TEMPERATURE AND THERMAL EXPANSION

Size: px

Start display at page:

Download "TEMPERATURE AND THERMAL EXPANSION"

Rosamond Flowers
6 years ago
Views:

1 TEMPERATURE AND THERMAL EXPANSION After boiling water, you will feel that the water is hotter than before, or you can say that the water temperature is higher than before. Otherwise, when you pick an ice, our hand will feel cold. Besides those examples, there are many phenomena related to the temperature. Changes in temperature cause thermal expansion. Have you ever made an ice in plastic bag filled with water? After it freezes, the plastic bag will get bigger. From that fact we can see the relation between temperature and thermal expansion. 1. Temperature You will feel hot when the sun shines intensely on the day time, conversely you will feel cold at night. You will get experience when you touch the glass of hot water or an ice. Temperature is a numerical measure of hotness or coldness of an object. Hot object have high temperature, conversely cold object have low temperature. You can perceive how hot or cold an object is by using your touching sense, but is it able to measure the exact degree of hotness or coldness of an object? Your touching sense is unreliable for measuring temperature, because it cannot inform you the exact temperature of an object. An exact measurement of temperature can be obtained by using thermometer. 2. Thermometer Since we're going to be talking about heat, temperatures, and energy, we wanted to introduce you to how temperature is measured. The big three are Fahrenheit, Celsius and Kelvin. Even though scientists may use only a few scales to measure temperature, there are dozens of types of devices that measure temperatures. All of these devices are called thermometers because they measure temperature. There are thermometers to measure your body temperature, the temperature in your oven, and even the temperature of liquid oxygen. How to calibrate thermometers Whenever you purchased a new thermometer and about to use it for the first time, you need to calibrate it. Thermometers will also need to be calibrated whenever they are dropped or when going from one temperature extreme to another. Thermometers are a critical measuring equipments for temperature to ensure the safety and quality of many food products. Also remember to sanitize thermometers before using and in between using. You could clean and sanitize the thermometer by washing it in hot, soapy water, rinse and wipe dry or you could use a sanitizing solution suitable for food contact surfaces. Allow the thermometer to sit in a sanitizing solution for about two minutes. Then use hot water to rinse the thermometer. There are two methods that you could use to calibrate thermometers Ice Point Method Fill a container with crushed ice and water. Ensure that the container have enough crushed ice to provide an environment of 32 degrees F, so you may need to add more ice into the container during the process. It will takes about 4 to 5 minutes for the mixture of water to stabilized. Then insert the thermometer which needs to be calibrated into the

2 appropriate immersion depth. Hold the thermometer away from the bottom and sides of the container to avoid error. If your thermometer is not accurate at 32 degrees F, adjust the thermometer accordingly. Boiling Point Method Fill a container with distilled water and heat it up. After the temperature in the water has reached the boiling point, insert the thermometer. Be sure to leave at least a two inch gap between the thermometer sensing element and the bottom and sides of the container to avoid error. If your thermometer is not accurate at 212 degrees F, adjust the thermometer accordingly. There are four kinds of thermometer scale as follows a. Fahrenheit is the Classic Fahrenheit is the classic English system of measuring temperatures. Water freezes at 32 degrees Fahrenheit and boils at 212 degrees. The scale was created by Gabriel Daniel Fahrenheit in 1724 and divides the difference between the boiling point and freezing point of water into 180 equal degrees. You will probably be asked to convert temperatures back and forth from Fahrenheit to Celsius. Here's the formula: (Fahrenheit-32)*5/9=Celsius. b. Celsius Based on Water Celsius is the modern system of measuring temperature. It fits in with much of the metric system and has nice round numbers. In Celsius, we call the freezing point of water 0 degrees Celsius, and the boiling point 100 degrees Celsius. Then the scale is divided into 100 equal degrees between those two points. The scale used to be known as centigrade but the name was changed several years ago. Both Celsius and Fahrenheit are used when discussing our day-to-day weather temperatures. Celsius degrees are larger than Fahrenheit degrees. c. Kelvin to Absolute Zero Kelvin is an important scale used in most of science. The big difference is that it is based on a single point (absolute zero) which is given a value of 0 degrees. From there, the scale increases by degrees that are the same size as Celsius degrees. It is a scale that is based on energy content, rather than on arbitrary temperature values like the other two scale (based on water). Water freezes at the value and boils at Kelvin. The word "Kelvin" comes from Lord Kelvin, who did a lot of work with temperatures. d. Reamur Scale Reamur scale was made by Rene Antoine Ferchauldt de Reaumur. The number 0 as a lower fixed point was assigned to the temperature at which pure ice melt, and the number 80 as an upper fixed point was assigned to the temperature at which water boils. The space between those marks was divided into 80 scales, so every scale represent 1 0 R. Types of Thermometer Thermometers are widely used to measure the temperature in various applications. Different types of thermometers are available. To know more about the different types of thermometers. a. Clinical Thermometers

