Physics - Unit By Unit! Trivia Quiz

Answer: Length

Named after Swedish physicist, Anders Jonas Ångström (1814-1874), the Ångström is a unit of length corresponding to 10^-10 meters (10 raised to the -10 meters). This incredibly small distance is appropriate for describing the separation of atoms in molecules.

For example, the oxygen to hydrogen bond length in water is around 0.96 Ångströms. It was defined in 1907 by using the red atomic emission line of cadmium being 6438.46963 Å, where Å is the symbol for Ångströms. Although there are many variations listed in the end-points, visible light is typically in the range 3800 to 7200 Å, or 380 to 720 nm.

Answer: Length

In 1913, British astronomer Herbert Hall Turner, created the term 'parsec' as a contraction (portmanteau) of "parallax of one second", corresponding to the distance at which the mean radius of the earth's orbit subtends an angle of one second of arc. To give this a little more meaning to non-astronomers, the nearest star to earth is Proxima Centauri, which has a parallax of 0.77 seconds of arc. Thus, its distance from the Sun and Earth is 1/parallax = 1.30 parsec. One parsec equals 3.26 light-years, or 3.09 × 10 raised to the power 13 km (1.9 × 10 raised to the power 13 miles).

These distances are literally 'astronomical'! The mean distance from the earth to the sun is 'just' 5 millionths of a parsec.

Answer: Energy

Although there is some debate about the first usage of the word, most sources identify French chemist and physicist Nicholas Clément using 'calorie' in the early 1820s in reference to heat engines. A calorie was defined as the heat needed to raise the temperature of 1 kg of water from 0 to 1°C in Ganot's 'Traité de Physique' (Treatise on Physics), first published in 1851.

More commonly, people now come across 'calories' when considering the energy provided by foods and balancing their energy gained with the energy consumed by metabolic activities including exercise. VERY confusingly, food energy is presented on food labels as calories when in fact they are kilocalories (so 1000 times greater); the capital 'C' on the label ('Cal') actually represents a kilocalorie, but is still called a 'calorie', whereas a lower case 'c' ('cal') represents the original calorie unit. Although there is a lot of variation based on individual differences and activities performed, the typical adult requires around 2000-2500 kcalories (or 2000-2500 Calories!) per day. Non-adherence to this 'protocol' in labelling adds further to this confusion....

Answer: Energy

Unlike the aforementioned confusion associated with 'calorie', the Joule unit (which is a different measure of energy), is well-defined; 1 Joule (symbol 'J') is the energy released through a resistance of one ohm by a current of one ampere over a duration of one second.

It is also defined as the work done by a force of one newton over a distance of 1 metre. Energy in thermodynamics is expressed in Joules (or kilojoules). The unit is named in honour of English physicist James Prescott Joule (1818-1889), who determined using different materials that heat was a form of energy irrespective of the type of substance being heated.

Answer: Energy

BTU is an initialism for 'British Thermal Unit', a unit of energy. As an aside, an initialism is where each letter in the abbreviation is pronounced separately, as distinct from an acronym where the letters are pronounced like a single word, e.g. NATO. British railway engineer Thomas Tredgold (1788-1829) was interested in heating and ventilation and developed the concept of the BTU, originally based on the amount of heat needed to raise a cubic foot of water by one degree Fahrenheit.

This changed to one pound of water as the amount of water in a cubic foot (the volume) is dependent on the temperature of that water whereas a pound of water (the mass) is temperature independent and thus a more reliable measure. 1 BTU is equivalent to 1055 J or 252 calories.

Answer: Luminous Intensity

Hopefully the similarity between the S.I. (metric) unit of luminosity, candela, and the word 'candle' made this one pretty easy - candela is actually the Latin word for candle. The symbol for this unit is 'cd' rather than 'Cd' to avoid confusion with the chemical symbol for cadmium. Interestingly, the light emitted by a typical candle is about 1 cd.

The specific definition of the candela is the luminous power emitted by a light source in a particular direction per unit solid angle. This is then weighted by the sensitivity of the human eye to each wavelength of light (our eyes are generally most sensitive to green light). A solid angle is best described by thinking of a sphere with the light source at the centre of it. The solid angle is described by the 'steradian' - and represents the area of that sphere equal to the square of the radius of the sphere. The entire surface area of the sphere is covered by 4 x pi (or approximately 12.57) steradians. This is much easier to visualize with a diagram!

