Download Physics 3 - Westmount High School

Activity
5
People in physics
What kind of person is a physicist?
People often think of scientists as men with crazy hair
glasses, wearing white lab coats and playing with test
(see Figure 5.1). Does this fit your idea of a physicist or
scientist? In your mind, describe common
characteristics of physicists. Are they young or old?
or female? Fun or boring?
and
tubes
Male
Repeat the above exercise with a little more thought,
listing the personality traits you think are required to be
physicist. Is a physicist patient or easily frustrated? Does
she rely more on logic or on intuition?
a
he or
Figure 5.11
Optional activity: People in physics investigation
Now that you have had the opportunity to step into the shoes of a physicist, if you would like, you
can take the time to learn more about people in physics during this activity. On the following pages,
you will find files on 12 different physicists in history. You will notice that a great deal of
information about them is missing: the photographs, names and other biographical information,
and even some of the words in the descriptions of their work and discoveries. Your job is to figure
out who these physicists are by understanding the work they did. Start by filling in the missing
words in the “Contributions to Physics” paragraphs by reading the “Physics information” section
(pages 56 to 64). Then check whether your answers are correct by consulting the Answer Key and
identifying the physicists. By reading their biographies (pages 65 to 77), you will be able to fill in
the remaining missing information in their files, along with any other information of interest.
Of course, there are many other ways you could find out about the work these physicists have
done. As a general rule, you should read as much as possible and allow your curiosity free rein.
After completing each file, ask yourself whether details about the physicist were very different
from the way you had imagined him or her while you were learning about the physics, and if so,
in what respect.
1.
Caricature of a mad scientist, image drawn by J. J. McCullough, Wikimedia Commons (http://commons.wikimedia.org : accessed
August 22, 2007). This image is under GNU Free Documentation Licence.
People in Physics files
Name:
Date of birth:
City of birth:
Nationality:
Gender:
Biographical notes:
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Contribution to physics:
In 1967, this physicist was a graduate student at the University of Cambridge helping in the
construction of a brand new radio telescope. S/he was put in charge of its operation and of
analyzing by hand the 128 metres of paper that the telescope recorded every four days. After a few
weeks, the physicist noticed a very regular signal coming from the same part of the sky. Since the
physicist and his/her advisor, Dr. Antony Hewish, were not sure what caused the signals they
detected, they labelled them “LGM” (for Little Green Men), thinking they could be radio signals
from an alien civilization. However, they soon realized that there were similar signals coming from
other parts of the sky. Eventually, they concluded that these signals must be coming from some
sort of astrophysical objects. These objects were given the name of “__ __ __ __ __ __ __,” for
“pulsating radio stars.” We now know that __ __ __ __ __ __ __ belong to a group of very small,
very massive (and thus very dense) stars called “__ __ __ __ __ __ __ stars.” Theoretical physicists
had predicted the existence of these stars; this discovery represented the first evidence that their
prediction was correct. Dr. Hewish received the Nobel Prize in 1974 for this discovery, while the
physicist in question was famously left out of sharing the prize.
Name:
Date of birth:
City of birth:
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Gender:
Biographical notes:
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Contribution to physics:
On October 1, 1847, this physicist sneaked away from a dinner party, fuelled by a compulsion to
check on the skies through the telescope in the makeshift observatory set up on the roof. The
physicist noticed a smudgy light in the sky that had not been there before; in fact the physicist
knew that there were no stars that bright at that location. Suddenly, s/he realized the object was a
comet! The blurriness was caused by the comet’s __ __ __ __. At the time astronomers all over
the world were on the lookout for comets that could only be seen through a telescope; the King of
Denmark offered gold medal prizes to the first person to observe each such comet. However,
Denmark was so far away, and the mail was so slow, that by the time news of this physicist’s
observation had reached Europe, four European astronomers had seen the comet. It took over a
year, but the medal was eventually awarded to this physicist whose name was also given to the
comet.
