## SAT Physics Historical Figures and Contemporary Physics - Historical Figures

HISTORICAL FIGURES
Newtonian Mechanics
GALILEO GALILEI (1564-1642)
• Bodies dropped from the same height will all fall with the same acceleration, g. The distance they travel is proportional to the square of time, y =
• Principle of inertia: The natural state of motion is uniform constant velocity.
ISAAC NEWTON (1642-1727)
• 1st law of motion (law of inertia): Modifies Galileo’s principle of inertia. The natural state of motion is constant velocity unless acted upon by an unbalanced force.
• 2nd law of motion (ΣF = ma): The acceleration of an object is directly proportional to the net force acting on the object. Acceleration is inversely proportional to the mass of the object.
• 3rd law of motion: When two objects interact (action-reaction pair), an equal and oppo­site force acts on each object. This causes an opposite reaction. However, the actual motion depends on the masses involved.
• Law of gravity (Fg = Gm1m2/r2): Two masses, m1 and m2, at a distance apart of r attract each other. The magnitude of the attraction is proportional to the masses and inversely proportional to the square of the distance (inverse square law).
JAMES WATT (1736-1819)
• Power: While working with steam engines to improve their efficiency, Watt developed the concept of power. The units of power are named after him.
JOHANN KEPLER (1571-1630)
• 1st law of planetary motion: Planetary motion is elliptical.
• 2nd law of planetary motion: A line drawn from the central body (Sun) to an orbiting body (planet) will sweep equal areas of space in equal time intervals.
• 3rd law of planetary motion: The square of the period (time of one orbit) is proportional to the radius of the orbit,
T2 α r3.
Electricity and Magnetism
CHARLES-AUGUSTIN DE COULOMB (1736-1806)
• Coulomb’s law (FE = kq1q2/r2)\ Two charges, q1 and q2, at a distance of r, will attract/ repel each other. The magnitude of the attraction/repulsion is directly proportional to the magnitude of the charges and is inversely proportional to the square of the distance (inverse square law).
GEORG SIMON OHM (1789-1854)
• Ohm’s law: Determined the relationship among potential, current, and resistance: V=IR.
• Electromagnetic fields: Introduced a way to visualize electric and magnetic fields as lines extending through space.
• Electromagnetic induction: Discovered the principle of electromagnetic induction where an emf, ε = ΔΦ/t, can be induced (created) in order to stimulate the flow of a current in a loop of conducting material. The induced emf is a potential. It is created by changing the flow of the magnetic field (changing the flux ΔΦ) passing through the loop of conducting material.
HEINRICH LENZ (1804-1865)
• Lenz’s law: Dictates the direction of an induced current in a closed loop of conducting material, based on conservation of energy.
JAMES CLERK MAXWELL (1831-1879)
• Electromagnetic waves: Mathematically demonstrated that light is an electromagnetic wave moving at the speed of light.
Waves and Optics
THOMAS YOUNG (1773-1829)
• Young’s double-slit experiment: Shined monochromatic light through two narrow slits to create an interference pattern. The resulting pattern demonstrated that light is a wave phenomenon.
CHRISTIAN DOPPLER (1803-1853)
• Doppler effect: The shift in wavelength and frequency perceived in sound and light when the source is moving toward or away from an observer.
Thermal Physics
LORD KELVIN (1824-1907)
• Absolute zero: Developed the concept of absolute zero and its associated temperature scale. The units of temperature for this scale are named after him.
JAMES JOULE (1818-1889)
• Heat and work equivalence: Devised an experiment to show that the temperature of water could be increased by applying a flame or by doing mechanical work on it (rapidly stirring it). Showed that heat and work are both methods of adding energy to a system. This means that the energy associated with heat (calories) can simply be converted during the work. The units of work and energy are named after Joule.
Modern Physics
ALBERT M1CHELSON (1852-1931) AND EDWARD MORLEY (1838-1923)
• Michelson-Morley experiment: Designed an elaborate device known as an interferometer to detect the motion of Earth through the invisible ether. The ether was believed to be the medium that allowed light to travel through space. The experiment failed to prove the existence of the ether and opened the door for new lines of thinking, such as Einstein’s special theory of relativity.
J. J. THOMSON (1856-1940)
• Discovered the electron: While working with cathode rays passing through electric plates, he deduced the existence of the electron. He suggested that the atom is similar to a plum pudding. In this model, the atom is viewed as having an overall positive charge (pudding) and the negative electrons are distributed randomly (plums) throughout the atom.
MAX PLANCK (1858-1947)
• Founder of quantum theory: While working with emitted light spectra, Planck suggested that light energy can be emitted only in multiples of specific quantities and are thus quantized. He derived a constant (Planck’s constant) that determines the energy associated with the specific quantities he observed.
ALBERT EINSTEIN (1879-1955)
• Einstein’s miracle year, 1905: Published four papers that changed physics. Three of these are addressed in introductory physics courses.
• Photoelectric effect: Suggested that light acts like a particle (photon) and that the energy of the photon is quantized. The energy of the photon can be calculated by multiplying Planck’s constant by the frequency of the photon: E=hf.
• Special relativity: All the laws of physics are the same in inertial frames (frames of reference that have constant velocity). The speed of light in a vacuum is a constant 3 × 108 m/s regardless of the motion of the light source.
• Mass-Energy Equivalence: Matter can be converted into energy and vice versa according to the equation E = mc2.
ERNEST RUTHERFORD (1871-1937)
• Gold foil experiment: Fired alpha particles (positive helium nuclei without their electrons) at a very thin piece of gold foil. The scattering of the alpha particles revealed that the atom is mostly empty space consisting of a dense positive nucleus surrounded by orbiting electrons.
NIELS BOHR (1885-1962)
• Planetary model of the atom, including specific energy levels: Combining the concepts of his predecessors, mentioned above, Bohr deduced a model of the atom that explains why electron orbits do not decay and fall into the nucleus. His calculated energy levels, which electrons must occupy, match completely with the observations of light spectra emitted from these atoms.
CONTEMPORARY PHYSICS
Astrophysics
Astrophysics is the physics of celestial objects such as stars, planets, galaxies, etc. Research is conducted with a variety of telescopes and arrays that receive and analyze electromagnetic radiation. These include ground-based and space-based telescopes, such as the Hubble Space Telescope, X-ray telescopes, and radio wave arrays. Astrophysics seeks to resolve the origin of the universe and to explain its properties.