3 The clinical thermometers are used to measure the body temperature of the patient. There are again three types of clinical thermometers depending on the body part used to measure the temperature. Ear (Tympanic) thermometers Human ear is located near the brain. This makes it an accurate point to measure the body temperature. The temperature of the eardrum is measured by the ear thermometers. However, the eardrum is most fragile and delicate body part. Therefore, the body temperature cannot be measured by touching the eardrum. For temperature measurement, infrared sensors are used to remotely sense the temperature of the eardrum. Thermopile, an infrared sensor, is commonly used in ear thermometer. Pacifier Thermometers The pacifier thermometers are used to check the body temperature of babies or infants. They help measure the body temperature without irritating the baby. The thermometer is held in the mouth of the baby and the baby's natural sucking instinct is used to check its body temperature. The pacifier thermometers are very safe for checking the body temperature of babies. Underarm or Oral Thermometers The underarm thermometers are kept in the underarms to measure the body temperature of the patient. Likewise, the oral thermometers are held in the mouth for temperature measurement. b. Food Thermometers There are many food thermometers like the dial oven-safe thermometers, digital instant thermometers, pop-up thermometers and disposable thermometers. The dial oven-safe thermometers are used for thick foods and can be placed in the food while you are cooking the food. However, the dial oven-safe thermometers are not suitable for thin and watery foodstuffs. The digital instant thermometers cannot be kept bin the food while cooking. These thermometers read the temperature within 10 seconds. The pop-up thermometers are used to measure the temperatures of the turkeys and chicken. The disposable thermometer strips are used to measure the temperature of food after they are cooked and they change the color according to the temperature. The color of these thermometer strips can be matched to a chart, which gives the corresponding temperature of food. c. Outdoor Thermometers The outdoor thermometers are used to measure the temperature of the surrounding air. Wireless outdoor thermometers are very popular these days. 3. Temperature Scale Conversion a. Conversion Between Celsius and Fahrenheit Temperature Scale T C = (T F 32 ) 0 C T F = ( T C + 32) 0 F

4 b. Conversion Between Celsius and Reamur Temperature Scale T C =( TR)0 C T R =( TC)0 R c. Conversion Between Celsius and Kelvin Temperature Scale T K = ( TC + 273)K T C = (TK 273 ) 0 C D. Thermal Expansion If you heat a pan full of water, you will see that when the water is boiling, the pan lid starts to jiggle and some water will spill out of the pan. Why does it happen? The fact indicates that the water volume is expanding which makes the pan unable to retain the water anymore. The increasing volume of water is called thermal expansion. Besides the water, the pan is expanding either, which make the pan volume is also increasing. But why does the water spill out from the pan? It happen because the volume of the water is increasing greater than the pan. So we conclude that the thermal expansion of liquids are greater than that of solid. Thermal expansion is the tendency of matter to change in volume in response to a change in temperature. When a substance is heated, its particles begin moving and become active thus maintaining a greater average separation. Materials which contract with increasing temperature are rare; this effect is limited in size, and only occurs within limited temperature ranges. The degree of expansion divided by the change in temperature is called the material's coefficient of thermal expansion and generally varies with temperature. Common engineering solids usually have coefficients of thermal expansion that do not vary significantly over the range of temperatures where they are designed to be used, so where extremely high accuracy is not required, calculations can be based on a constant, average, value of the coefficient of expansion.

5 Thermal Expansion Over small temperature ranges, the linear nature of thermal expansion leads to expansion relationships for length, area, and volume in terms of the linear expansion coefficient. Thermal Expansion of Solids L = L 0 α T linear A = A 0 2α T superficial (areal) L = L 0 3α T volumetric Thermal Expansion of Liquids Liquids have higher expansivities than solids Thermal Expansion Gases V = βv 0 T β 10 3 /K, 3α 10 5 /K

6 Have you ever seen a hot air balloon? Hot air balloon flies because the hot air inside the balloon is less dense than the surrounding air. that phenomenon shows that the air expand when temperature increased. Helium and nitrogen also undergo the same thing. Gases become lighter because their molecules take greater spaces while the amount remains constant. In other words, the volume of gas expands. Generally, if the amount of gasses in the balloon decreases, then its volume will also decrease and the gas constracts. behavior of gases is more complicated, gases will expand as much as pressure will allow Coefficients of Thermal Expansion for Selected Materials material linear volume α (10 6 K 1 material ) β (10 6 K 1 ) aluminium 23.1 alcohol, ethyl 1120 barium ferrite 10 gasoline 950 brass 20.3 jet fuel, kerosene 990 carbon, diamond 1.18 mercury 181 carbon, graphite 6.5 water, liquid (1 ) 50 carbon, graphite 0.5 water, liquid (4 ) 0 chromium 4.9 water, liquid (10 ) 88 concrete 8 ~ 12 water, liquid (20 ) 207 copper 16.5 water, liquid (30 ) 303 germanium 6.1 water, liquid (40 ) 385 glass 8.5 water, liquid (50 ) 457 gold 14.2 water, liquid (60 ) 522 iron 11.8 water, liquid (70 ) 582 lead 28.9 water, liquid (80 ) 640 nickel 13.3 water, liquid (90 ) 695 platinum 8.8 plutonium 54 silicon 4.68 silver 18.9 solder, lead-tin 25 steel, stainless 17.3 steel, structural 12 tin 22 titanium 8.5 tungsten 4.5 uranium 13.9 water, ice (0 ) 51 zinc 30.2

Heat and Temperature

Chapter 4 Heat Heat and Temperature Heat is a form of energy Heat is the energy of random motion of molecules constituting the body. It flows from a hot body to a cold body. Unit of heat is joule (J) and