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Test your thermal knowledge
Thermal effects in everyday life
     Ice skating
     Food physics
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1.   The energy produced from incinerating 10 kg of household waste is enough to keep a 60 W light bulb burning for:

a)   10 minutes
b)   100 hours
c)   10  hours
d)   10 000 hours
2.   How many Joules are there in 1 calorie?

a)   4,186
b)   9.81
c)   0.981
d)   4.186
3.   Which of the following is not a "greenhouse gas"?

a)   CO2
b)   N2
c)   CH4
d)   N2O
4.   What does chemical potential represent in a thermodynamic system?

a)   The partial molar derivative of internal energy versus temperature
b)   The change in internal energy associated with the change of its molar content
c)   The change of potential due to increase of entropy
d)   The chemical energy release during a reaction
5.  What is the agreed target of the European Union (EU-15) for greenhouse gas reduction in the framework of the Kyoto Protocol?

a)   8% below the level of 1990 between 2008 and 2012
b)   5% below the level of 1995 between 2010 and 2012
c)   8% below the level of 1985 between 2005 and 2010
d)   5% below the level of 2000 between 2010 and 2015
6.   Which of the following statements concerning the use of double-glazing for thermal insulation is correct?

a)   The thermal resistance provided by double glazing is approximately double that of a single pane of glass
b)    Single and double-glazing provide the same insulation if the thickness of the single pane of glass equals the sum of the thicknesses of the two panes forming the double-glazing
c)   Double-glazing provides better insulation but only if the two panes of glass are at the same temperature
d)   The thermal resistance provided by double glazing is nearly half that of a single pane of glass
7.   The coefficient of thermal expansion of stainless steel at room temperature is approximately:

a)   1 x 10-6 K-1
b)   1 x 10-6 mK-1
c)   20 x 10-6 K-1
d)   90 x 10-6 K-1
8.   Which of the following is not true?

a)   Soduim melts at about 78° Réaumur
b)   Sodium melts at about 208° Fahrenheit
c)   Sodium melts at about 271 Kelvin
d)   Sodium melts at about 98° Celsius
9.   Which one of the following combination of thermocouples is not feasible?

a)   Platinum  (+), Chromel (-)
b)   Chromel (+), Alumel (-)
c)   Iron (+), Constantan (-)
d)   Platinum-Rhodium (+), Platinum (-)
10.  Which one of the following materials has the highest melting point?

a)   Sintered alumina
b)   Pure graphite
c)   Sintered titanium carbide
d)   Silica glass
11.  The universal gas constant R is:

a)   8.314 J K-1 mol-1
b)   6.0225 x 104 J K-1 mol-1
c)   6.0225 J K-1 mol-1
d)   8.314 x 104 K-1mol
12.   How can the temperature of ice be increased slightly above 0° C without it melting?

a)   By increasing the pressure
b)   By decreasing the pressure
c)   By adding salt
d)   It is not possible as ice always melts at 0° C


1c, 2d, 3b, 4b, 5a, 6a, 7d, 8c, 9a, 10b, 11a, 12b.

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Ice skating

Well OK, not everybody goes skating everyday, but it is still interesting to ask why skating "works".  The answer is more difficult than what one might initially think.

It is often claimed that one can skate on ice because the pressure of the skate causes the ice to melt (in accordance with the Le Châtelier's Principle and Clausius-Clapeyron Theorem), thus dramatically reducing the friction between skate and ice. While this makes a good story, it is not quite correct. If one takes the skater to have a mass of 75 kg (weight of 165 lbs), and the skate to be 3 mm wide and 20 cm long, one can calculate that entire force exerted on the area of one skate is only a pressure of about 12 atmospheres (it takes a pressure of about 121 atmospheres (1.22 MPa in SI units)) to reduce the melting temperature by 1 °C. Although the force is concentrated in a somewhat smaller area due to the sharp edges of the blades, the effect of pressure alone is enough to shift the melting temperature of the ice by only a few tenths of a degree. Since common experience is that ice skating is possible even when the ambient temperature is well below the normal freezing point, the pressure induced lowering of the melting point clearly does not explain this every day observations.


What is responsible then for the melting? Scientists have far from a complete understanding of this everyday phenomenon. It is likely partially related to an effect known as surface melting. The stability of solids is due to the regular structure that allows for each molecule to have multiple attractive interactions. At the surface of a solid, this is not the case, since there are no molecules 'above' the surface to bind to. As a result, the surface molecules will often distort to make the best of a bad situation by trying to increase their bonding to each other and those below. This is known as surface reconstruction.

It is also known that the molecules on the surface can become disordered and liquid like at a temperature below the normal melting point of a solid, this is the phenomenon known as surface melting. Bringing up another surface (such as the metal of a skate) will influence this surface melting, since now the water molecules on the surface can bind to the metal surface atoms as well. Another important effect is friction, which can generate enough heat to melt a thin layer of ice in contact with the skate.



