Истории и востоковедения
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- Бұл бет үшін навигация:
- Absolute Temperature scale
- How to make a thermometer
| Thermal equilibrium
Thermal equilibrium is simply another way of saying that two or more objects are at the same temperature. For instance, your best friend and you have never met, not even shaken hands. Yet if you are in good health you can bet that your body temperatures are at 37 ºC. You are both in thermal equilibrium. Ignoring the fact that our extremities (e.g hands, feet and nose) may be colder than the rest of our body. This is sometimes called the zeroth law of thermodynamics. The reason for this is that physicists first found the first and second laws, then realised that there is a more fundamental law so they decided to give it the number zero. More formally the law can be quoted as follows: «Zeroth law of thermodynamics: If object A and object B are in thermal equilibrium with object C, then they are in thermal equilibrium with each other». Absolute Temperature scale There is a physical lower temperature limit of matter. Nothing can be cooled below -273.15 ºC. For convenience, scientists have devised the absolute temperature scale which starts with -273.15 ºC and called it Kelvin (not degrees Kelvin). So the relationship between Celsius and Kelvin is: TK=TC+273.15, where TK is the temperature in Kelvin, and TC is the temperature in Celsius. Examples: Ice freezes at 0 ºC or TK= 0 +273.15= 273.15 Kelvin. Normal room temperature is at 20 ºC or TK = 20 + 273.15 = 293.15 Kelvin. How to make a thermometer To measure temperature we have to measure another macroscopic quantity that is directly influenced by temperature. There are many ways to do this. Thermometers use physical properties ranging from electrical resistance to radioactivity. But the oldest of all is the mercury thermometer. All materials change their physical dimensions when heated or cooled. The change in length is a direct measure of temperature. To make a thermometer, we fill a thin glass tube with mercury (some use alcohol). Place the tube in ice that's been sitting in the room for a while 43 and starting to melt. We then note the position of the mercury column on the glass tube. We can call this level whatever we like, but we choose to call it 0 ºC. We then place the tube in boiling water and watch the mercury column expand to another length. We then call this level 100 ºC for convenience. We can then divide the length between the two positions into 100 equal segments and call each one 1 ºC. Note that there is nothing special about this way of putting marks on a thermometer. It is simply for human convenience. We could have equally chosen a material other than water to calibrate our thermometer. We could have chosen wax. That it, we could have chosen 0 ºC to be the temperature when wax just starts to melt, and 100 ºC when it boils. There is also no need to call one 0 and one 100 such as the Fahrenheit scale which we will not be using here. жүктеу/скачать 1,14 Mb. Достарыңызбен бөлісу:
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