Name:
Date of birth:
City of birth:
Nationality:
Gender:
Biographical notes:
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Contribution to physics:
In 1930, this physicist was on a ship headed to England to start his/her PhD studies. During the
trip, the physicist started to work out mathematically what the properties of stars much more
massive than our Sun would be as they age. At the time, it was known that massive stars would
end their lives by collapsing in on themselves due to the force of gravity, and physicists thought
that all these stars would collapse to the size of the Earth to make what is called a “__ __ __ __ __
dwarf.” However, the physicist in question realized that once the effects of general relativity and
quantum physics were taken into account, very massive stars would keep collapsing to form even
denser objects such as neutron stars and __ __ __ __ __ holes. In 1933, this physicist presented
these results to a group of colleagues. Immediately after, a mentor and well-known physicist, Sir
Arthur Eddington, publicly contradicted the results. Disappointed, the physicist in question
decided to leave England and continue to work in the United States. During his/her life, the
physicist established him/herself as one of the most important theoretical astrophysicists of the
century, and his/her work was given the respect it deserved. In 1983, s/he was awarded the Nobel
Prize for contributing to our understanding of the structure and evolution of stars.
Name:
Date of birth:
City of birth:
Nationality:
Gender:
Biographical notes:
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Contribution to physics:
This physicist was trained to be a nuclear physicist. S/he worked on the Manhattan Project during
WWII, the secret project that built the first atomic bombs. One of the uranium isotopes (Uranium235, which has a mass of 235 atomic mass units) is a useful chemical element to split into lighter
elements. This split releases enough energy to trigger the chain reaction required for a nuclear
bomb. However, U-235 is present in just a small percentage (0.7%) of all uranium isotopes—most
of them are U-238. Separating the two isotopes is crucial for creating nuclear fission reactions.
This physicist developed a method to do this through a process known as gaseous diffusion. S/he
also improved radiation detectors (called Geiger counters). It was after the war, however, that s/he
made the discovery that made him/her famous. While the theoretical physicists Tsung Dao Lee
and Chen Ning Yang proposed that parity might be violated in certain interactions, it was this
physicist who showed conclusively that parity was not conserved in the decay of __ __ __ __ __ __60, a __ __ __ __ __ __ isotope. Parity is a __ __ __ __ __ __ __ __ under spatial inversion, so the
laws of physics governing this decay take into account the differences between left and right. The
only way you could unambiguously explain left and right to an extraterrestrial being would be
through these weak interactions that violate parity, such as in the experiment performed by this
physicist. Many believe s/he should have won the Nobel Prize for this work. S/he performed this
experiment in 1956 and his/her book Beta Decay (1965) is still a standard reference for nuclear
physicists.
Name:
Date of birth:
City of birth:
Nationality:
Gender:
Biographical notes:
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Contribution to physics:
In 1926, this physicist discovered the statistical laws that govern the particles that obey the Pauli
exclusion principle. Both the statistics and the particles are now named after this person. In 1934,
this physicist created a theory explaining beta decay (a kind of radioactive decay) in particle
physics by merging previous work on radiation theory with Pauli’s idea of the neutrino. As soon
as the Joliot-Curies discovered artificial radioactivity in 1934, s/he proved that every element
bombarded with neutrons suffers a nuclear transformation. Through this work s/he discovered the
existence of slow neutrons, which helped the discovery of nuclear fission and the production of
new artificial chemical elements. In 1939, Otto Hahn and Fritz Strassmann in Germany discovered
nuclear fission, and this physicist understood that secondary slow neutrons are emitted that may
start new fissions, thus leading to a nuclear __ __ __ __ __ reaction. On December 2, 1942, beneath
a stadium at the University of Chicago, s/he succeeded in setting off the first controlled nuclear
__ __ __ __ __reaction. This achievement led to the __ __ __ __ __ __ __ __ __ Project that built
the first atomic bomb. In the last part of his/her life, this physicist built a model that explains the
acceleration of cosmic rays by a giant magnetic field in space.