Misconception about boiling point elevation

Adding salt to water when boiling foods is not to speed up the cooking.  It is true that an increase in cooking temperature will shorten cooking times  (typically by half for temperature rise of 10 - 20 °C, achievable in a pressure cooker). However, even adding salt to the level of sea water the boiling point will only increase by 0.6 °C, shortening the cooking time by a few percent.  Adding too much salt to food increases the risk of having high blood pressure from consuming consuming too much sodium.


Taking the mystery out of boiling eggs

Boiling eggs to a given texture (not too runny or not too hard) is not always successful.  Away from the kitchen, modelling the problem for several eggs is even more difficult. However, Charles D. H. Williams (University of Exeter)  gives a solution of a simpler problem, how to soft boil one egg.  He assumes that the egg is spherical and that the properties of the egg (specific heat, density, porous nature of egg’s surface, etc.) do not change over time during boiling.  His equation is:

t = a M2/3 ln [ 0.76 (Tegg - Twater)/(Tyolk - Twater)]     where

t is the time in seconds (s) to boil the egg
a is a group of constants put together (s g1/3)
ln is the natural logarithm or log base e of a number
Tegg is the initial temperature of the egg (°C)
Twater is the temperature of the water  (°C)
Tyolk is the temperature of the yolk-white boundary of the egg when fully cooked (°C).

According to this formula, for eggs taken out of a fridge:

  • a small (size 6, M=47 g) egg takes four minutes to cook
  • a medium egg (M=57 g)  takes four and a half minutes to cook and
  • a large egg (size 2, M= 67 g) takes five minutes.

A medium sized egg (M=57 g) takes three and a half minutes if it had been stored at room temperature (Tegg=21°C). The graph below illustrates the dependence of boiling time on size and the initial temperature of the egg.

Graph of boiling times


Ohmic heating

In the usual heating methods heat from outside the food is transmitted to the food by conduction and/or convection. For products containing particles (such as fruit or vegetable pieces in a liquid surrounding the particles) the conduction methods often cause overheating of the outer liquid before the heat sufficiently penetrates into the food particles floating in the liquid. This impairs the nutritional as well as sensory characteristics. 

An alternative heating process uses ohmic heating, where the heat is generated through passing an electric current through the food heating it as a result of electric resistance. This avoids overheating and enables producing  food with improved taste and nutritional content, and in some cases improved microbial safety.  This method is being developed  mainly for industrial use although it has been tried in the UK and Japan for domestic cooking.


What to do when your freezer breaks down or there is a power cut?

Do not open the freezer. The food will not warm up too much if left there (especially if the freezer is full and has a large thermal inertia) in a short time (one to two hours). For longer periods of freezer not working, take the food out and insulate it well by wrapping it in blankets.


Why is temperature of food important?

Temperature determines how microorganisms (bacteria, viruses ond other) grow or are destroyed.  Freezing foods does not kill microorganisms whilst heating foods above 70 °C for 3 minutes kills most microorganisms apart spores. Most bacteria grow best at the temperatureof human body, 37 °C.  Therefore food on display in catring outlets should either be kept chilled (between 0 °C and 5 °C) so that bacteria do not grow or else kept warm so that they are destroyed.  The legislation recommends to keep food 'piping hot' although it is not clear what this excatly is (see Quotes).

Apart from the microbiological safety aspects, temperature and heat are important in food processing and for making it digestible (starch conversion in potatoes and rice).  Heating of foods increases the vapour pressure of volatile compounds and hence the olfactory appeal during serving of foods (colour being the other attribute that we sense before tasting the food).

Some anthrophologists go as far as saying that mankind owes its suceess (in terms of proliferation and controlling the planet Earth) to the fact that the human species have mastered fire and preparing hot meals, postulating that people are obligate 'cookivores' and that civilisation started in the regions where fire or heat were available naturally (near volcanos).



The good news is - the reading is normal.
The bad news is - the thermometer leaks.



The nice thing about standards is that there are so many to choose from.
Andrew S. Tannenbaum

To measure is to know. If you can not measure it, you can not improve it.
Lord Kelvin

Well we can try to measure democracy, just as you measure temperature with a thermometer, or pressure with a barometer.
Hugo Chavez

It doesn't matter what temperature the room is, it's always room temperature.
Steven Wright

If you aren't at room temperature, your situation can be improved.
Steven Singer

One does not allow the plumbers to decide the temperature, depth and timing of a bath.
Jack Gould

Any time you get a mouthful of hot soup, the next thing you do will be wrong.
Zall's First Law

The idea is that a dish that is piping hot is one so hot it makes a sizzling or hissing noise, perhaps not closely similar to the sound of the pipes, but at least audible. It is first recorded near the end of the fourteenth century, in Chaucer's Canterbury Tales. In the Miller's Tale it says (in modernised spelling): "Wafers piping hot out of the gleed", where a wafer is a kind of thin cake, baked between wafer-irons, and gleed is the hot coals of a fire.



Volume 7 of the Touloukian and DeWitt series that gave rise to the evitherm database.