"B"e "B"old with Physics Vocabulary Quiz

Answer: Boost

A boost, also known as a Lorentz transformation after an early 20th-century physicist, is a quick way of converting data from one inertial reference frame into data that makes sense in another. Since one of the basic principles of special relativity is that all inertial (that is, not accelerating) reference frames experience the same laws of physics, boosts are a powerful physics tool: problems that are hard in one reference frame are often mathematically much easier in another reference frame! Particle physicists use this trick a lot when working at accelerators; they'll speak of boosting between the "lab frame" (the frame in which the accelerator is stationary) and the "rest frame" of the accelerated particles (in which the particles are stationary).

Working through several problems with these boosts is an indispensable way to understand length contraction, time dilation and other bizarre phenomena of relativistic motion.

Answer: It provides a DC voltage.

A voltage is a difference in electrical potential between two parts of a circuit -- in this case, between the positive and negative terminals of a battery. We can understand it as the amount of work per charge that needs to be done in order to move charged particles (like electrons) across that potential difference. When we connect one end of a circuit to one battery terminal, and the other end to the other terminal, the voltage across the battery provides the "oomph" that makes electrons flow through the circuit, much like the role that water pressure plays in making water flow through a pipe. A battery generates this voltage via chemical reactions in its interior; rechargeable batteries use reversible chemical reactions, which can be run backwards when an outside current is applied.

The voltage supplied by a battery is always DC (direct current) -- it is ideally a constant voltage (though it will fall off from the advertised value as the battery starts to die). AC (alternating current) voltages are the type that come out of wall outlets, oscillating between negative and positive extremes tens of times per second (the exact frequency depends on the country's electrical standard). AC batteries don't exist.

Answer: A black hole concentrates a high electrical charge into an infinitesimally small volume.

Strongly electrically charged black holes are not found in nature, since like electrical charges repel each other and resist compression into a black hole; this repulsive force is much stronger than gravitational attraction. Natural black holes generally form when some other astrophysical object is so massive that it collapses; this happens to many aging stars, after their internal nuclear engines are no longer powerful enough to resist gravitational collapse. (It won't happen to our Sun, however, since it doesn't have enough mass.)

Ever since the early twentieth century, when general relativity made it clear that black holes were possible, people have been fascinated by them; imagine something with such a powerful gravitational field that even light cannot escape if it gets too close! Since that time, astronomers have observed incredibly massive black holes at the centers of galaxies (including our own), as well as smaller ones in more average locations. The gravitational lensing effect is one way that black holes can be detected, although other massive objects (such as stars or even galaxies) also act as lenses in this way.

Answer: A black body absorbs all the light that strikes its surface.

A perfectly "white" object reflects all light that strikes it; a perfectly "black" object absorbs it. That doesn't mean that a black body appears black, however: it can still emit light, based on its internal heat energy. A star is a good approximation of a black body: nearly all the light you see from a star was emitted by that star. Our moon is the opposite: what we call moonlight is actually reflected sunlight.

By treating thermal photons as oscillators in a box, we can predict the spectrum of black body emissions at any temperature. Human beings give off thermal radiation according to the black body laws, emitting light mainly in the infrared: this is why infrared cameras can see people without ambient infrared light. (We people are not true black bodies, though, because we reflect so much ambient visible light -- which is lucky, because that's how we see each other!) The light of a star takes on a different tint depending on its temperature; understanding black body radiation allows us to realize that bluish stars are hotter than reddish ones. This is a fine example of the time-honored physics technique of using an excellent approximation to understand a complicated real-world phenomenon.

Answer: The energy needed to split a nucleus into free protons and neutrons

It is a general rule in physics that particles and systems tend to seek the state with the lowest potential energy. A ball at a great height will fall, minimizing its gravitational potential energy; a compressed spring will expand. If a system is going to stay together in the absence of external forces, it must have a lower total potential energy than its constituent parts would have if they were disassembled. This difference between energy levels -- the energy that would be required to split a system up into separate, unbound constituent parts -- is called binding energy.

Nuclear binding energy is what makes nuclear fission and nuclear fusion produce energy. Some nuclei are more tightly bound than other nuclei; fission (splitting a nucleus) and fusion (joining two nuclei into one) take loosely bound nuclei and turn them into tightly bound nuclei. The energy that's released in the reaction can be understood as the difference in binding energy between these two states.