Name:
Date of birth:
City of birth:
Nationality:
Gender:
Biographical notes:
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Contribution to physics:
This physicist demonstrated that radioactivity is the spontaneous disintegration of atoms. S/he
noted that it takes a certain amount of time for half of a radioactive material to decay, which is
called its half-__ __ __ __. Applying this knowledge to the concentration of the remaining
radioactive material in minerals, s/he helped determine that the Earth was much older than most
scientists believed at the time. In addition, this physicist went on to name two by-products of
radioactive decay, alpha and beta particles. Furthermore, s/he helped determine the correct
structure of the atom by providing experimental evidence that atoms are made up of a dense
positively charged __ __ __ __ __ __ __ surrounded by a cloud of electrons. The famous
experiment involved bombarding very thin metal foils with positively charged alpha particles. To
his/her amazement, some of the particles bounced back. “It was as if you had fired a 15-inch naval
shell at a piece of tissue paper and the shell came right back and hit you,” exclaimed the physicist.
Because the alpha particles had bounced back, it became clear that nearly all of the positively
charged protons were located in a densely packed core called the __ __ __ __ __ __ __.
Name:
Date of birth:
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Nationality:
Gender:
Biographical notes:
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Contribution to physics:
This physicist chose a fascinating research topic for his/her doctoral thesis: Henri Becquerel had
discovered that the rays emitted by uranium salts could be recorded by photographic plates (in the
same way that light rays are recorded by film in an old-fashioned camera). He believed them to be
phosphorescent in nature, but the rays were mysteriously able to blacken the plates even when they
were separated by black paper and had been kept in a dark drawer. This physicist undertook to
discover what was responsible for these rays. In trying to concentrate the activity of the salts, s/he
found that the actual strength of the rays emitted by the salts was directly proportional to the
amount of uranium present. S/he thus became convinced that radiation was an atomic property.
When other substances were tested for their ability to emit “Becquerel rays,” this physicist was
astonished to find that something in pitchblende, the ore that is left over when uranium is extracted
from mines, had a much greater activity than uranium. In fact by their activity, s/he was eventually
able to identify the substances responsible: two new elements that s/he named
__ __ __ __ __ __ __ __ (after his/her native country) and radium. S/he postulated that these rays
were
a
property
of
the
atoms
themselves
and
named
this
property
__ __ __ __ __ __ __ __ __ __ __ __ __. The rays in fact accompany a transformation from one
element to another, a dramatic secret of nature that has been of use in medicine, carbon dating and
bomb building!
Name:
Date of birth:
City of birth:
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Gender:
Biographical notes:
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Contribution to physics:
This physicist became fascinated with the discovery of radium in 1898. After completing his/her
PhD studies, s/he worked with Otto Hahn, a German chemist researching radioactivity. Their
research indicated that the uranium nucleus can split to form two other atoms, releasing a
considerable amount of energy when it does so. This process was later called nuclear
__ __ __ __ __ __ __, and its discovery laid the groundwork for the study of nuclear energy and
the atomic bomb. The experimental evidence was obtained by Hahn and Fritz Strassmann, but
only after years of research with this physicist, who also did the corresponding calculations and
interpreted the experimental results correctly, as set out in the paper “Disintegration of Uranium
by __ __ __ __ __ __ __ __: A New Type of Nuclear Reaction” published in Nature with Otto
Frisch. Hahn alone received the Nobel Prize in Chemistry for his work in nuclear fission. When
he gave his acceptance speech in 1946, he did not even mention this physicist’s name or their 30
years of collaboration. This physicist was not recognized for his/her work during his/her lifetime.
Name:
Date of birth:
City of birth:
Nationality:
Gender:
Biographical notes:
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Contribution to physics:
This physicist entered university intending to study mathematics but soon changed his/her mind
and decided to study physics, saying later, “Mathematics began to seem too much like puzzle
solving. Physics is puzzle solving, too, but of puzzles created by nature, not by the mind of man.”
