anode v. cathode

An electrolytic cell. An electrode is a conductor that makes contact with a nonmetallic medium (e.g., an electrolyte). In an electric circuit, an electrode is referred to as either an anode or a cathode. Current enters a device through the anode, and leaves a device through the cathode.

If a device (e.g., an LED) is consuming power, the anode polarity is positive and the cathode is negative. If a device (e.g., a battery) is providing power, the anode is negative and the cathode is positive. In a rechargeable battery, the anode and cathode swap positions depending on whether the battery is being discharged or recharged.

By convention, current flows in the direction of positive charges. In metals, negatively charged electrons flow in the direction opposite the conventional current.

In electrochemistry, oxidation, or the loss of electrons, occurs at the anode. Reduction, or the gain of electrons, occurs at the cathode.

John Denker on anode v. cathode

conduction v. convection v. radiation

Heat is transferred from high-temperature regions to low-temperature regions in a few fundamentally different ways: conduction, convection, and radiation.

Conduction

Conduction is the transfer of heat between substances that are in direct physical contact. When warm, fast-moving molecules collide with cooler neighboring molecules, some of their vibrational energy is transferred. You experience conduction when you touch a hot stove or an ice cube. Conduction is most effective in solids, particularly metals. Poor conductors (or insulators) include air, wood, and Styrofoam.

Convection

Convection is the bulk circulation of liquids or gases as hot, less dense material rises, and cool, denser material falls. Convection occurs in the oceans and the atmosphere, as well as within buildings and tea kettles. Conduction only involves molecular-scale particle motion, whereas convection involves the macroscopic movement of the entire medium.

Radiation

An infrared image of microwave burritos. Radiation is the transfer of thermal energy through space by electromagnetic waves. Radiative heating requires neither a medium nor the exchange of matter. Dull, black surfaces are better at absorbing thermal radiation than reflective white surfaces. The Sun heats the Earth by radiation. Microwave ovens, radiators, fires, and light bulbs transfer heat by both radiation and convection.

Wisc-Online interactive heat transfer lesson

Wikipedia on heat transfer

refraction v. diffraction

Refraction

When a wave (of light, sound, water, etc.) passes from one medium (e.g., glass or air) into another, its speed will change. If the wave hits the boundary between the substances at an oblique angle, the change in speed will cause it to bend. This phenomenon is called refraction, and is described by Snell’s law. Waves bend toward the normal when they slow down, and away from the normal when they speed up.

Refraction is a key principle used in the manufacture of lenses for eyeglasses, telescopes, and cameras. It is also responsible for mirages, rainbows, and the perceived bending of straws in drinks.

Ordinary (left) and negative refraction (right). Image courtesy of Nature.com.

Diffraction

Diffraction occurs when waves bend around obstacles or spread out after passing through aperatures. This effect is most pronounced when the scale of the obstacle is about the same as the wavelength of the waves. If the wavefront passes through several closely-spaced openings, an interference pattern of light and dark regions will form.

Diffraction causes the edges of shadows to blur, and allows sound to travel around objects. The iridescent colors observed on peacock feathers, sea shells, and CDs are caused by these materials acting as diffraction gratings.

Astronomy 162 on refraction v. diffraction

ontogeny v. phylogeny

Ontogeny is the biological development of an individual organism from an embryo to its adult form. Phylogeny is the evolutionary history of an entire species or higher taxonomic group. In 1866, Ernst Haeckel, a German zoologist, formulated a theory of recapitulation. He conjectured that the stages in the embryonic development of an individual corresponded to the adult stages of prior species in its evolutionary history. For example, a human embryo with gill slits was said to represent a fishlike ancestor. Haeckel’s law is usually presented in a concise form as, ‘ontogeny recapitulates phylogeny’. This theory has since been discredited.

speed of light v. speed of sound

Speed of Light

The speed of light is 299,792,458 meters per second, or approximately 1 billion kilometers per hour (670 million miles per hour). Represented by the letter c, it is the speed at which electromagnetic radiation propagates in a vacuum. It is also the maximum speed at which matter, energy, or information can travel.

The speed of light is a fundamental physical constant, and remains the same for observers in all inertial reference frames. When light moves through transparent materials, it is constantly absorbed and re-radiated, making its average speed in a medium less than c.

Speed of Sound

The speed of sound in air at 20° C, also called Mach 1, is 343 meters per second (1,236 kilometers per hour, or 768 miles per hour). This makes light about 900,000 times faster than sound. For phenomena occurring within several kilometers of an observer, light’s travel time will be perceived as functionally instantaneous, but sound from the same source will arrive about 3 seconds later for every kilometer separating the observer from the event.

Unlike electromagnetic radiation, sound requires a medium through which to travel, and its speed varies depending on the composition and temperature of that medium. The speed of sound increases with increasing stiffness, decreasing density, and increasing temperature. Sound travels faster in liquids than gases, and faster still in solids.