Answer: The temperature of the system and the energy of the state

Like most powerful quantities in thermodynamics, the Boltzmann factor is deceptively simple. Where e=2.718... is the base of the natural logarithm, k is Boltzmann's constant, T is the temperature of the system and Ei is the energy of a state labeled i, Boltzmann's factor is e^(-Ei/(kT)). By itself, this is not a probability (because the Boltzmann factors of all the states don't necessarily add up to 1), but it is a relative probability: you can divide the Boltzmann factors of two states to calculate the ratio of their probabilities. For states i and j, this becomes e^(-(Ei-Ej)/(kT)).

A close look at the Boltzmann factor reveals several important intuitions about thermodynamics. For a system at a constant temperature T, we see that the higher the energy of a state, the smaller its Boltzmann factor and hence its probability. As the temperature increases, thermal energy becomes larger than the energy difference Ei-Ej, and the energy difference thus becomes less important. The Boltzmann factor has been used to explore systems as diverse as stellar atmospheres, black body radiation and superconductors.

Answer: Light polarized perpendicular to the surface

We can think of a surface as a boundary between two materials -- for example, the boundary between air and water, or air and glass. The materials each have an index of refraction, which dictates both the speed of light in that material and the angle at which it bends (refracts) in crossing the surface; we'll call these two indices n1 and n2, with n1 belonging to the first material the light passes through and n2 belonging to the material on the other side of the surface. Brewster's angle is equal to the inverse tangent of n2/n1.

Brewster's angle has the effect of sorting light by its polarization. A photon (the smallest unit of light) is essentially a traveling electromagnetic field; if the electric-field portion is perpendicular to the surface, a photon striking that surface at Brewster's angle CANNOT reflect. The effect is to split the light into two beams: the refracted beam, continuing past the surface, is somewhat polarized perpendicular to the surface, and the reflected beam is 100% polarized parallel to the surface. This is why cleverly designed sunglasses and camera filters are so good at cutting out the glare from puddles of water: by using filters that transmit only the non-reflected polarization, they block the reflections from reaching the viewer.

Answer: Boson

A boson is so named because it follows Bose-Einstein statistics (first described by Satyendra Bose and published with Albert Einstein). There is no theoretical limit on the number of identical, indistinguishable bosons that can occupy a single quantum state, so as a system cools down (thus losing heat energy), the bosons tend to accumulate in the ground or lowest-energy state. This accumulation, known as a Bose-Einstein condensate, is the phenomenon that makes superconductors and superfluids possible.

Particles with half-integer spin, known as fermions, must obey the Pauli exclusion principle -- which does not apply to bosons. Thus, only one identical fermion can occupy a given energy state, no matter how low the temperature has dropped. Different quantum materials behave quite differently at low temperatures!

Answer: Hydrogen

The structure of the hydrogen atom -- and an explanation of the curious spectrum of light emitted by excited hydrogen -- was a primary concern of early twentieth-century physicists. In 1913, Niels Bohr proposed a model whereby an electron could follow one of several prescribed orbits around the nucleus, similar to a planetary orbit around the sun. More distant orbits corresponded to higher-energy (excited) states of the atom, and transitions between these orbits resulted in the atom's distinctive spectral lines. As the radius of the innermost orbit, the Bohr radius thus gave the size of the hydrogen atom in its ground state.

Although we now know the Bohr model to be incorrect, the Bohr radius is still important in atomic physics. Electrons do not orbit the nucleus in well-defined paths; the uncertainty principle tells us that we cannot meaningfully consider both their position and their momentum at once. Instead, we can define an orbital, a region around the nucleus where we are likely to find the electron; for the ground state of the hydrogen atom, we can envision a spherical electron "cloud" containing all the electron's possible positions. The Bohr radius, which can be calculated from fundamental physical constants, describes the size of that electron cloud. The model is different, but the role of this constant is the same.

Answer: Magnetic field vector

The Biot-Savart Law is a bit complicated to write without the benefit of Greek letters and mathematical symbols, but it essentially says that the magnetic field vector (that is, the magnitude and direction of the magnetic field) at a point can be calculated as the sum of contributions from different "pieces" of current. Each current "piece" depends on the current and its direction, its distance and direction from the point of interest, and a constant (the permeability of space); it's most useful to calculate the integral of these elements, so that the pieces can be treated as infinitesimally small. This law is extremely useful for calculating and understanding magnetic fields, but it's only valid in certain limited situations: if the current is changing, you'll need an equation that's a bit more powerful!

Thank you for joining me on this journey through a "b"rave new world of physics vocabulary. I hope you've enjoyed the ride!