You Should Buy Your Petrol in the Morning


Petrol expands as it gets warmer, and it expands a lot: 950 parts per million per kelvin rise in temperature. This may not seem like a lot, but you might be surprised.

Energy is released when petrol is burnt because the energy required to break the bonds between the carbon and hydrogen atoms in the petrol is less than the energy released when new bonds are formed between the carbon and hydrogen and the oxygen atoms from the air. See, for example, the combustion of octane (one of the main components of petrol) shown below.

2\,\textrm{C}_{8}\textrm{H}_{18} + 25\,\textrm{O}_{2} \rightarrow 16\,\textrm{CO}_{2} + 18\,\textrm{H}_{2}\textrm{O} *

This means that the amount of energy released by burning petrol depends on the number of atoms in the petrol, and therefore on the petrol’s mass, but you pay for petrol by the litre. You use mass but pay for volume.

Imagine buying petrol on a day when there is a fifteen degree Celsius difference in temperature between morning and afternoon; the petrol will expand by 14250 parts per million. One litre of petrol bought in the morning will expand to 1.014 litres in the afternoon. This means that the same amount of energy will cost you 1.43% more if you buy it in the afternoon rather than in the morning.


This is all a bit of a moot point, as petrol is stored in large underground tanks, and the temperature underground is a lot more stable than the temperature above ground. Perhaps you’d be better stockpiling petrol in the winter when ground temperatures are lower.

* Breaking eighteen carbon-hydrogen bonds, seven carbon-carbon bonds and twenty-five oxygen-oxygen double bonds requires (18×413)+(7×348)+(25×498) = 22320 kJ. Forming thirty-two carbon-oxygen double bonds and thirty-six hydrogen-oxygen bonds releases (32×360)+(36×459) = 28044 kJ.


“Sugar” is one of those terms that scientists and the general public use in different ways. A scientist would be far more likely to refer to “sugars”, as sugar is actually a group of different molecules. All sugars are carbohydrates, i.e. they contain carbon, hydrogen and oxygen, but the arrangement of these atoms differs between sugars.



The monosaccharides are the simplest sugars. Shown above is the straight-chain form of glucose.

Glucose (aka dextrose)

Glucose is what fuels cellular respiration in living things, and as such is an essential nutrient. It is one of the main products of photosynthesis, created when carbon dioxide and water are combined (using energy from the Sun’s light), producing oxygen as a by-product.

Glucose is found in most foods that contain carbohydrates. It can be created through the breakdown of starch and glycogen (see below), or in more complex disaccharide sugars like sucrose and lactose.

The name dextrose comes from the scientific name for one of glucose’s isomers, D-glucose. The D- prefix comes from the fact that a solution of D-glucose rotates the plane of polarised light to the right (i.e. from the Latin for “right”, which is “dexter”).


Fructose is a sugar that is commonly found in many plants, especially in fruits. It is the sweetest of the naturally occurring carbohydrates.


Galactose is less sweet than glucose and fructose, and is found in dairy products and sugar beets.



Dissaccharides are formed when two monosaccharides are combined. Shown above is lactose.

Sucrose (aka “Table Sugar”)

Sucrose is the sugar that sits on your kitchen table. It comes from cane and beets, and is formed when glucose and fructose are combined.


Maltose is formed of two glucose molecules combined together. It is created in the mouth when starch (a polysaccharide) is broken down by amylase in saliva. You can demonstrate this by chewing a piece of bread for a long time – after a while the starch will be broken down into maltose and it will begin to taste sweet. Maltose is contained in cereals, pasta and potatoes.


Lactose is a combination molecule of glucose and galactose, and is found in milk, and therefore in milk products like cheese. A dietary intolerance to lactose is found in more than half of the world’s population.


Oligosaccharides are formed when between three and nine monosaccharides are combined.

Fructo-oligosaccharides are formed from combinations of fructose molecules, and are found in vegetables, and are an important component of fibre. Galacto-oligosaccharides are formed from combinations of galactose molecules, and are found in soybeans; they stimulate the action of “friendly” bacteria in the gut, but cannot be digested by humans.


A section of a cellulose molecule

Polysaccharides are longer chains (more than ten units) of monosaccharide molecules; common polysaccharides include starch, glycogen, cellulose and chitin. Starches are broken down in the body into more “useful” glucose molecules, and glycogen is used as secondary store of energy (after fat tissue) in humans (the same role that starch plays in plants). Cellulose and chitin are structural, used in the creation of the structure of a plant or animal.

Cold Welding

Welding is the joining together of two materials, usually two metals. This is usually done through the application of heat: the two metals melt, their liquid stages coalesce and mix, and then the combined liquid stage is left to re-solidify by cooling down.


But what if you just placed two pieces of metal on top of each other? The two sets of metal atoms at the interface between the two pieces of metal wouldn’t know which atoms belonged to which piece of metal. As Richard Feynman wrote:

“When the atoms in contact are all of the same kind, there is no way for the atoms to ‘know’ that they are in different pieces of [metal]. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms ‘know’ when they are not on the same part.”

So if you take two very clean and very flat pieces of metal, and place them in contact with each other, in a vacuum (so that there is no air between the two pieces) they will join themselves together, in the same manner as they would if they were welded together conventionally using heat. (Usually the application of large pressures is required, so that the two pieces of metal are fully in contact throughout the entire overlapping area.)

Space is a very good vacuum, and spontaneous cold welding of metal parts aboard spacecraft is something that spacecraft designers have to plan to avoid. It is suggested that the failure of the Galileo spacecraft’s high-gain antenna was due to cold welding that had not been spotted in ground testing due to the quick formation of a protective oxide layer, which did not form in space due to the absence of oxygen.


