Unit 2 Heat And Temperature

Unit 2 Heat and temperature 2.1 Definition of Heat and temperature 2.2 Temperature scale 2.3 Body heat and body temperature 2.4 Conduction, convection, radiation and evaporation 2.5 Temperature measurement 2.6 Regulation of temperature

2.1 Definition of Heat and temperature Heat is a form of energy. It is produced by burning fuels, by human bodies, and by friction when two surfaces are rubbed against each other. Temperature is a measure of the tendency of a body to transfer heat from or to other bodies. This can be viewed by imagining that atoms are in constant random motion. Particles at higher temperature are moving with more energy. When two objects touch, energy (heat) is transferred from one with higher energy (temperature) to the other one, due to collision of particles.

2.2 Temperature scale The three temperature scales commonly in use today are Fahrenheit 華氏 / °F, Celsius 攝氏 / °C and absolute scales (or Kelvin scale / K).

T(°F) = 9/5 T(°C) + 32 T(°C) = 5/9[T (°F)-32] T(K) = T(°C) + 273.15

2.3 Body heat and body temperature Body heat is the result of a balance between heat production and heat loss, which occurs through conduction 傳導, convection 對流, radiation 輻射 and evaporation 蒸發. Balancing heat production within the body against heat loss to the surroundings determines the body temperature. Body fat is energy, and it is part of the heat storage process in our bodies. Body temperature

2.4 Conduction, convection, radiation and evaporation Conduction How heat is Heat passes from transferred one molecule to the next Where the heat Occurs most rapidly transfer occurs in solids. Metals are good thermal conductors Characteristic Does not pass of the method through thermal insulators such as glass, wool, plastic, etc. Examples

Heat produced by friction passes by conduction through the skin to underlying muscle during massage

Convection

Radiation

Heat passes by the movement of heated molecules

Heat travels in straight line as heat rays. All hot objects give out heat rays Occurs in gases and space

Occurs in liquids and gases

Natural convection: Expansion of the heated gas or liquid increases its volume and reduces its density. The hot gas or liquid therefore rises. A stream of heated molecules forms a convection current. Forced Convection: movement of fluid due to external forces. E.g. a fan. The movement of hot water in a hot water system and movement of warm air in a sauna are due to convection

Dull, dark surfaces in the path of heat rays absorb them and become hotter. Shiny and light surfaces reflect heat rays and remain cool Nichrome heating element 鎳鉻發熱元件 and infrared 紅外線 heaters give out radiant heat. heat from the sun reaches the earth’s atmosphere by radiation.

The human body at 37 °C loses about 25% of its heat to the surroundings by conduction and convection as against 43% by radiation. Most of the remaining heat loss is by evaporation.

The greenhouse effect 溫室效應 The greenhouse effect is a name given to the trapping of heat in the earth's atmosphere by a process very similar to that used in greenhouses. The atmosphere, like window glass, is transparent to incoming visible radiation and most of the sun's infrared. These are absorbed by the earth, and reemitted as infrared. Since the earth's temperature is much lower than the sun's the infrared radiated by the earth has a much longer wavelength. The atmosphere, like glass, traps these longer infrared rays, keeping the earth warmer than it would otherwise be. The amount of energy trapped depends on trace gases like carbon dioxide. Changes in quantities of these gases could therefore affect the entire earth's temperatures. Cooling by evaporation When a liquid changes to a vapour it requires an additional quantity of heat energy to do so. This is because the molecules of a liquid are packed close together and are, therefore, able to exert mutual attractions to each other. Any molecule that is to escape from the liquid and become part of a vapour needs to be travelling at high speed when it reaches the surface, in order to overcome the attraction of the molecules left behind. To reach this high speed, energy is required and is supplied in the form of heat. Since no rise in temperature is produced by this additional energy we call it latent heat 潛熱, which means hidden heat. This latent heat must be obtained from somewhere and the nearest source is the surface from which the liquid is evaporating. This causes a cooling effect on the surface and is particularly noticeable if the liquid evaporating does so rapidly. Such a liquid is said to be volatile 揮發性. The effect is noticed if perfume or eau de cologne is spilled on the skin and may be used purposely to cool a person's forehead during illness. Its most important biological application is perspiration 流汗 , when the evaporating sweat helps to remove heat from the body, thus assisting in the control of body temperature. Factors that can affect evaporation rate include: temperature, wind, surface area, and humidity. Humidity is measured by relative humidity (RH) %RH =

actural amount of water vap our in the air ×100 % amount of water vap our saturating the air at the same temperatu re

For comfort, air should have a %RH between 40 and 50. If it reaches 70, sweat will not evaporate to cool the body adequately and heat fatigue will cause headache, tiredness and irritability. High humidity results in a lowering of oxygen level in the blood. Yawning then occurs to inject more oxygen into the blood stream.

