Electricity and Magnetism

1

• No Lab Activity This Week
• B2-WNL next week
• Chapter 8 Mallard HW quiz Due by 12 AM Tuesday
  04/15
• Chapter 6 quiz end of class Today 04/03
• Exam 3 (Chapters 5 and 6) on TUE 04/08

Thursday, April 03 Spring 2008
2
The Smallest Pieces

• Democritus
• Cutting matter to reach smallest piece, called
  the atom or uncuttable
• All material formed from atoms

3
Elements

• John Dalton
• Father of modern atomic theory
• Cannot break down elements
• Molecules composed of atoms
• Example Water molecule is 1 oxygen atom and 2
  hydrogen atoms

water
caffeine
4
Are Atoms Real?

• Evidence for the reality of atoms
• Behavior of a gas
• Chemical combinations
• Radioactivity
• Brownian motion
• X-ray crystallography
• Atomic-scale microscopy

5
The Behavior of a Gas

• Bernoulli
• atoms have mass velocity and thus kinetic
  energy
• Decreasing volume increases pressure (A)
• Increasing temperature increases pressure (B)

6
Chemical Combinations

• Dalton applied law of definite proportions
• Elements combine in a specific ratio of weights
• Ex Water is 8 parts oxygen to 1 part hydrogen
• Ratio of weights when two elements combine in
  more than one way are small whole number
  multiples of each other
• Ex 12 lbs carbon can combine with either 16 lbs
  or 32 lbs of oxygen
• Implication some units of elements are
  fundamentally indivisible

7
Radioactivity

• Radioactivity Becquerel (1896)
• Phosphors flash when hit by radiation
• twinkling caused by this effect is demonstrated
  in 1903

8
Brownian Motion

• Brownian motion erratic, jiggling motion
• pollen grains suspended in water
• Einstein (1905) motion is random collisions of
  atoms
• Predicted particle movement
• Perrin tested and confirmed Einsteins
  predictions (1909)

9
X-ray Crystallography Atomic-scale Microscopy

• Developed in 1912
• Demonstrates arrangement of atoms in crystals
• 1980s STM,
• detection of tiny
• flows of electrons

10
Discovering Chemical Elements

• Electrolysis
• Mendeleev (1869)
• Current Periodic Table
• 112 elements
• 92 found in nature
• Everyday elements
• helium, carbon, aluminum, copper, gold

11
The Structure of the Atom

• Thomson identified electron (1897)
• Negatively charged
• Smaller and lighter than smallest atom
• Atoms are NOT fundamental building blocks, but
  are made up of smaller more fundamental particles
• First atomic model was called the plum pudding
  model

12
The Atomic Nucleus

• Rutherford determined atomic structure (1911)
• Concluded
• atom has nucleus at center surrounded by
  electrons
• Definitely not like a plum pudding
• Later discoveries found nucleus is composed of
  protons and neutrons

Most particles traveled through foil
Some deflected
13
Why the Rutherford Atom Couldnt Work

• Object in circular orbit is accelerating
• Accelerated electrical charge emits
  electromagnetic radiation
• If electrons give off energy while orbiting
• Result electrons spin toward nucleus eventually
  atom ceases to exist
• Rutherford atom exists for lt 1 min

14
The Bohr Atom

• More realistic model of atom (1913) explains
  how matter absorbs and emits radiation
• Energy levels for electrons
• Specific distances from nucleus
• Electrons exist with no radiation
• Electrons cannot exist between allowed distances
  (energy levels)

Niels Bohr (1885 1962)
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Example Figure 8-7
Energy changes when level changes
Any level above ground state
Lowest energy level
16
Photons Particles of Light
Used to move electron to higher energy state
Photon emitted as electron moves to lower energy
state