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ElectricityGreeks learned about charge by rubbing amber (fossilized tree resin)
Greek word for amber = “elektron”
Picturefrom
wikipedia
Atomic Structure - A Who’s Who
J. J. Thomson1856-1940
Ernest Rutherford1871-1937
Robert Millikan1868-1953
Discovered electron(Measured charge divided by mass)
Discovered nuclearstructure
Measured charge ofelectron (thereby finding its mass)
Who’s Who in Electricity
Benjamin Franklin 1706-1790
Charles-Augustinde Coulomb1736-1806
Henry Cavendish1731-1810
Defined positive vs.negative charge
Discoveredelectricforce law
Who’s Who in Electricity
Michael Faraday
1791-1867
• Left school at age 13• became book binder • read books he bound• learned science (but not math!)• asked author he read to work in his lab• became great scientist and inventor
• invented notion of “fields”
Who’s Who in Electricity
Karl Gauss 1777-1855
Numerous contributionsto electromagnetic theory
Who’s Who in Electricity
Alessandro Volta1745-1827
Invented battery
Who’s Who in Electricity
Otto von Guericke1602-1686
http://www.hp-gramatke.net/history/english/page4000.htm
Credited with first electrostatic generator - Put ball (made of sulphur) on wooden cradle, spin it and rub by hand
Who’s Who in Electricity
Ewald Jürgen von Kleist1700-1748
Pieter van Musschenbroek1692-1761
Independently developedthe “Leyden jar,” an early capacitor (1745)
Who’s Who in Electricity
Alessandro Volta1745-1827
Invented battery - “Voltaic pile”
Batteries - from Volta to today
Cathode
Anode
Who’s Who in Electricity
André-Marie Ampère1775-1836
Who’s Who in Electricity
Georg Ohm1789-1854
Who’s Who in Electricity
Gustav Kirchhoff1824-1887
Contributed to:Electric CircuitsSpectroscopyBlack-body radiation
Who’s Who in Electricity
John Ambrose Fleming1849-1945
Vacuum tubediode
Russell Ohl1898-1987
Diodes with “p-n junctions”- Toward modern electronics!
Who’s Who in Electricity
Transistors (1940s-50s, 50s-60s, and today)Lee De Forest
1873-1961Triode
Who’s Who in Electricity
Modern transistors - npn and pnp
junctions
Integrated circuits -
William Shockley1910-1989
John Bardeen1908-1991
Walter Brattain1902-1987
Who’s Who in Magnetism
William Gilbert1544-1603
Said Earth is magnet with
iron core
Who’s Who in Magnetism
Hans Christian Oersted1777-1851
Discovered thatmagnetism can be created by currents
Who’s Who in Magnetism
Nikola Tesla1856-1943
Scientist and inventor, AC power, AC motors, wireless transmission
Who’s Who in Magnetism
Hendrik Lorentz1853-1928
Worked on electricity, optics, relativity
Who’s Who in Magnetism
Joseph Larmor1857-1942
Who’s Who in Magnetism
Wilhelm Weber1804-1891
Invented the telegraph(with Gauss)
Karl Gauss 1777-1855
Numerous contributionsto electromagnetic theory
Who’s Who in Magnetism
Edwin Hall1855-1938
Discovered effect while graduate student at
Johns Hopkins
Who’s Who in Magnetism
Jean-Baptiste Biot1774-1862
Felix Savart1791-1841
Developed law in 1820
Who’s Who in Electricity
André-Marie Ampère1775-1836
Who’s Who in Magnetism
Michael Faraday
1791-1867
Joseph Henry 1797-1878
electromagnets, First Secretary of the
Smithsonian
Who’s Who in Magnetism
Luigi Galvani 1737-1798
early studies in bioelectricity,
colleague of Volta
Who’s Who in Magnetism
Heinrich Lenz1804-1865
Who’s Who in Magnetism
James Clerk Maxwell1831-1879
one of the greatest physicists of all time, electromagnetism, thermodynamics/
statistical mechanics
Who’s Who in Optics
Heinrich Hertz1857-1895
Who’s Who in Optics
John Poynting1852-1914
Who’s Who in Optics
Willebrord Snellius
1580-1626
Rene Decartes1596-1650
Thomas Harriot
1560-1621
ibn Sahl ~940-1000
Who’s Who in Optics
Christiaan Huygens1629-1695
Who’s Who in Optics
Arthur Compton1892-1962
Who’s Who in Optics
Lord Rayleigh(John William Strutt)
1842-1919
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
PowerPoint® Lectures forUniversity Physics, Twelfth Edition – Hugh D. Young and Roger A. Freedman
Lectures by James Pazun
Chapter 21
Electric Charge and Electric Field
Announcements
Forces of NatureAll interactions in the entire universe (!) can be described by four forces
You’ve studied only the “gravitational”force
This semester, we’ll do “electricity and magnetism”
Amazingly the two turn out to be part of the same force - the “electromagnetic” force
We’ll hint at a third force soon, the last one is covered in Modern Physics class
Supermassive black hole at the center of the Milky Way (Sagittarius A)
Review of the Gravitational ForceObjects have an intrinsic property called “mass”
An object’s mass can be 0 (light - a photon)Two objects with mass attract each other, the force is
This works for any two objects: “Universal gravitation” is a vector force along line connecting masses - sign force is attractive (always!)
is a measure of strength of gravitational force (“coupling constant”)
is small gravity is very weak
Force goes like weaker for masses that are further apart
Force proportional to mass more mass means stronger force
F = −Gm1m2
r2r̂
⇒⇒
⇒G
G
⇒1/r2 ⇒
F (r̂)
m
Electric ChargeObjects have another intrinsic property called “charge”
Greeks learned this by rubbing amber (fossilized tree resin)
Greek word for amber = “elektron”
q
Do “Balloons” Demo
Amber from wikipedia
Electric ChargeTwo kinds of charges
Called positive (+) and negative (-)By convention (due to Benjamin Franklin)
Objects with charge feel a forceLike charges repel, opposites attract
This is known from experiment/observation
Objects can also be charge neutralMost matter is net charge neutral
Neutral matter contains the same amount of positive and negative charges
Why is most matter neutral?