Chaos Theory
Chaos theory is a mathematical theory that attempts to explain the behavior of complex and chaotic systems. When a complex series of events are set in motion the results can vary drastically depending on small initial changes in the system. A popular phrase describing this is “the butterfly effect.” Chaos theory is not limited to physics. It is involved in processes such as erosion and fluctuations in stock market prices. In astrophysics, the universe we now live in was greatly influenced by the very initial conditions under which the universe was formed.

Dark Matter
The total mass of the universe appears not to match the gravitational effects observed throughout the universe. Scientists have hypothesized that a form of matter, which cannot be seen with telescopes, accounts for the missing mass. This matter is under investigation and has been called dark matter.

Microprocessor
A microprocessor is a complex, single circuit consisting of many miniaturized components. Examples of microprocessors are the chips that run computers and smartphones. The entire central processing unit of a computer is often a single microprocessor. Microprocessors are based on semiconductor and transistor technology.

Semiconductor
A semiconductor is a material that can act as a conductor or as an insulator. It is the key to modem miniaturized circuit design. Silicon is the most widely used semiconductor and is used in the manufacture of transistors and microprocessors.

Superconductivity
A superconductor is a material that has zero electrical resistance when cooled below a critical temperature.

String Theory
String theory hypothesizes that the elementary particles making up matter are actually linear oscillations or strings. The theory attempts to explain how everything interconnects and hopes to eliminate inconsistencies among other earlier theories.

Transistor
A transistor is an electrical component used in integrated circuits of all modern electrical devices. A transistor can both amplify the electrical signal it receives and act as a switch. A transistor typically has three leads: the base, the collector, and the emitter. A small amount of I current passing through the base can control a larger current at the collector. This determines the amount of current leaving the transistor at the emitter. The amplification characteristic is used in electronic amplifiers, such as those found in stereo equipment. Transistors are used in the computer/semiconductor industry as a switch to establish the on/off (1,0) binary code used by computers.

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