Lect4_1-17_

Lect4_1-17_ - Atom An atom consists of a tiny positive...

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Atom • An atom consists of a tiny positive nucleus surrounded by negative electrons in quantum motion. • The nucleus consists of positive protons and electrically neutral neutrons. • Most atoms we deal with on Earth are electrically neutral. The number of negative electrons balances the number of positive protons. • An atom with missing electrons is said to be an ion. The atom is ionized. • A gas of ionized atoms and free electrons is a plasma. (Don’t confuse with blood plasma.) • Most of the universe is in the plasma state. Stars are all plasma.
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Elementary Particles • Electrons with charge – e • Protons with charge +e and neutron with charge 0. • As far as we can tell electrons are fundamental; they are point charges with no structure. • Protons and neutrons are composite; they are composed of quarks. • Quarks may also be fundamental • There are dozens of other elementary particles (examples: photons, muons, neutrinos) Most are unstable and last for only tiny fractions of a second.
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Electrical Charge • Charges attract or repel each other with an inverse square law force like gravity F = kqQ/r 2 • We say that the charge creates an electrical field that causes forces on other charges. • If the charge is moving it also creates a magnetic field which causes “sideways” forces on other moving charges.
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Vibrating Charges and EM waves • A vibrating electric charge emits a linked electric and magnetic field moving away at the speed of light. • This IS “light”, electromagnetic radiation. • In quantum mechanics we learn that light can also be described as particles, photons. •I t i s v e r y d i f ficult to make any mental picture of how particles can be waves or vice-versa. They are said to be complementary descriptions.
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E field of a charge Fig 2.5 EM wave from a vibrating charge Fig 2.7
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Maxwell's Rainbow The Electromagnetic Spectrum Increasing wavelength ( ± 29 Radio waves Television •M i c r o w a v e s Infrared (IR) red Visible green blue Ultraviolet (UV) X-rays •G a m m a r a y s ( ² ) Increasing frequency ( f 29 c= ± f = wavelength X frequency c = 300,000 km/s = 186,000 miles/s = 680 million mph A l ltrave latthespeedofl ight. All can travel in a vacuum. No physical distinction between the categories; They blend into one another; There is no upper or lower limit to the spectrum.
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transparent to visible light and part of the radio spectrum. To “see” radiation like UV, X and ± - rays, we have to put telescopes in space that can detect these kinds of radiation. Infrared is an
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Lect4_1-17_ - Atom An atom consists of a tiny positive...

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