How Much Do You Know About Physics?

Question: What does the liquid-drop model attempt to describe?

Answer: In nuclear physics, the liquid-drop model is a description of atomic nuclei formulated in 1936 by Niels Bohr. According to the model, the nucleons (neutrons and protons) behave like the molecules in a drop of liquid. If given sufficient extra energy (as by the absorption of a neutron), the spherical nucleus may be distorted into a dumbbell shape and then split at the neck into two nearly equal fragments, releasing energy. Although inadequate to explain all nuclear phenomena, the theory underlying the model provides excellent estimates of average properties of nuclei.

Question: What does the candela measure?

Answer: In the International System of Units, the candela (cd) is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 terahertz and has a radiant intensity in that same direction of 1/683 watt per steradian (unit solid angle).

Question: Which subatomic particles make up protons and neutrons, come in various flavours and colours, and took their name from a passage in James Joyce’s novel Finnegans Wake?

Answer: In 1964 Murray Gell-Mann of the United States introduced the concept of quarks as a physical basis for a particle classification scheme, adopting the fanciful term from a passage in James Joyce’s novel Finnegans Wake. Gell-Mann’s model postulated the existence of various types of quarks, distinguished by distinctive “flavours” such as “up,” “down,” and “strange.” The interpretation of quarks as actual physical entities posed problems, but the problems were resolved in 1977 by the introduction of the concept of colour, a special quantum property of quarks. All combinations of quarks must contain equal mixtures of these imaginary colours so that they will cancel out one another, with the resulting particle having no net colour.

Question: In 1895 Aleksandr Popov of Russia demonstrated a receiver that detected lightning flashes. What name can be given to this device?

Answer: Russian engineer Aleksandr Popov built his first primitive radio receiver, a lightning detector, in 1895, apparently independently of the Italian inventor Guglielmo Marconi, who is given precedence for the invention of radio.

Question: What relation between heat and gas is formally recognized in Charles’s law?

Answer: Charles’s law is a statement that the volume occupied by a fixed amount of gas is directly proportional to its absolute temperature, if the pressure remains constant. It is a special case of the general gas law and can be derived from the kinetic theory of gases under the assumption of a perfect (ideal) gas.

Question: What is the term for the disappearance of electrical resistance in materials that have been cooled below their transition temperature?

Answer: Superconductivity is the complete disappearance of electrical resistance in various solids when they are cooled below a characteristic temperature. This temperature, called the transition temperature, varies for different materials but generally is below 20 K (–253 °C).

Question: Carl David Anderson won a Nobel Prize for Physics in 1936 for his discovery of the first known particle of antimatter. What was it?

Answer: While studying cloud-chamber photographs of cosmic rays, Carl David Anderson found a number of tracks whose orientation suggested that they were caused by positively charged particles, but they were too small to be protons. In 1932 he announced that they were caused by positrons, positively charged particles with the same mass as electrons. The claim was controversial until verified the next year by British physicist Patrick M.S. Blackett and Italian Giuseppe Occhialini.

Question: Who won the 1939 Nobel Prize for Physics for his invention of the cyclotron, the first particle accelerator to achieve high energy?

Answer: Ernest Orlando Lawrence first conceived the idea for the cyclotron in 1929 at the University of California, Berkeley. One of his students, M. Stanley Livingston, undertook the project and succeeded in building a device that accelerated hydrogen ions (protons) to an energy of 13,000 electron volts (eV). Lawrence then set out to build a second cyclotron; when completed, it accelerated protons to 1,200,000 eV, enough energy to cause nuclear disintegration. He won the 1939 Nobel Prize for Physics for his invention of the cyclotron.

Question: In 1827, during a microscopic study of plant fertilization, what did Robert Brown see that is now called Brownian motion?

Answer: Brownian motion is any of various physical phenomena in which some quantity is constantly undergoing small, random fluctuations. “Classical Brownian motion” is the random movement of microscopic particles suspended in a liquid or gas. It was named for the Scottish botanist Robert Brown, the first to study such fluctuations. In 1827 Brown was investigating the fertilization process in Clarkia pulchella, a newly discovered species of flower, when he noticed a “rapid oscillatory motion” of the pollen grains suspended in water under the microscope. Initially, he believed that such activity was peculiar to the male sexual cells of plants, but then he was startled to observe that pollen of plants dead for over a century showed the same movement.

Question: What did Italian physicist Evangelista Torricelli invent in the 17th century when he inverted a mercury-filled tube into a dish?

Answer: In 1643 Evangelista Torricelli, pursuing a suggestion by Galileo, filled a glass tube 4 feet (1.2 m) long with mercury and inverted the tube into a dish. He observed that some of the mercury did not flow out and that the space above the mercury in the tube was a vacuum. After much observation, he concluded that the variation of the height of the mercury from day to day was caused by changes in atmospheric pressure. He had invented the barometer, though he never published his findings.

Question: What did Wilhelm Conrad Röntgen discover when he switched on a cathode-ray tube and noticed that some nearby materials were giving off light?

Answer: In 1895, while experimenting with electric current flow in a partially evacuated glass tube (cathode-ray tube), German physicist Wilhelm Conrad Röntgen observed that a nearby piece of barium platinocyanide gave off light when the tube was in operation. He theorized that when the cathode rays (electrons) struck the glass wall of the tube, some unknown radiation was formed that traveled across the room, struck the chemical, and caused the fluorescence. Further investigation revealed that paper, wood, and aluminum, among other materials, are transparent to this new form of radiation. He found that it affected photographic plates, but, since it did not noticeably exhibit any properties of light, such as reflection or refraction, he mistakenly thought the rays were unrelated to light. In view of its uncertain nature, he called the phenomenon “X” radiation—hence X-rays. He took the first X-ray photographs, of the interiors of metal objects and of the bones in his wife's hand.

Question: What measuring device counts the transitions between two quantum states of certain atoms?

Answer: The atomic clock is a type of clock that uses certain resonance frequencies of atoms to keep time with extreme accuracy. The electronic components of atomic clocks are regulated by the frequency of the microwave electromagnetic radiation emitted or absorbed by the quantum transition (energy change) of an atom or molecule. In an atomic clock these quantum transitions produce extremely regular waves of electromagnetic radiation; like the recurrent events in other types of clocks, these waves are then counted.

Question: Who introduced the first successful detector of individual alpha particles and other ionizing radiations?

Answer: Hans Geiger built the first version of his particle counter and used it and other radiation detectors in a variety of important experiments. In 1925 Geiger and Walther Müller improved the sensitivity, performance, and durability of the particle counter; the Geiger counter (or Geiger-Müller counter) detects not only alpha particles but beta particles (electrons) and ionizing electromagnetic photons.

Question: In the human perception of sound, what musical interval is equal to a doubling of the sound wave’s frequency?

Answer: Relating frequency to pitch as perceived by a human, two notes will sound similar if they are spaced apart in frequency by a factor of two, or an octave. This means that the frequency interval between 100 and 200 hertz sounds the same as that between 1,000 and 2,000 hertz or between 5,000 and 10,000 hertz.

Question: What law of physics was Hermann von Helmholtz among the first to articulate when he argued that force could never be destroyed?

Answer: In the 1840s Hermann von Helmholtz concluded that so-called vital heat must be the product of mechanical forces within an organism. From there he went on to generalize his results to state that all heat was related to ordinary forces and, finally, to state that force itself could never be destroyed. His paper “On the Conservation of Force,” which appeared in 1847, marked an epoch in both the history of physiology and the history of physics. For the physical sciences, it provided one of the first statements of the law of the conservation of energy.