Electric Forces and Electric Fields

Electric Forces and Electric Fields.

AP PHYSICS B - Chapter 18.

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.

The Ancient Greeks Found Amber.

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!

The ancient Greeks saw no connection between Electricity and Magnetism. Today we know that: Electric forces hold atoms and molecules together. Electricity controls our thinking, feeling, muscles, and metabolic processes. Electricity and magnetism determine much of

our current technology (e.g. computers). Electricity and magnetism are linked on a fundamental level.

Electric ChargesElectric Charges• Evidence for electric charges is everywhere, e.g.

Ex. static electricity and lightning.

• Objects may become charged by contact and frictional forces. Ex. clothes in dryer

• Benjamin Franklin (1700’s) discovered that there are two types of charges:Ex. positive charge and negative charge.

• Franklin also discovered that like charges repel and unlike charges attract one another.

• Electric charge was found to be both quantized (Millikan) and conserved (Franklin).

Classes of Materials

• CONDUCTORS are materials in which charges may move freely (e.g. copper).

• INSULATORS are materials in which charges cannot move freely (e.g. glass).

• SEMICONDUCTORS are materials in which charges may move under some conditions (e.g. silicon).

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

Unlike Charges Attract.

1706 - 1790

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LIKE CHARGES REPEL.

Charging an Object by Induction Which Involves No Contact but Includes Grounding.

The Theory About How an Insulator Works.

Charles Coulomb1736-1806

Discovered Coulomb’s Lawexperimentally

Charles Coulomb Measured the Force Between Charged Objects.

(q1 q2)

d2

F = k ·

k = 9 x 109 N m2/C2

Quantum of Electric Charge

• Electric charge is quantized. The smallest possible unit is the charge on one electron or one proton: 1e- = 1.602 x 10-19 Coulombs

• No smaller charge has ever been detected in an experiment.

Electric Charges in Atoms

• Atoms consist of a nucleus containing positively charged protons.

• 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).

Nuclear diameter ~ 10-15 m (femtometer)

Atomic diameter ~ 10-9 m (nanometer)

Electric field lines

Lines of force

Field Lines Always Point Away from the Positive and Toward the Negative.

The Electric Field Around Two Unlike Charges.

The Electric Field Around Two Like Charges.

Electric Field Lines: Conventions Positive Point Charge Negative Point Charge

Electric Field lines

Two Ways to Measure Electric Field Strength

F

E = F q0

d

E = k · Q d2

What happens if q0 is larger than a test charge?

+ q0’ >> q0

Charging by Induction (no contact with charged object)

Charged Rod Two Metal Spheres

Charged Rod Two Metal Spheres

(separated)

Now, Two Charged Metal Spheres

Charged Rod One Metal Sphere

(polarized)

An Electric Field Can Accelerate a Charged Particle.

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

Setting-up a Capacitor.

+ + + + + + + + ++ + + + + + +

- - - - - - - - - - - - -

Q

V

QC =

VE

VE =

d

V

W = q·V

The Effect of Adding an Insulator to a Capacitor.

insulator

Three Different Types of Capacitors.

A Stud-Finder is Really a Capacitor.

A Water Molecule.

Water Molecules in an Electric Field.

No Electric Field.Electric Field, Eo