father to allow him to drive the chariot of the sun across the sky for a day, he drove too close to the earth, setting it on fire. To save the earth, Jupiter struck Phaeton out of the sky with his thunderbolts and he died, plunging out of the sky. His mother and sister turned into trees in their grief but still cried mourning him. Their tears, dried by the sun, are amber.The Greeks called amber elektron, or sun-made, perhaps because of this story, or perhaps because it becomes electrically charged when rubbed with a cloth and can attract small particles. Homer mentions amber jewelry - earrings and a necklace of amber beads - as a princely gift in the Odyssey.Another ancient writer, Nicias, said that amber was the juice or essence of the setting sun congealed in the sea and cast up on the shore.Amber ranges in age from 30 to 50 Million years old. Listen.
The ancient Greeks did not perform any quantitative Electricity experiments. Today we know that: Electric forces hold atoms and molecules together. Electrical impulses control our thinking, feeling, muscles, and metabolic processes. Electronics determine much of our current technology.
Electric Charges • Evidence for electric charges is everywhere. (static electricity and lightning) • Objects may become charged by contact and frictional forces. (clothes in dryer) • Benjamin Franklin (1700’s) discovered that there are two types of charges: (positive charge and negative charge) • Franklin also discovered the Rules of Charge. (like charges repel and unlike charges attract) • Electric charge was found to be conserved and quantized. (Franklin, late 1700’s and Millikan, early 1900’s)
Classes of Materials • CONDUCTORS are materials in which charges may move freely (Cu, Hg, Ag, Au, Al, Fe). • INSULATORS are materials in which charges cannot move freely (rubber, plastic, wood, glass). • SEMICONDUCTORS are materials in which charges may move under some conditions (e.g. Si, Gd, Ge, Y).
Did not happen 1706 - 1790 • Famous Quotations: 1. Haste makes waste. • 2. Never leave ‘til tomorrow which you can do today. • 3. A penny saved is a penny earned. • 4. The sleeping fox catches no poultry. • And lesser known ones … • 1. Come to class with the knowledge of the day before. • 2. If you can’t calculate what you’re talking about, you don’t know what you’re talking about.
Charles Coulomb Measured the Force Between Charged Objects. (q1 q2) d2 F = k · Charles Coulomb 1736-1806 k = 9 x 109 N m2/C2 LNK2LRN
Electric charge is quantized. The fundamental unit of charge is the charge on one electron or one proton: 1e- = 1.602 x 10-19 Coulombs Millikan’s Oil-Drop Experiment Robert Millikan 1900, while a professor at the University of Chicago
Two Ways to Measure Electric Field Strength 1. Using a test charge q0to probe the field. 2. Coulomb’s Law. (they agree) E = F q0 F d E = k · Q d2 LNK2LRN
Electric Field lines always point away from the positive and toward the negative.
Atoms consist of a nucleus containing positively chargedprotons. The nucleus of an atom is surrounded by an equal number of negatively charged electrons. The net charge on an atom is zero. An atom may gain or lose electrons, becoming an ion with a net negative or positive charge. Polar molecules have zero net charge but their charges are unevenly distributed in space (e.g. water). Electric Charges in Atoms
Charges and the Earth • The earth acts as a near-infinite source or sink of charges, and therefore its net charge cannot easily be changed. • Any conductor in contact with the earth is said to be GROUNDED and cannot receive a net charge. (principle of lightning rod)
Induced Charge • Charged objects brought close to a conductor may cause charge to redistribute (polarize the conductor). • If a polarized conductor is momentarily grounded, charge will be transferred to/from the earth, and it may be left with a net charge (by INDUCTION). • Objects may be charged by • conduction (requires contact with another charged object. • induction (requires no contact with another charged object).
Benjamin Franklin determined that there are only 2 types of charge. Rubber Rod - Negative Glass Rod - Positive LNK2LRN Unlike Charges Attract.
LIKE CHARGES REPEL. LNK2LRN
Charging an Object by Induction Which Involves No Contact but Includes Grounding. LNK2LRN
Charging by Induction (no contact with charged object) Charged Rod Two Metal Spheres (Polarized)
Charged Rod Two Metal Spheres (separated)
Charges Redistribute, and we have Two Charged Metal Spheres. Another way
Charged Rod One Metal Sphere (polarized)
What happens if q0 is larger than a test charge? + q0’ >> q0 LNK2LRN
Field Lines Always Point Away from the Positive and Toward the Negative. LNK2LRN
Electric Field Lines: Conventions Positive Point Charge Negative Point Charge
Electric field lines Lines of force
An Electric Field Can Accelerate a Charged Particle. LNK2LRN RESULT: Charge on a proton = 1.6x10-19 C
This could occur in Millikan’s Oil-drop Experiment. RESULT:Elementary unit of charge = charge on an electron =-1.6x10-19 C LNK2LRN
Setting-up a Capacitor. Q C = V E + + + + + + + + ++ + + + + + + - - - - - - - - - - - - - Q V V V E = d LNK2LRN W = q·V
The Effect of Adding an Insulator to a Capacitor. insulator LNK2LRN
A Water Molecule. LNK2LRN
Water Molecules in an Electric Field. No Electric Field. Electric Field, Eo LNK2LRN
Magnetite: From Magnesia (Turkey). Formula: Fe3O4. Description: Dark grey, slightly shiny. Magnetite is naturally magnetic. It is also called Lodestone. In Middle Ages, pilots were called lodesmen. The lodestar is the Polar star, the leading star by which mariners are guided. The name probably comes from Magnesia, but there is a fable of Magnes, a Greek shepherd, who discovered magnetite when the nails in his shoes stuck to the ground!
