Fun stuff |
Test your thermal knowledge Thermal effects in everyday life Ice skating Food physics Cartoon caption competition Quotes Photo gallery TEST YOUR THERMAL KNOWLEDGE 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 a) 4,186 a) CO2 a) The partial molar derivative of internal energy versus temperature a) 8% below the level of 1990 between 2008 and 2012 a) The thermal resistance provided by double glazing is approximately double that of a single pane of glass a) 1 x 10-6 K-1 a) Soduim melts at about 78° Réaumur a) Platinum (+), Chromel (-) a) Sintered alumina a) 8.314 J K-1 mol-1 a) By increasing the pressure Answers 1c, 2d, 3b, 4b, 5a, 6a, 7d, 8c, 9a, 10b, 11a, 12b. Your score: 9 - 12 - Excellent! Have you thought about registering as an evitherm expert? Or maybe you would like access to data, to promote your own services, to know what's going on (via forum or newsletter) or to join an R&D project? If so, subscribe to evitherm now. 6 - 8 - Very good, and why not subscribe to evitherm now to improve your thermal knowledge even further? 3 - 6 - You have some thermal knowledge but would no doubt benefit from having quick and easy access to knowledge, experts and services in thermal metrology. May we suggest that you subscribe to evitherm now. 1 - 3 - Perhaps you have forgotten what you learnt some years back and would benefit from refreshing your memory, or some training, or communicating with others in a forum environment. Whatever, we are sure that you will benefit if you subscribe to evitherm now. 0 - Really, are you sure you're on the right website? Don't despair, subscribe to evitherm now. THERMAL EFFECTS IN EVERYDAY LIFE 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 According to this formula, for eggs taken out of a fridge:
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. 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 nice thing about standards is that there are so many to choose from. To measure is to know. If you can not measure it, you can not improve it. Well we can try to measure democracy, just as you measure temperature with a thermometer, or pressure with a barometer. It doesn't matter what temperature the room is, it's always room temperature. If you aren't at room temperature, your situation can be improved. One does not allow the plumbers to decide the temperature, depth and timing of a bath. Any time you get a mouthful of hot soup, the next thing you do will be wrong. 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. PHOTO GALLERY
Volume 7 of the Touloukian and DeWitt series that gave rise to the evitherm database.
|