Transcript Document

Scientific Method
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Predictive
Testable -- Hypotheses
Experimentally Reproducible
Quantitative -- Uncertainties
Objective and Unbiased
Confirmable -- Peer Review Evaluation
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Scientific Theory
• Speculation – untested!
o The abominable snowman is a prehistoric ape
o Extra-terrestrial intelligent life exists
o God created the universe in 7 days
• Hypothesis – tested!
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Gravity bends light– general relativity theory
Science is statistical – quantum theory
A “big bang” began the universe – big bang theory
Life has evolved – theory of evolution
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Scientific Discovery
• Directed
o Human genome
o Laser
o Transistor
• Serendipitous
o Superconductivity
o Neutrino
o Penicillin
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Scientific Serendipity
The Neutrino
Until the early part of the 20th century neutrinos were unknown. Only the study
of nuclear decays revealed the existence of a yet unknown particle. Nuclear
physicists studied three types of radioactivity: alpha (charged helium), beta
(electrons), and gamma (high-energy light) radiation. They found that both the
alpha and gamma spectrum emitted in nuclear decays were discrete. In 1930,
however, James Chadwick observed that the energy spectrum of electrons
(the beta particle) emitted in nuclear beta-decay was continuous. This
observation could not be explained at the time. In an attempt not to abandon
the fundamental conservation law of energy Wolfgang Pauli postulated a new
particle to explain Chadwick’s observation. Three years later, Enrico Fermi
called this new particle the neutrino and incorporated it in his theory of weak
interaction.
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Scientific Serendipity
The Neutrino
Twenty six years after Pauli’s postulate of the neutrino Frederick Reines and
Clyde Cowan made the first experimental observation of neutrinos in their
pioneering experiment at the Savannah River reactor for which Reines received
the Nobel Prize in 1995 [9]. In their experiment they used the inverse of the
beta-decay reaction which led to the neutrino postulate in 1930. Through inverse
beta-decay (in which anti-neutrinos interact with protons to produce a positron
and neutron) they detected anti-neutrinos from a nearby nuclear reactor. Their
neutrino detector was placed at a distance of a few meters from the core of the
reactor. Subsequent accelerator experiments found that there are three distinct
types (or flavors) of neutrinos depending on the type of charged particle found in
the neutrino-producing interaction.
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Scientific Serendipity
Superconductivity
On July 10, 1908 at the University of Leiden, Heike Onnes succeeded in liquifying
helium at a temperature of 4.2 K. By 1911, having developed a cryostat capable
of maintaining the liquid at a constant low temperature, he was able to investigate
the behavior of other substances at liquid helium temperatures.
It was well known by this time that the electrical resistance in a metal decreased
with temperature. Exactly what would happen to resistance at temperatures
approaching absolute zero, however, was hotly debated. Lord Kelvin believed
that the flow of electrons might stop altogether, with the electrons becoming
frozen in place. The resistance at absolute zero would thus be infinitely high.
Others, including Onnes, assumed that the decrease in resistance with falling
temperature would continue in an orderly fashion, ultimately reaching zero at 0 K.
Onnes used mercury, which he could distill into a very pure sample. He poured it
into a U-shaped glass capillary tube equipped with electrodes on both ends to
measure the current passing through it while it was still a liquid. As he cooled the
mercury (even into a solid) Onnes found a regular decrease in resistance. At
liquid-helium temperatures well higher than 0 K (absolute zero) the resistance
suddenly disappeared. He assumed that a “short circuit” had occurred and
replaced the U-tube with a W-shaped tube with electrodes at both ends and at the
kinks, presenting four segments for measurement. He observed the same
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anomalous behavior and serendipitously discovered superconductivity.
Scientific Serendipity
Penicillin
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In the early 1920s, the British scientist Alexander
Fleming reported that a product in human tears could
lyse bacterial cells. Fleming’s finding, which he called
lysozyme, was the first example of an antibacterial agent
found in humans. Like pyocyanase, lysozyme would
also prove to be a dead end in the search for an
efficacious antibiotic, since it typically destroyed
nonpathogenic bacterial cells.
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Fleming’s second discovery, though, would change the
course of medicine. In 1928, Fleming serendipitously
discovered another antibacterial agent. Returning from a
weekend vacation, Fleming looked through a set of old
plates that he had left out. On one such plate, he found
that colonies of Staphylococcus, which he had streaked
out, had lysed. Fleming observed that bacterial cell lysis
occurred in an area adjacent to a contaminant mold
growing on the plate and hypothesized that a product of
the mold had caused the cell lysis.
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Scientific Serendipity
Penicillin
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While Fleming generally receives credit for discovering penicillin, in fact
technically Fleming rediscovered the substance. In 1896, the French
medical student Ernest Duchesne originally discovered the antibiotic
properties of Penicillium, but failed to report a connection between the
fungus and a substance that had antibacterial properties, and Penicillium
was forgotten in the scientific community until Fleming’s rediscovery.
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Through follow-up work, Fleming showed experimentally that the mold
produced a small substance that diffused through the agar of the plate to
lyse the bacteria. He named this substance penicillin after the Penicillium
mold that had produced it. By extracting the substance from plates,
Fleming was then able to directly show its effects. Important to its discovery
was the penicillin had destroyed a common bacterium, Staphylococcus
aureus.
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Through follow-up work, Fleming showed experimentally that the mold
produced a small substance that diffused through the agar of the plate to
lyse the bacteria. He named this substance penicillin after the Penicillium
mold that had produced it. By extracting the substance from plates,
Fleming was then able to directly show its effects. Important to its discovery
was the penicillin had destroyed a common bacterium, Staphylococcus
aureus, associated with sometimes deadly skin infections.
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Uncertainties
Gaussian Distribution
μ = Mean (Average) σ = Standard Deviation
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Measurements and Precision
m±s
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Measurements and Precision
m±s
133.005 ± 2
What’s wrong?
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Measurements and Precision
m±s
133 ± 2
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Combining Uncertainties
If c  a  b then s c 
2
sa
2
 sb
s a  s b 
If c  a  b then
    
 a  b
c
sc
2
2
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Assignment for 09/09/09
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The volume, V, of a small box of length a, width b and height c is
given by V = abc. Calculate the mean and standard deviation of
the volume if the measured values of the box dimensions are given
by a = 3.00±0.03 cm, b = 2.50±0.05 cm and c = 10.00±0.1 cm.
A campus walkway has four straight segments of lengths a, b, c
and d. Calculate the total length (mean and standard deviation) of
the walkway if the measured values of the segments are given by
a = 15.0±0.5 m, b = 52±1 m, c = 28.5±0.7 m and d = 44.8±0.9
m.
Which of the following measured quantities are expressed
correctly and which are not? Explain your answers.
(a) 5.06±0.1 kg (b) 7.009±0.004 mm (c) 3.0067±0.0055 km
(d) 8±0.05 g (e) 85±2 mL
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