Transcript History of the Atom

History of the Atom:
Physics Chapter 27
Early work in Electricity and
Magnetism
 Oersted:
 Current makes magnetic field
 Faraday/Henry:
 Magnetic fields moving make currents
 Maxwell:
 Electricity, magnetism and light are all parts of the
electromagnetic field
 Hertz:
 Experiments supported Maxwell’s work
James Clerk Maxwell
 1831-1879
 Showed that electricity and
magnetism were related, and
were related to atoms
 Predicted that accelerating
charges would make waves
(electromagnetic radiation)
Cathode Ray Tube Experiments
 Glass tube with wire at each end; as much air pumped out as
possible
 Charge passed across tube makes fluorescent glow
 William Crookes
 Tube coated with fluorescent material can be made to glow in
one focused dot
 Rays travel in straight lines
 Ray carries negative charge
Joseph John Thomson
 1856-1940
 Used a study of the cathode
ray tube to determine the
presence of electrons 1897
 Suggested the plum pudding
model of the atom and the
existance of isotopes
 Won the Nobel Prize in
Physics in 1906
J. J. Thomson’s Experiment
 Thomson used both a
magnetic field and electric
field to deflect the electrons
 He measured the deflection
of the ray and calculated the
charge:mass ratio of
electrons
J. J. Thomson’s Experiment
 Used magnetic field to show cathode rays had negative charge
 Used electric field to show cathode rays were particles with
negative charge
 Used varying electric currents to determine charge to mass
ratio
Force caused by electric field: qE
Force caused by magnetic field: Bqv
When these forces are equal Bqv=qE
Then v = E/B
When electric field removed, particles given centripetal force by
magnetic field Bqv = mv2/r
 Solved for mass/charge ratio: m/q = Br/v
 Thomson calculated m/q as 5.686 x 10-12 kg/C
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 Evidence suggested particles very small and came from atom
J. J. Thomson’s experiment
 Thomson calculated m/q as 5.686 x 10-12 kg/C
 Millikan calculated q = 1.602 x 10-19 C
 Can be used to calculate m:
 m=(5.686 x 10-12 kg/C) q
 m calculated as 9.109 x 10-31 kg
 Method can be used for any charged particle
Example
 A beam of electrons travels an undeflected path in a cathode
ray tube. E is 7.0 x 103 N/C. B is 3.5 x 10-2 T. What is the
speed of the electrons as they travel through the tube?
 What we know:
 E = 7.0 x 103 N/C
B=3.5 x 10-2 T
 Equation:
 v = E/B
 Substitute:
 v = (7.0 x 103N/A s) / (3.5 x 10-2 N/A m)
 Solve!
 v = 2.0 x 105 m/s
Example
 An electron of mass 9.11 x 10-31 kg moves with a speed of 2.0
x 105 m/s across a magnetic field. The magnetic induction is
8.0 x 10-4 T. What is the radius of the circular path followed by
the electrons while in the field?
 What we know:
 M = 9.11 x 10-31 kg
B=8.0 x 10-4 T
 Equation:
 Bqv = mv2/r so r = (mv) / (Bq)
 Substitute:
 R = (9.11x10-31kg)(2.0x105 m/s)

(8.0x10-4N/Am)(1.6x10-19As)
 Solve! r = 1.4 x 10-3 m
v=2.0 x 105 m/s
Robert A. Millikan
 1858-1953
 Used the 'falling drop method'
to determine the charge of the
electron (-1.6022 x 10-19 C) and
mass of electron as 9.10 x 10-28
g
 Investigated photoelectric effect
and spectroscopy of elements
 Won the Nobel Prize in Physics
in 1923