1. History of the Atom: Physics Chapter 27

2. 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

3. James Clerk Maxwell
Showed that electricity and magnetism were related, and were related to atoms
Predicted that accelerating charges would make waves (electromagnetic radiation)

4. 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

5. Joseph John Thomson
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

6. 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

7. 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 x kg/C
Evidence suggested particles very small and came from atom

8. J. J. Thomson’s experiment
Thomson calculated m/q as x kg/C
Millikan calculated q = x C
Can be used to calculate m:
m=(5.686 x kg/C) q
m calculated as x kg
Method can be used for any charged particle

9. 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

10. Example
An electron of mass 9.11 x 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 kg B=8.0 x 10-4 T v=2.0 x 105 m/s
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

11. Robert A. Millikan
Used the 'falling drop method' to determine the charge of the electron ( x C) and mass of electron as 9.10 x g
Investigated photoelectric effect and spectroscopy of elements
Won the Nobel Prize in Physics in 1923