The work that was to win this physicist recognition (and the Nobel Prize) was the solution of a
longstanding puzzle in physics—or nature. Some atoms are extremely stable, something that could
not be explained by the liquid drop model at the time. This physicist explained why these atoms,
with __ __ __ __ __ numbers of nucleons (protons or neutrons) are exceptionally stable, by
introducing a nuclear __ __ __ __ __ model in which the nucleus has discrete energy levels, and,
as in the atomic __ __ __ __ __ model of electrons, filled energy levels are the most stable.
Name:
Date of birth:
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Gender:
Biographical notes:
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Contribution to physics:
This physicist was a Nobel Prize winner known for revolutionary thinking in theoretical physics,
a sense of wit and an ability to explain even the most advanced science to a general audience. The
work completed by this physicist on __ __ __ __ __ __ __ field theory is still influential today.
This physicist won the Nobel Prize for the theory of __ __ __ __ __ __ __ electrodynamics, which
enables physicists to make precise predictions about the behaviour of subatomic particles. A
significant contribution in this regard was the path integral formalism, which allows physicists to
calculate the probability of observing a process by summing all the possible paths between its
initial and final states (which could be labelled by position or energy, for example). In 1969, this
physicist also gave a lecture in which s/he set out challenges for the building of micromachines.
Part of the lecture was the intriguing description of how the entire Encyclopaedia Britannica could
be printed onto the head of a pin coupled with an explanation of how this might be achieved. Many
attribute the field of nanotechnology to the ideas suggested in a talk given by this physicist entitled
“There’s Plenty of Room at the Bottom: An Invitation to Enter a New Field of Physics.”
Name:
Date of birth:
City of birth:
Nationality:
Gender:
Biographical notes:
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Contribution to physics:
This physicist was an experimentalist, who helped to greatly advance our understanding of
electromagnetism. His/her name was given to one of the fundamental equations of classical
electromagnetism, relating current and magnetic fields, which represents his/her greatest
discovery, electromagnetic __ __ __ __ __ __ __ __ __. Interestingly, s/he was very involved with
chemistry, but this work complemented his/her work in electromagnetism very well; for example,
s/he discovered rules concerning dissociation of ions and voltage. This physicist also worked in
several other related fields. For example, s/he investigated some of the properties of electro__ __ __ __ __materials and showed that magnetic fields can affect materials through which light
passes by demonstrating that the materials change the polarization of the light when a strong
magnetic field is applied. This physicist coined many standard terms in electrical science, such as
ions and electrodes, and conceptualized electric field lines.
Name:
Date of birth:
City of birth:
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Contribution to physics:
Until the work of this physicist, the nature of light and colour was considered a matter for
philosophers, who believed they could understand the truth about nature through pure thought.
René Descartes believed that light was a disturbance of an invisible medium filling the Universe,
much like sound waves are disturbances in air or other media. Colour was also poorly understood:
Isaac Barrow wrote that blue consisted of black and white particles arranged alternately. Although
it had long been known that a ray of light impinging on a prism would produce a
__ __ __ __ __ __ __ __ of colours, it was believed that the prism itself somehow created them.
This physicist, who was a student of Barrow’s, discovered that all the colours in the
__ __ __ __ __ __ __ __ are present in white light and that the prism simply separates the different
components by __ __ __ __ __ __ __ __ __ __. His/her experimental approach changed the face
of natural philosophy, laying the groundwork for the scientific revolution.
Physics information
Atoms
The atom is the smallest particle of a chemical element that
its chemical properties. Atoms are composed of a nucleus made
protons and neutrons surrounded by a cloud of electrons (see
5.2). Electrons are very small and negatively charged, while
are positively charged and about 1800 times more massive than
electrons. Neutrons have about the same mass as protons, but
neutral.
retains
up
of
Figure
protons
are
Figure 5.22
Bosons and fermions
There are particles that are like dogs; they like to stay close together as a group. These particles
are named “bosons” after the physicist Satyendra Nath Bose, who discovered the statistical laws
that govern them. On the other hand, there are particles that are like cats; they go as far away as
they can from their own kind. Scientists say these particles obey the Pauli exclusion principle.
