PH504: Introduction
1. Preparation
The entire course is delivered in 3 chunks with 3 class tests.
There will be plenty of assignments with answers provided.
Electrostatics
Electromagnetism
Optics
Prof Michael Smith will deliver the first 10 lectures and a class test.
Lecture notes for electrostatics will be posted on:
http://astro.kent.ac.uk/mds/Modules/modules.htm
Previous years’ notes and other details are also available.
Experiments on electrostatics will not be undertaken. Therefore, you might
wish to watch Walter Lewin’s lectures on video (not a requirement). You can
download and save from:
http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring2002/video-lectures/
http://ocw.mit.edu/ans7870/8/8.02/videolectures/wl-802-lec1-220k.rm
Also online on youtube, and at
http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-andMagnetismSpring2002/CourseHome/
MDS Topics:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Intro, Complex numbers,
Partial differentiation
Vectors
Coulomb’s law, Gauss’ law
Potential energy and electric potential
Electric dipole
Gauss law
Capacitance
Dielectrics; polarisation
Boundary conditions for E, D
Useful webnotes:
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http://farside.ph.utexas.edu/teaching/em/lectures/lectures.htm
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http://www.david-mowbray.staff.shef.ac.uk/
Recommended Text Book
Many textbooks cover the subject matter of this part of the course.
Recommended: 'Electricity and Magnetism' by W J Duffin
(McGraw-Hill), currently in its fourth edition. This provides a good,
clear description of the physics with a comprehensive discussion
of the underlying mathematics.
An alternative book is 'Electromagnetism' by I S Grant and W R
Phillips (Wiley) 1990, part of the Manchester physics series. This is
of a similar level to Duffin although the general layout, diagrams
etc are less clear in places.
A more recent book is ‘Classical Electromagnetism’ by R H Good.
The mathematics level of this book is quite high and subjects are
covered in a very different order to most other books. However it
does contain many worked problems and more detailed
discussions of some of the conceptual points and assumptions.
2. Why Bother with electrostatics?
1. Strength: Even though electrostatically induced forces seem to
be rather weak, the electrostatic force between e.g. an electron
and a proton, that together make up a hydrogen atom, is about 40
orders of magnitude stronger than the gravitational force acting
between them.
For most objects, the number of negative charges equals the
number of positive charges, giving a net charge of zero.
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2. Annoyance.
Static charge, bad hair days, wrinkled clothes, dog hair that I
can’t get off of my pants.
3. Application
Ink Jet Printer – words written by deflection of ink droplets in
electrostatic fields
Ion Mass Spectrometers. Why do they swab your bags at the
airport?
The compounds acquired are desorbed and ionized. The ions are
accelerated toward the detector by a constant electric field
gradient in the drift region.
Since the ‘flight’ depends on ion mass and charge, explosives can
be detected.
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3. In a Nutshell
There are two types of charges: positive and negative.
Like charges repel and opposite charges attract.
Coulomb’s law:
F = Q1Q2/(4 0r2)
where
0
is the permittivity of free space.
A conductor allows electric charge to travel through it with ease.
Charges can flow through a metal with very little resistance.
An insulator does not, it blocks it. Rubber, for instance, has its
atoms arranged such that it is very difficult for electric charge to
flow through it. Rubber is considered to be an insulator.
All materials have a certain specific conductivity (Siemens/m).
The conductivity of a material
depends on the material’s arrangement of free electrons.
Below is a chart of some common materials and their conductivities. Electrical Conductivity
Material
(1/(ohm*m))
60 x 106
60 x 106
5 x 106
10-8 - 10-14
10-10 - 10-14
10-13 - 10-16
Silver
Copper
Lead
Wood
Glass
Hard Rubber
Values taken from Tipler, Paul A. Physics, Third Edition. 1991.
A person's body acts as a conductor.
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The skin has a very large resistance (on the order of MegaOhms).
The salt water (electrolyte) solution that is in your body is designed
specifically to conduct electricity, so that your nerve impulses can
reach the muscles.
Water with absolutely no impurities conducts electricity very
poorly.
Triboelectric Series. When certain types of materials are rubbed
against other certain types, charge may be transferred from one to
the other. The amount and type of charge is determined by the
Triboelectric Series.
The Triboelectric Series is a list of materials which determines
which materials will become positively charged and which will
become negatively charged when rubbed together. A sample
portion of the Series is shown below.
To read the chart, those materials nearer the top will become
positively charged and those nearer the bottom will become
negatively charged. For instance, when Teflon is rubbed against
silk, the Teflon becomes negatively charged and the silk becomes
positively charged.
Positive Charge
Human hair
Nylon
Wool
Silk
Paper
Cotton
Wood
Hard rubber
Teflon
Negative Charge
Aircraft & Space launch Vehicles can become charged.
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Polarisation. When an uncharged object is placed near a charged
object its charges rearrange themselves. Those charges attracted
to the charged object move towards the charged object and those
charges repelled move away. This effect is known as polarization.
Charges on a conductor tend to gather at sharp points. A
metal cone, for instance, which has been charged will have a lot of
charge at the point, and much less charge elsewhere.
WHY ??
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