5.111 Lecture Summary #2 Friday, September 5, 2014
Readings for today: Sections A.2-A.3 (5th ed pgs F9-13, 4th ed pgs F10-13) – Force and
Energy, Sections B.1-B.2 (5th ed pgs F15-17, 4th ed pgs F15-18) – Elements and Atoms ,
Section 1.1 (5th ed and 4th ed pgs 1-3) – The Nuclear Atom.
Read for Lecture #3: Section 1.2 – The Characteristics of Electromagnetic Radiation,
Section 1.4 – Radiation, Quanta, and Photons (focus on pgs 10-12 in the 5th or 4th ed).
Topics: I. Discovery of the electron and the nucleus
II. (Failure of) the classical description atoms/intro to quantum mechanics
I. A) DISCOVERY OF THE ELECTRON
J.J. Thomson (English physicist, 1856-1940) in 1897 discovers the electron and
determines the charge to mass ratio. In 1906, he wins the Nobel Prize.
The Experiment:
deflection plate (+)
cathode(-) anode(+) ∆V >> 0
H2(g) ∆x deflection
cathode rays
∆V = 0
deflection plate (-)
high voltage
difference
If ΔV (the voltage difference between the two deflection plates) = 0 there is no deflection,
but when ΔV is >> 0, there is Δx deflection towards the positive plate.
cathode rays = c harged particles = electrons
From classical electromagnetism, Thompson knew that
Δx(-) ∝ e(-)= of the negative particle
m(-) = of the negative particle
1
, Then Thomson applied a very large ΔV and a slight deflection towards the negative
plate was observed. Therefore there is also a positively charged particle!
Δx(+) ∝ e(+)= charge of the positive particle
m(+) = mass of the positive particle
BUT ⏐Δx(-)⏐ >>>⏐ Δx(+)⏐
Since H is electronically neutral,
⏐ e(-)⏐ = ⏐ e(+)⏐
So: ⏐ Δx(-) ⏐ =
⏐ Δx(+)⏐
Since the ratio of Δx is large, the ratio of masses will be large: m(-) m(+)
This negative particle from the cathode ray tubes was later named the electron and its mass
was later determined to be very small (m = 9.11 x 10-31 kg).
Atoms are NOT indivisible!
I. B) DISCOVERY OF THE NUCLEUS
Ernest Rutherford (Cambridge University, England, 1871-1937) studied α emission
from newly-discovered radioactive elements that he got from his friend Marie
Curie.
• RaBr2 gave off α particles
• α particles are ions. This information was unknown in 1911.
In 1911, Rutherford experimented with this new radioactive material and discovered
the nucleus. (Experiments were done with a postdoc, Hans Geiger, and an
undergraduate, E. Marsden.)
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Readings for today: Sections A.2-A.3 (5th ed pgs F9-13, 4th ed pgs F10-13) – Force and
Energy, Sections B.1-B.2 (5th ed pgs F15-17, 4th ed pgs F15-18) – Elements and Atoms ,
Section 1.1 (5th ed and 4th ed pgs 1-3) – The Nuclear Atom.
Read for Lecture #3: Section 1.2 – The Characteristics of Electromagnetic Radiation,
Section 1.4 – Radiation, Quanta, and Photons (focus on pgs 10-12 in the 5th or 4th ed).
Topics: I. Discovery of the electron and the nucleus
II. (Failure of) the classical description atoms/intro to quantum mechanics
I. A) DISCOVERY OF THE ELECTRON
J.J. Thomson (English physicist, 1856-1940) in 1897 discovers the electron and
determines the charge to mass ratio. In 1906, he wins the Nobel Prize.
The Experiment:
deflection plate (+)
cathode(-) anode(+) ∆V >> 0
H2(g) ∆x deflection
cathode rays
∆V = 0
deflection plate (-)
high voltage
difference
If ΔV (the voltage difference between the two deflection plates) = 0 there is no deflection,
but when ΔV is >> 0, there is Δx deflection towards the positive plate.
cathode rays = c harged particles = electrons
From classical electromagnetism, Thompson knew that
Δx(-) ∝ e(-)= of the negative particle
m(-) = of the negative particle
1
, Then Thomson applied a very large ΔV and a slight deflection towards the negative
plate was observed. Therefore there is also a positively charged particle!
Δx(+) ∝ e(+)= charge of the positive particle
m(+) = mass of the positive particle
BUT ⏐Δx(-)⏐ >>>⏐ Δx(+)⏐
Since H is electronically neutral,
⏐ e(-)⏐ = ⏐ e(+)⏐
So: ⏐ Δx(-) ⏐ =
⏐ Δx(+)⏐
Since the ratio of Δx is large, the ratio of masses will be large: m(-) m(+)
This negative particle from the cathode ray tubes was later named the electron and its mass
was later determined to be very small (m = 9.11 x 10-31 kg).
Atoms are NOT indivisible!
I. B) DISCOVERY OF THE NUCLEUS
Ernest Rutherford (Cambridge University, England, 1871-1937) studied α emission
from newly-discovered radioactive elements that he got from his friend Marie
Curie.
• RaBr2 gave off α particles
• α particles are ions. This information was unknown in 1911.
In 1911, Rutherford experimented with this new radioactive material and discovered
the nucleus. (Experiments were done with a postdoc, Hans Geiger, and an
undergraduate, E. Marsden.)
2