These particles are called “fermions,” since it was Enrico Fermi who discovered the statistical laws
that govern them. Bosons have integer spin, and fermions have half-integer spin. Electrons and
protons are fermions, but helium nuclei are bosons.
Chandrasekhar limit
By the early 1930s scientists had concluded that, after converting all of their hydrogen to helium,
stars like our Sun lose energy and contract under the influence of their own gravity. They contract
to about the size of the Earth, and the electrons and nuclei of their atoms are compressed to a state
of extremely high density. They are then known as white dwarf stars. However, the astrophysicist
Subrahmanyan Chandrasekhar determined what is now known as the “Chandrasekhar limit”: that
a star having a mass more than 1.44 times that of the Sun does not form a white dwarf but instead
continues to collapse. Such a star blows off its outer parts in a supernova explosion and becomes
a neutron star. An even more massive star collapses even further and becomes a black hole. These
calculations contributed to the eventual understanding of supernovae, neutron stars, black holes
and the life cycle of stars in general.
“Atom with orbiting electrons,” digital image, Tinka Sloss and NASA/ESIP, Science Education Resource Centre, Using Data in
the Classroom (http://serc.carleton.edu/usingdata/index.html : accessed August 22, 2007). This image is under Creative Commons
licence (Attribution-NonCommercial-ShareAlike 1.0).
2.
Comets
A comet is a small object that orbits our Sun. It is usually made of rock, dust and ice. The loose
ice and particles surrounding the rock and ice form a “coma,” which becomes bigger and brighter
when the comet approaches the Sun. A tail that can extend for millions of kilometres develops near
the Sun because of solar radiation pressure: the pressure exerted on any surface by electromagnetic
radiation from the Sun. (Light, radio waves, microwaves and others are all emitted by the Sun.)
This tail faces away from the Sun and gives the comet its name, which comes from the Greek word
komē meaning “hair of the head.” Figure 5.3 is a picture of Comet Hyakutake, which was
discovered by an amateur astronomer from southern Japan.
Historically, comets frightened observers on Earth who thought they were unlucky or signalled
attacks by extraterrestrial beings. Because comets are in orbit, they will reappear in our skies after
making a complete path around the Sun. You may have heard of Halley’s Comet, which comes
close to the Earth every 75 or 76 years. It was last seen in 1986 and will next appear in 2061.
Figure 5.33
Induction
Electromagnetic induction is the creation of an electric field due to a changing magnetic field. The
effect is observed through the currents created when a magnet moves past a coil (see Figure 5.4)
because an electric field causes charges to accelerate parallel to it. This is the principle behind
electrical power generators (see Figure 5.5); for example, water can move turbines, which rotate
in a magnetic field, thereby creating current in them. A transformer, which is used to change
voltages and currents in power lines, is also possible because of electromagnetic induction.
Recently, LED flashlights powered by shaking have become available; a magnet moves near an
electric circuit that charges the battery.
There is also a reciprocal effect, where a current flowing through a wire causes the wire to turn. A
motor can be made with this principle. However, this effect is not due to electromagnetic induction,
but rather to the Lorentz force.
3.
Comet Hyakutake, digital image, NASA, Wikipedia (http://en.wikipedia.org : accessed August 22, 2007). This image is in the
public domain.
Figure 5.44
Figure 5.55
Nuclear fission
Fission is a term borrowed from biology, where it refers to the splitting of a cell in two. Nuclear
fission, then, is a process in which a nucleus splits into two or more smaller nuclei. Since it is the
number of protons in the nucleus that determines which element the substance is, this is a process
whereby one element can change into other elements. Other kinds of particles are released as well:
gamma rays and alpha and beta particles. This can happen spontaneously or be engineered when
a heavy nucleus is bombarded with slow neutrons, as shown in Figure 5.6.