Answer: Radioactivity

The Curie, a non-SI (non-metric) unit of radioactivity, was named in honor of the Curies but it's actually not clear whether it refers to Pierre (most common attribution), Marie, or both, as there is disagreement in the reference texts! Nevertheless, both are very deserving of this accolade. Along with Henri Becquerel (the subject of another question in this quiz), the Curies are credited with the discovery of radioactivity and did much research on this exciting new area of physics in the very early 20th century. According to an American contemporary, Bertram Boltwood, Marie was initially happy with the unit being the amount of radon gas in equilibrium with a definite but very small amount (1/100 of a microgram) of radioactive radium, but she soon wanted it to be 1 gram as this was not such an insignificant amount! Her wish was met by the Radiation Standards Committee, chaired by eminent physicist and fellow Nobel Prize winner, Ernest Rutherford in 1910.

More recently, the unit has been redefined as 1 Curie (symbol Ci) = 3.7 × 10 raised to the power 10 decays per second of the radium isotope, 226Ra. The Curie is still in relatively common use into the 21st century.

An interesting reference for the development of the Curie as a unit can be found at www.orau.org/health-physics-museum/articles/how-the-curie-came-to-be.html

Answer: Radioactivity

The becquerel is the S.I. (Metric) unit of radioactivity. It is defined as 1 atomic disintegration per second. The amount of energy released by that disintegration depends upon the isotope involved and the mechanism for that radioactive decay (disintegration) - alpha, beta or gamma radiation. One Curie (see another question) is equal to 3.7 × 10 raised to the power 10 becquerels (symbol Bq). The unit is named in honor of French physicist and engineer Henri Becquerel (1852-1908) who first discovered radioactivity and shared the 1903 Nobel Prize in Physics with other pioneers in this field, Pierre Curie and Marie Skłodowska-Curie. Becquerel was very interested in Röntgen's discovery of X-Rays and he worked on exposing samples including some uranium salts to light to examine phosphorescence. He soon discovered that penetrating radiation was emanating from the uranium salts without any need for stimulation by light.

Later he showed that there were different types of this radiation (radioactivity) by placing different samples in a magnetic field - some radiation was positively charged (alpha particles), some was negative (beta particles) and some was neutral (gamma rays).

Answer: Mass

English Physicist and Chemist John Dalton (1766-1844) is responsible for two major scientific breakthroughs - the identification and description of color blindness and recognition of its hereditary nature plus the development of atomic theory. He postulated that matter was made up of small, indivisible entities called atoms which cannot be created, destroyed or sub-divided (this is still held as true with the exception of nuclear reactions). These atoms differed in size depending on the chemical element and then these atoms of different elements would combine in simple ratios to form chemical compounds, for example the well-known representation of water as H2O - there are two hydrogen atoms for every oxygen atom. Finally, chemical reactions involved the movement of atoms between what we now call reactants and products.

The most profound aspect of this seminal work was the idea that atoms were the basis of matter. The mass of individual atoms is very small, typically in the range of 2-200 x 10 raised to the power -27 kg (roughly double this for the mass in pounds). Rather than use such incomprehensibly small numbers, the mass of atoms is given in atomic mass units (amu) also known as Daltons (symbol Da) in honour of John Dalton. We see the atomic mass of each element in the Periodic Table where carbon for example has an atomic mass of 12.011 Da. Dalton's findings are even more remarkable given that he was shunned by almost all educational institutions as he was a Quaker. He also made significant contributions in meteorology and physical chemistry.

Answer: Mass

A hectogram, symbol 'hg', is simply a more unusual, and rarely used, mass measurement. It is comprised of the prefix 'hecto' meaning 100 (just like kilo means 1000 and centi means 1/100) and 'gram'. Thus a hectogram is 100 grams or 1/10 of a kilogram (and 0.220 pounds). Perhaps the most common occurrence of the 'hecto' prefix is in the pressure unit, hectopascals, where 1 hPa = 1 millibar.