Benjamin Franklin
1706-1790
Matter as ChargesMatter is made up of protons, neutrons and electrons
The unit of charge is Coulombs:
The charge of a proton is
Note,
Memorize this!
q = e
q = 0
q = −e
[q] = Coulombs
e = 1.60217653(14)× 10−19Ce � 1.6× 10−19C
melectron � mprotonmelectron
mproton� 1
1836
Make-up of AtomsSince ,they behave very differently!
When the charges are “free,”electrons move much easier than protons
They feel the same force(Newton’s Third Law pairs),but their acceleration is different!
Inside atoms, we think of the lighter electrons “orbiting” protons like the Earth orbits the Sun
This will change when we learn quantum mechanics (Modern Physics)
Protons and neutrons reside in the nucleus of the atom
melectron � mproton
Fe on p Fp on e
motion
How Do We Know?Electrons discovered in 1897 by Thomson
It was thought that the positive chargeswere distributed evenly throughout the matter (Thomson’s “plum pudding” model)
Rutherford experiment 1907His assistant (Geiger) developed a counter of alpha particlesAlpha particles (helium nuclei, discovered in 1895) shot into a foil made of gold, they found more particles bounced backwards than expected
Conclusion - Almost all the mass is localized in small regionsIf an atom was the size of a football stadium, the nucleus would be basketballs at the 50-yard line and the electrons would be in the highest seatsMost of what makes up matter (your desk, your fingers, ...) is almost entirely empty space!
Atoms - Neutrals and Ions
Neutral matter is neutral not because it is made up of uncharged particles, but because it is made up of
equal numbers of positive and negative charges.
The Electric ForceHow would you determine the electric force between charges?
Same way Cavendish did for gravitation in 1787- torsional balance!Coulomb measured the electrostatic force(for charges at rest) in 1784.
Cavendish already did it, but didn’t publish!
Charles-Augustinde Coulomb1736-1806
Henry Cavendish1731-1810
Coulomb’s Law• The result (Coulomb’s Law) is astounding!
• Looks just like gravitational force!• Again an “inverse square law”• Mass m => Charge q• Gravitational constant G =>
Coulomb’s constant k(electrostatic coupling constant)
• --------
F =kq1q2r2
r̂
Finish this
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
PowerPoint® Lectures forUniversity Physics, Twelfth Edition – Hugh D. Young and Roger A. Freedman
Lectures by James Pazun
Chapter 21
Electric Charge and Electric Field
Announcements
Coulomb’s Law• The result (Coulomb’s Law) is astounding!
• Looks just like gravitational force!• Again an “inverse square law”• Mass m => Charge q• Gravitational constant G =>
Coulomb’s constant k(electrostatic coupling constant)
•
F =kq1q2r2
r̂
Measure with voltmeter and plasma ball?
Examples of electrical force calculated—IA fascinating comparison of gravitational force to electrostatic force is shown in Example 21.1 and Figure 21.11.
Regard Problem-Solving Strategy 21.1.
See also Example 21.2 and Figure 21.12.
Examples of electrical force calculated—IIConsider Example 21.3 and Figure 21.13.
See also Example 21.4 and Figure 21.14.
Movement of charges—charging by conduction• Materials that allow
easy passage of charge are called conductors. Materials that resist electronic flow are called insulators. The motion of electrons through conducts and about insulators allows us to observe “opposite charges attract” and “like charges repel.”
Electrons move freely and charges may be induced• Take a child’s toy, a rubber balloon. If you rub the balloon
vigorously on a fuzzy sweater then bring the balloon slowly toward a painted concrete or plaster wall, the balloon will stick to the wall and remain for some time. The electrostatic force between static electrons and the induced positive charge in the wall attract more strongly than the weight of the balloon.
Static electricity about an insulator can shift• The motion of static charges about a plastic comb and light bits
of paper can cause attractive forces strong enough to overcomethe weight of the paper.
Electric fields may be mapped by force on a test charge
If one measured the force on a test charge at all points relative to another charge or charges, an electric field may be mapped.
This experiment is often done in one’s mind (called a “gedanken experiment”).
Electric fields I—the point chargeFields of force may be sketched for different arrangements of charge.Consider Example 21.6 and Figure 21.19.
Electric fields II—charges in motion within a fieldConsider Example 21.7.
Consider Example 21.8 and Figure 21.21.
Electric fields add as vectorsRegard Figure 21.22.
Review Problem-Solving Strategy 21.2.Follow Example 21.9 and Figure 21.23.
A field around a ring or line of chargeReview Example 21.10 and Figure 21.24.
Review Example 21.11 and Figure 21.25.
A field around a disk or sheet of chargeReview Example 21.12 and Figure 21.26.
Review Example 21.13 and Figure 21.27.
Electric field lines map out regions of equivalent force I
Electric dipoles and water• As mentioned
in the introduction, the dipole force of water is vital to chemistry and biology.
Consider force and torque on a dipole• Regard Figure 21.32.
• Follow Example 21.14 and Figure 21.33.
The electric field of a dipole revisited
Consider Example 21.15.
Figure 21.34 illustrates the example.