Figure 5.66
Fission can be used as a power source because some materials generate neutrons as part of the
fission process, and these can trigger fresh fission events. This can give rise to a chain reaction
4. Changing
the magnetic field by moving a magnet. Image created by Marjorie Gonzalez. Used with permission from the McGill
Let’s Talk Science Partnership Program.
5. Changing the flux by moving a current loop through a magnetic field. “The alternating current (AC) dynamo” (Electricity module), digital image, G. R. Delpierre and B. T. Sewell, PhysChem
(www.physchem.co.za : accessed July 11, 2007). Copyrighted image reproduced with permission.
6.
Fission processes. Left: Uranium fission. Right: Fission products include new particles that can trigger new reactions. Images
created by Marjorie Gonzalez. Used with permission from the McGill Let’s Talk Science Partnership Program.
that is self-sustaining. In a nuclear reactor, the chain reaction is carefully controlled so that energy
is released at a controlled rate (and nothing blows up), while in a nuclear weapon, the reaction is
uncontrolled and proceeds very rapidly. In fact, news of the discovery of fission in 1939 spurred
several famous scientists including Albert Einstein to write a letter to President Roosevelt warning
him that Nazi Germany might be working on developing nuclear weapons. This led directly to the
creation of the Manhattan Project.
Nuclear fusion
Nuclear fusion is the opposite phenomenon to nuclear fission. Rather than split atoms apart,
nuclear fusion is the process by which two or more atomic particles combine to form a heavier
nucleus, as, for example, when four hydrogen atoms merge to create a helium nucleus. This process
produces even more energy than nuclear fission and does not result in radioactive waste material.
This is what keeps stars burning and how chemical elements are created. Despite many efforts,
scientists have not yet been able to create electrical energy from nuclear fusion. Perhaps the ITER
Project currently under construction in France will succeed in this.
Nuclear shell model
The nuclear shell model describes the structure of the nucleus, which is made up of nucleons
(protons and neutrons), in terms of discrete energy levels. The nucleons can be thought of as filling
nuclear energy shells in the same way that electrons fill up atomic shells in the atomic shell model.
This model explains the stability of certain atoms with specific numbers of nucleons, called “magic
numbers.” The magic numbers are 2, 8, 20, 28, 50, 82 and 126, and correspond to filled energy
shells. The shells exist for both protons and neutrons, so a magic nucleus would have a magic
number of protons or a magic number of neutrons, and a doubly magic nucleus would have a magic
number of both. This model was developed in 1929 following independent work by several
physicists who were jointly awarded the Nobel Prize for its development. It superseded the liquid
drop model of the nucleus, in which the nucleus is modelled as a drop of an incompressible nuclear
fluid. This was a useful model because the nucleons are bound together like molecules in a liquid,
but it did not explain the stability of magic nuclei.
Prisms
A prism is a specially shaped piece of transparent material (like glass) that bends light. This is
called refraction. Light travels slower in a denser medium like glass or water than it does in air.
Like other waves, it changes direction when it changes speed (depending on the angle of incidence
on the different medium). This is why a coin at the bottom of a glass of water looks closer when
you look at it from above than from the side, for example. White light is a mixture of different
colours of light. (Light is a kind of electromagnetic radiation that we can see, and each colour has
a different frequency, defined as the speed of the wave divided by its wavelength. The frequency
of the wave tells you how many wavelengths pass through a point in space per second.) Thus, the
different colours making up white light will change frequencies inside a prism and bend by
different amounts. This splits the light into many different colours called a spectrum (see Figure
5.7). This separation of white light into a spectrum of colours is also something you can see in
nature: a rainbow is produced when light is refracted by water droplets suspended in the air.
Figure 5.77
7.
A prism refracting white light. “Classic diagram of a dispersion prism,” digital image by Joanjoc, Wikimedia Commons
(http://commons.wikimedia.org : accessed July 11, 2007). This image is in the public domain.