2.5 Temperature measurement Core temperature vs. Surface temperature The average normal body temperature remains almost constant, with ± 0.6 °C for a healthy person. It is generally considered to be 37 °C when measured orally. Core temperature (body temperature): the temperature in the interior. It is maintained constant. The core temperature is an indicator of the health of a person.

Surface temperature: the temperature of the skin or subcutaneous tissues 皮下組織. This varies with the temperature of the surroundings. This temperature is an important factor when discussing heat loss through sweating (evaporation) from the skin. Clinical temperature measurement There are two kinds of clinical temperature measurements: surface or skin temperature measurement and core temperature measurement. We measure temperatures with a thermometer, which is a device that detects a change in temperature. This is done by detecting a change in the physical property of a sensor. This physical property can be a mechanical expansion of a temperature dependent liquid, such as mercury, or an electric resistance change of a thermally sensitive resistor (thermistor 熱敏電阻). Traditionally, we measure temperature from an oral cavity, rectum 肛 門 , or under the armpit by contact measurement. The measured temperature varies from site to site, ranging from the oral temperature, which is lower than the core temperature by about 0.5 °C to the rectal temperature that is lower than the core temperature by as much as 2 °C. The tympanic membrane 鼓膜 , or ear drum, that connects the outer ear (ear canal) and the middle ear provides an ideal location for temperature measurement. (This is close to the core temperature, and is not affected by interfering factors) In practice, non-contact measurement of the tympanic membrane temperature is not superior to other measuring sites in reflecting the real core temperature. The measured temperature is still lower than the core temperature by about 0.5 to 1 °C. However, it provides a convenient and comfortable site for reliable and accurate temperate measurement for a variety of patients. Surface temperature measurement Radiation from the skin surface is the main mechanism of heat loss from the body. It accounts for 60% of the body heat loss under normal conditions. The surface temperature is a good indicator for bone fractures 骨折 and inflammation 發炎. Medical thermography 熱 像 圖 法 is a technique to scan the skin surface and map its thermal distribution. Physicans have used this technique to diagnosis tumors and breast cancers. Liquid crystal thermometer Liquid crystal thermometers are made of chemical compounds with the property of temperaturedependent reflection of light within a range of temperature (26 °C to 40 °C). They are often used for surface temperature measurements, especially when monitoring the temperature trend in an operation room. Liquid crystal thermometers are easy to use, inexpensive, and disposable, but less precise than other thermometers. In general, their resolution is around 0.2 °C to 0.5 °C.

Core temperature measurement Core temperature measurement can detect fever caused by pyrogens 致熱物 that are released from virus, bacteria, and degeneration of old tissues. High and lasting fever above 41 °C can cause damage to the brain and other internal organs that are vital to human life. Mercury thermometer The volume of mercury increases when the temperature rises, and the result of the change is read on a calibrated 校準 scale. The mercury thermometer is the most commonly used device to measure body temperature at home because of its low cost and ease of use. Its accuracy varies widely, depending on how well the expansion of mercury is calibrated. Infection 傳染 and cross-contamination 交叉感染 are its main disadvantages. Other disadvantages are potential damages to mucosal tissues 黏 膜 組 織 when temperatures are measured orally or rectally, and possible mercury intoxication 水銀中毒 due to the broken glass. Electronic thermometer Thermometers that use temperature-responsive sensors are called electronic thermometers. Because their sensor mass is smaller and they use electronic circuits, their response time is faster than that of mercury thermometers. Thermistors are composed of dissimilar materials with temperature-dependent electrical resistance. Dissimilar materials are mixed homogeneously 均 勻 地 and produced in the form of beads, rods, disks and other shapes. Thermistors are suitable for continuous temperature monitoring in patients during surgery and for infants’ skin temperature monitoring on the abdomen 腹部. Thermocouple is composed of dissimilar materials that are constructed as fused junctions of two materials (e.g., copper and constantan wires 康史登銅 線 , which is an alloy of nickel and copper). When the thermocouple is heated, a current flows from the measuring side (hot junction). It develops an electric potential, which can be measured when we place a voltmeter between open ends. Electronic thermometers use thermistors or thermocouples as sensors to measure temperature in the mouth, rectum or armpit. The major advantages of electronic thermometers are the small size of sensors, ease of use, and fast response time. Their disadvantages are similar to those of mercury thermometers, such as mucosal damage and cross-infection if not carefully handled. Infrared thermometer A basic infrared thermometer (IRT) design includes four parts: a waveguide 波導 (tube) to collect the energy emitted by the target, a sensor to convert the energy to an electric signal, an emissivity 發 射 率 adjustment to match IRT calibration to the emitting characteristics of the object being

