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Ørsted
Notice that we are starting to see
modern style professors as
scientists and researchers.
- professor at the University of Copenhagen.
On 21 April 1820, during a lecture, Ørsted noticed a compass
needle deflected from magnetic north when an electric current
from a battery was switched on and off, confirming a direct
relationship between electricity and magnetism.
[note that light and heat also emanate from electrified wires]
Hans Christian Ørsted
(1777 – 1851)
Danish physicist and chemist.
Ampére
Ampére felt that if a current in a wire
exerted a magnetic force on a compass
needle, two such wires also should interact
magnetically. In a series of ingenious
experiments he showed that this interaction
was simple and fundamental - parallel
(straight) currents attract, anti-parallel
currents repel. The force between two long
straight parallel currents was inversely
proportional to the distance between them
and proportional to the intensity of the
current flowing in each.
André-Marie Ampère (1775 – 1836)
We see people quantifying
the behaviour of magnetic
and electric interactions.
Faraday -- background
Michael Faraday
(1791 -- 1867)
- Faraday was born in Newington Butts.
- apprenticed to a bookbinder
- In 1812, at the age of twenty, and at the end of his
apprenticeship, Faraday attended lectures by the eminent
English chemist Humphrey Davy of the Royal Society.
- Faraday subsequently sent Davy a three-hundred-page
book based on notes that he had taken during these
lectures. Davy hired him as a secretary and he eventually
rose to take Davy’s position.
- Although Faraday received little formal education he was
one of the most influential scientists in history.
Treated by Davy’s wife as a
ser vant.
He refused many honours
including being buried at
Westminster Abbey, a
knighthood, and the presidency
of the Royal Society.
Faraday -- accomplishments
Michael Faraday
(1791 -- 1867)
- discovered two compounds of chlorine and carbon
- investigated diffusion of gases
- investigated alloys of steel
- produced new types of glass for optical purposes
- determined that magnets can affect light (rotating their
polarisation)
- invented a type of Bunsen burner
- discovered benzene
- liquified chlorine gas
- determined the laws of electrolysis
Faraday -- accomplishments
He did not credit Davy (his
boss) with discussions.
Michael Faraday
(1791 -- 1867)
- designed the first electric motor (for which he got in
trouble)
- discovered electromagnetic induction (a changing
magnetic field produces an electric field)
- constructed the first dynamo
- proposed that electromagnetic fields extend into
empty space
- discovered diamagnetism
- worked on mining safety, corrosion, environmental
science, & educational reform.
Faraday -- fields
"On the physical character of the lines of magnetic force"
Phil. Mag. June 1852, pg 407 ff.
Michael Faraday
(1791 -- 1867)
Faraday -- fields
"On the physical character of the lines of magnetic force"
Phil. Mag. June 1852, pg 407 ff.
Michael Faraday
(1791 -- 1867)
Not sure what a
radiation force is ... he
seems to distinguish it
from electric force.
Faraday -- fields
Faraday imagined that electric
charges created electric fields
and magnets created magnetic
fields. These fill space, and take
time to change.
Field lines are defined by the direction in
which a positive “test charge” would move.
Faraday -- fields
Faraday -- fields
There is an
energy density
associated with
these fields.
1 2
u= E
2
[Notice that
symmetry fixes
the shape of
the field.]
Faraday -- fields
scalar fields
T (x, y, z, t)
P (x, y, z, t)
Faraday -- fields
vector fields
B(x, y, z, t)
E(x, y, z, t)
v(x, y, z, t)
Maxwell -- background
- family was comfortable and had connections to the peerage
- arrival at private school with homemade shoes and a
provincial accent earned him the nickname ‘Daftie’.
- published his first scientific paper at age 14
- died of abdominal cancer at age 48
James Clerk Maxwell
(1831 – 1879)
Scottish physicist.
- predicted (correctly) that the rings of Saturn are made up
of particles (rather than a solid or a liquid)
- also worked on colour theory, control theory, kinetic
theory, and thermodynamics
[Maxwell is regarded by most modern physicists as the scientist of the 19th century who
had the greatest influence on 20th-century physics, and he is ranked with Sir Isaac
Newton and Albert Einstein for the fundamental nature of his contributions. In 1931, on
the 100th anniversary of Maxwell's birth, Einstein described the change in the
conception of reality in physics that resulted from Maxwell's work as 'the most profound
and the most fruitful that physics has experienced since the time of Newton.']
Maxwell -- accomplishments
James Clerk Maxwell
- derived “Maxwell’s equations” that unify all previous work
on electricity and magnetism (and unifies those forces as
electromagnetism)
- from his equations showed that an wave of oscillating
electric and magnetic fields exists. Derived that the speed of
this wave is 310,740,000 m/s (c=2.99 x108 m/s) (1864)
- published A Treatise on Electricity and Magnetism (1873)
(1831 – 1879)
Scottish physicist.
"We can scarcely avoid the conclusion that light consists in the transverse undulations of
the same medium which is the cause of electric and magnetic phenomena." -- Maxwell
MAXWELL, James Clerk. A Treatise on Electricity and Magnetism. Oxford: Clarendon Press, 1873. Octavo, original russet
cloth; largely unopened. Custom cloth box. Two volumes. $10,500.
Maxwell -- accomplishments
Since all wave known phenomena require a medium to oscillate, Maxwell
believed that the propagation of light required a medium for the waves,
dubbed the luminiferous aether. One is tempted to identify this with the frame
of absolute space.
Light
[ a changing electric field makes a magnetic field;
a changing magnetic field makes an electric field]
μ and ε are conversion
factors to yield units of
energy density.
1 2
uE = E
2
1 B2
uB =
2 µ
A = amp = unit of electric
current = 6.241×1018 electrons /
second.
µ0 = 1.26 · 10
0
6
kg m
A2 s 2
2 4
A
s
12
= 8.85 · 10
kg m3
c=
1
µ
c = 2.99 · 108 m/s ✔
Scientific Notation
3.41 · 103 = 3410
3.41 · 106 =
3410000
3.41 · 10
1
= 0.341
3.41 · 10
5
= 0.0000341
Units
distance (m)
time (s)
mass (kg)
electric charge (C)
temperature (K)
luminous intensity (cd)
[amount of substance (mol)]
Dimensional Analysis
circumference = 2 π r
area of a circle = π r2
Light
Light is a transverse wave.
Sound is a longitudinal wave.
Ocean waves are both.
The Speed of Light
“This second inequality appears to be due to
light taking some time to reach us from the
satellite; light seems to take about ten to
eleven minutes [to cross] a distance equal to
the half-diameter of the terrestrial orbit.” -Romer
Ole Christensen Rømer (1644 – 1710)
The orbit of Io about
Jupiter is like a clock. As
we move away from Io, it
will appear to take longer
to ‘tic’.
Rømer compared the apparent
duration of Io's orbits as Earth
moved towards Jupiter (F to G)
and as Earth moved away from
Jupiter (L to K). (1677)
The Speed of Light
Armand Hippolyte Louis Fizeau
(1819 – 1896))
The Spectrum of Light
f=
c
E = hf
c = 299, 792, 458m/s
h = 6.62606896(33) · 10
34
Q: why are there 7 colors?
Js