There is an understandable tendency to believe that all names work like your own, but this isn’t the case. I can’t hope to cover all the minutiae of naming in different countries and cultures, so this article is, at best, a collection of interesting facts about names in different cultures.

To make this post easier to understand I’ll be sticking to some simple terminology: the first name is the name that is written first when the name is written down, and the second name is the name that is written second when the name is written down.

In the “Western system” the first name is a given name, and the second name is a family name (AKA a “surname”). People are also frequently given a second given name, which is placed in between the first and second names (i.e. a middle name). In the “Eastern system” the order is reversed, so the family name is the first name and the given name is the second name. For example, the President of China is Xi Jinping and is referred to as “Mr Xi”, and his daughter is Xi Mingze and is referred to as “Ms Xi”. To avoid confusion, in lists where both Western and Eastern system names appear, it is common to capitalise the name that should be used formally.* It is also common for Eastern system names that do not use the Latin alphabet to be converted to the Western order when the name is transliterated. For example, the Prime Minister of Japan is Shinzō Abe, where “Abe” is the family name and comes first when written in Japanese.

In a large number of countries, when a man and woman are married, the woman takes the man’s family name. Thus if Jane Doe marries John Smith, she becomes Jane Smith. Sometimes the family names are joined or “double-barrelled”, so Jane Doe and John Smith could become Jane and John Smith-Doe, or Jane and John Doe-Smith. In many Spanish and Latin American countries double-barrelled family names are obligatory, and children take the first of each of their parents’ family names. The President of Spain, Mariano Rajoy Brey and his wife, Elvira Fernández Balboa, have two children: Mariano and Juan Rajoy Fernández. In practice, most use only their first family name, so names are patrilinear as with most other countries.

Patronymic and matronymic second names take themselves from the name of the father and mother respectively, and women generally do not change their names when they are married. Perhaps the most well-known example of a qpatronymic name system are Icelandic names, where sons and daughters take a different second name to their parents (i.e. they do not continue-on a “family name”). For example, if a man named Jón (the most common Icelandic first name) had a son, Sigurður, and a daughter, Guðrún, they would take the names “Sigurður Jónsson” (literally “Sigurður, Jón’s son”) and “Guðrún Jónsdottir” (“Guðrún, Jón’s daughter”) respectively. Phonebooks in Iceland list people alphabetically by first name, and people are usually addressed formally by their first name (so the siblings above would never be introduced as “Mr Jónsson” or “Ms Jónsson”). Within in a large family there will therefore be a wide range of second names, and this occasionally causes trouble for Icelanders in foreign countries where people expect children to have the same second name as their parents. The other Nordic/Scandinavian countries, who used to use the same naming system, have generally moved away from it, but in those countries there are a smaller number of surnames and people are often referred to by both their first and second given (middle) names.

In some Eastern Slavic countries (Russia, Belarus, Ukraine, Macedonia, Bulgaria and Kazakhstan) a patronymic name is usually used as a middle name. In Russian, Abram, the son of Anatoly Ivanov, would have the full name Abram Anatolyevich Ivanov, and Anatoly’s daughter Darya would be Darya Anatolyevna Ivanov. In at least Russian, Ukraine and Belarus a person must have three names, including a patronymic.

Arabic names do not follow a given name and family name, or first name and second name system. Rather they indicate the heritage of a person and that person’s hoped-for characteristics. For example, the ruler of Dubai is Mohammed bin Rashid Al Maktoum. “Mohammed” is a religious given name literally meaning “praised”, “bin” is the colloquial form of “ibn” meaning “son of”, “Rashid” is his father’s (Rashid bin Saeed Al Maktoum) given name (meaning “integrity”) and “Al Maktoum” is the “House of Maktoum”, a de-facto family name. Some Arabic names are very long and contain a miniature family history with lots of “bin”s.

* For example, the leaders of the G22 countries are: Stephen HARPER, François HOLLANDE, Angela MERKEL, Pietro GRASSO, Shinzō ABE, Vladimir PUTIN, David CAMERON, Barack OBAMA, Cristina FERNÁNDEZ DE KIRCHNER, Tony ABBOTT, Dilma ROUSSEFF, XI Jinping, LEUNG Chun-ying, Pranab MUKHERJEE, Joko WIDODO, NAJIB Razak (“Najib” is the given name, and “Razak” is a patronym), Enrique PEÑA NIETO, Bronisław KOMOROWSKI, Tony TAN, Jacob ZUMA, PARK Geun-hye and PRAYUT Chan-ocha (Chan-ocha is the family name, but according to Thai custom he is referred to by his given name).

Ranking Things Properly

I keep seeing things ranked improperly, so here is how to do it right.

Imagine that we have six candidates for an exam, and they score as follows. Ranking these candidates is very easy.

Name Score Rank
Abel 90% 1
Bohr 80% 2
Curie 70% 3
Dirac 60% 4
Einstein 50% 5
Feynman 40% 6

But what if two candidates have the same score? The correct way of ranking is to give both of these candidates the same rank, but then the next rank is one place lower. In the example below, Abel and Bohr both score 90% and are therefore ranked in first place; Curie then remains in third place, rather than being elevated to second.

Name Score Rank
Abel 90% 1
Bohr 90% 1
Curie 70% 3
Dirac 60% 4
Einstein 50% 5
Feynman 40% 6

This prevents a situation in which we have six participants, but the person with the lowest score is ranked fifth. If more than two participants have the same score, or if this situation occurs more than once, the same rule is applied.

Name Score Rank
Abel 90% 1
Bohr 90% 1
Curie 90% 1
Dirac 60% 4
Einstein 60% 4
Feynman 40% 6