Lecture notes, lecture 1-18 PDF

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Lecture 1 - 18
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PHYC10004 Physics B Electromagnetism Professor Ray Volkas Assoc. Professor Harry Quiney School of Physics University of Melbourne http://www.lms.unimelb.edu.au

Course Outline



1. Electrostatics  Stationary charged particles  Electrostatic forces, fields, potentials 2. Circuit Theory  Capacitors  Electrons in circuits, currents, power, Ohm’s Law  Resistors, capacitors and batteries in circuits  Kirchoff’s Rules 3. Magnetism  Moving charged particles  Magnetic fields and forces  Magnetic induction  Textbook  

R. Knight, Physics for scientists and engineers Pre-reading and reading suggested for each lecture

9

3

6

Common problems  Vectors, dot and cross product  Units  SI prefixes:

µ = 10-6 (micro) n = 10 -9 (nano) p = 10 -12 (pico)  Problems with algebraic manipulation

Exam: formula sheet

 

Part 1 - Outline Chapters 26-30 – Electrostatics  Introduction to charge  Coulomb’s Law  The electric field  Gauss’s Law  Electric potential  Energy and the electric field  Applications...

Today’s Lecture  History and significance of electromagnetism  What is charge?  Conductors and insulators

 600 BC – Ancient Greeks discovered lumps of

amber, charged by friction (rubbing) would pick up small pieces of straw  Greek for amber = “elektron”  Magnetism discovered about the same time Geographica, Random House, 1999

 Region of middle-east called “Magnesia”

Encyclopedia Brittannica, Inc. © 1994-1999

In the beginning...

In the beginning...  1820 H.C. Oersted: electric current in wire

deflects compass  Late 19th century: Maxwell’s equations combine electricity and magnetism  1905 Electricity and Magnetism unified by Einstein through the theory of special relativity 

This course ends just before 1905!

Maxwell’s equations

Now...  Mid 20th century: Quantum

ElectroDynamics (QED)

 Basis of many areas of science,

e.g.:

Optics and photonics (light = EM wave)  Chemistry  Biomedical imaging  etc.  Basis of modern technology  Electric motors to particle accelerators  Domestic power and communications network  Global positioning system  Computer chips  etc. etc. etc. 

The four forces of nature

Strong force binds the nucleus

Weak force in radioactive decay

Electromagnetic force binds atoms

Gravitational force binds the solar system

The four forces of nature Electric force k qq F = e 12 2 rˆ r Coulomb’s Law 1798 (next lecture) Gravitational force Gm1m2 F= rˆ 2 r Newton’s Law

Electromagnetic force binds atoms

Gravitational force binds the solar system

Introductory Electrostatics  Gravitational “charge” comes in one type  Electric charge comes in two types

+

+ Like charges repel

– +

– –

Unlike charges attract Pith balls/Electroscope

Aside: Introductory atomic physics  Negative electrons move

around, bound to a positive nucleus  There are the same number

of electrons (-) as protons (+)

– –

++ ++

 So matter is neutral  Can charge matter by adding–

or removing electrons –

Introductory electrostatics  The unit of charge: The Coulomb  Charles Coulomb (1736-1806)  1 Coulomb = 1 C = 1 Amp x 1 Sec  Examples:   

Electron charge = -|e| = –1.6x10–19 C Proton charge = +|e| = +1.6x10–19 C Neutron charge = 0  

+

Charge created by cat fur = 1 µC Charge in thundercloud = 40 C

– Pith balls/Electroscope

Introductory electrostatics  The electric force is very strong! Gravitational attraction negligible

1% charge exchange

+

–

1% charge exchange leads to force strong enough to drag Earth from orbit!

Charge  Usually create charge by scraping electrons

from surfaces 

Also by touching surfaces together

+

+

+

+

Microscopic view of a surface

3. Cat Fur

– – ––

Charge is quantised  Matter is discrete

Charge that can be detected composed of integer multiples of elementary charge  q = ne, n = ±1, ±2, ±3, …  e =1.602×10-19 C  Charge is quantised 

Charge is conserved  Charge of any isolated system cannot change  Charge can be transferred, but not created  Examples  Radioactive decay  Electron (e-) - positron (e+) annhilation

e+ + e− → γ + γ Electron carries charge –e  Positron carries charge +e  γ high energy photons (uncharged)  Net charge 0 before and after 

Conduction of charge  A conductor contains electrons not strongly bound to

any particular nucleus  These “free electrons” can move under the influence of an electric field  electric current  There are huge numbers of free electrons available in an ordinary conductor  

e.g: copper >1022 per cubic cm Prevented from leaving surface by attractive force of nuclei

 Other conductors  Tap water, the human body, ionised gases ….

Conduction of charge + ++ + + + + + + + ++ + + + + + + + + + + + + Charged conductor

Uncharged conductor

Insulators  Electrons strongly bound to their own nuclei  Not free to move  Typically...