measured, and a sensor temperature compensation circuit to ensure that temperature variations within the IRT are not transferred to the final output. The main advantages of the infrared thermometer are that it only contacts the ear canal and not the eardrum (i.e. less infection), ease of use, and fast response. It is suitable for the temperature measurement of young or high-risk patients. However, inaccurate measurements occur when patients have curved ear canals. 2.6 Regulation of Temperature Thermostat 恆溫器 Most of the heating systems require a form of automatic regulator which controls the amount of heat produced, depending on the desired temperature. Such a device is called a thermostat and the majority of these operated by including a bimetal strip 雙金屬片. A bimetal strip is made from two layers of different metals fixed rigidly together, one of which expands considerably more than the other on heating. Brass and an alloy called invar 鎳鐵合金 are often chosen since the brass in the strip expands twenty times as much as the invar for the same temperature arise. These different expanding properties can only be accommodated if the strip bends with the greater expanding metal on the outside of the curve where it can be longer.

Regulation of body temperature Control of a heater The required temperature is set by hand on the thermostat. If the setting is above the required temperature, the thermostat switches on the electrical supply to the heater. The room warms up and when its temperature reaches the one selected, the thermostat responds to this information, which is fed back to it from the air in the room, by switching off the electrical supply. If the room cools and its temperature falls below the setting, the thermostat again responds and switches on the heater. In this way, temperature control by feedback is achieved. Control of body temperature If the body temperature rises above normal, the blood and temperature sensors in the skin feed back information to a part of the brain which responds in two ways. First it increases the rate of flow of blood to skin capillaries, so increasing the rate at which the blood is cooled. Second, sweating starts, producing cooling of the body by evaporation. If the temperature falls below normal, feedback

reduces the blood flow to the skin and no sweating occurs, and shivering is activated to generate heat in the muscles. Feedback 反饋 and control In many kinds of system, all or part of the output (or information about it) is fed back to the input and affects the output. Feedback is positive if it acts in the same direction as the input and increases the output. Feedback is negative if it acts in the opposite direction to the input and reduces the output.

Checklist After studying this chapter you should be able to 2.1 Definition of Heat and temperature • recall the definition of heat • recall the definition of temperature 2.2 Temperature scale • recall the names of the three different temperature scale • definition of the three temperature scale in terms of the boiling point and melting point of water • conversion between temperature in different scales 2.3 Body heat and body temperature • recall the factors affect the body heat • recall the typical body temperatures against different conditions 2.4 Conduction, convection, radiation and evaporation • Compare the conduction, convection and radiation in terms of i) how heat is transferred, ii) where the method occurs, iii) properties of the method and iv) example. • Explain greenhouse effect • explain how cooling by evaporation and give examples of evaporation • recall the factors affect the rate of evaporation • definition of relative humidity and its consequences on human comfort 2.5 Temperature measurement • explain the difference between core temperature and surface temperature • compare the merits of different temperature measurement sites (oral cavity, rectum, and tympanic membrane) • recall the use of surface temperature and core temperature measurement • compare the advantages and disadvantages of liquid crystal thermometer, mercury thermometer, thermistor, thermocouple, and infrared thermometer. 2.6 Regulation of temperature • describe the construction of bimetal strip and its use in thermostat with the aid of diagram • sketch the characteristic temperature of a room control by thermostat • describe how body temperature is controlled • recall the definition of positive and negative feedback control