Medical Physics - Temperature and temperature scale.pdf

Medical Physics Medical Physics Temperature & Temperature scale Temperature & Temperature scale Contents : Temperature and temperature scale 3 Thermometers and Temperature Scales 4 Electric thermometers 5 Liquid-in-glass Thermometers 10 Maximum thermometers 13 Heat Transfer 15 Heat Radiation 17 Cool...

Medical Physics Medical Physics Temperature & Temperature scale Temperature & Temperature scale Contents : Temperature and temperature scale 3 Thermometers and Temperature Scales 4 Electric thermometers 5 Liquid-in-glass Thermometers 10 Maximum thermometers 13 Heat Transfer 15 Heat Radiation 17 Cooling of the Human Body 26 Thermoregulation 33 Temperature scales 43 Temperature & Temperature scale Temperature and temperature scale: The SI unit of heat energy transferred is expressed in joule (J) while SI unit of temperature is kelvin (K), and ° C is a commonly used unit of temperature. When an object is heated, many changes may take place. Its temperature may rise, it may expand or change state. Temperature & Temperature scale Thermometers and Temperature Scales: There are many types of thermometers. The major ones employ the characteristics of expansion and contraction of substance according to the temperature, employ the valuable of electrical characteristics (electrical resistance) of substance according to temperature, or employ characteristics between temperature and heat radiation energy emitted from surface of substance. Temperature & Temperature scale Electric thermometers: Platinum resistance thermometer employs platinum characteristics which changes resistance according to the temperature. It allows us to obtain temperature by measuring electrical resistance. High purity platinum is used since contaminants greatly affect resistance. Temperature & Temperature scale Sensor for meteorological observation is made with thin sheet of mica or porcelain wrapped with platinum wire, and it is placed in stainless protective tube which has excellent thermal conductivity and corrosion resistant then made it to complete water proof. Resistance changes of platinum according to temperature is converted to electrical signals (current or voltage signal) by converter. Then the signal is sent to indicator or recorder and displayed or processed as atmospheric temperature. Temperature & Temperature scale Diagram of sensor and connection of platinum resistant thermometer Temperature & Temperature scale Example of the relation between resistance and temperature of platinum resistance thermometer Temperature & Temperature scale The advantage of resistance thermometer is: 1. It is very sensitive for measuring temperature, where it is easy to measure temperature changes of (0.01°C), for this it used quite often in medicine. 2. It has small mass, for this it has little effect on the temperature of the surrounding tissues and respond rapidly to temperature changes. 3. The electric measurement which allows remote measurement and automated. Temperature & Temperature scale Liquid-in-glass Thermometers: A liquid-in-glass thermometer measures temperature based on the thermal expansion of mercury or alcohol in a glass container. It is the most common way to measure temperature. The principle is that an increase in the temperature of different materials usually causes them to expand different amounts, for this a temperature increase causes the alcohol or mercury to expand more than the glass and thus produces an increase in the level of the liquid. Temperature & Temperature scale The boiling point of mercury is 356.72 °C, and its melting point is -38.86 °C. The boiling point of methyl alcohol is 64.65 °C, and its melting point is -97.78 °C. Because mercury has low thermal capacity, high heat conductivity, inertness in relation to a glass capillary tube and a high boiling point, it is an ideal thermometric liquid except for its relatively high melting point. Temperature & Temperature scale Accordingly, mercury thermometers are used for ordinary meteorological observations, and alcohol thermometers are used for those involving temperatures below the melting point of mercury. Temperature & Temperature scale Maximum thermometers: A maximum thermometer is a mercury thermometer used to measure the maximum temperature within a certain period. It has a narrow part in the capillary tube where mercury passage is constricted between the bulb and the starting point of the scale (Figure). As the air temperature rises, the mercury exits the bulb and passes through the constriction. When the air temperature falls, the mercury column breaks at this point. Constriction point of a maximum thermometer Temperature & Temperature scale Thus, the mercury in the capillary tube cannot return to the bulb, and remains in the column indicating the maximum temperature. Observation of maximum temperature is carried out once or twice a day. After measurement, the thermometer is held at the head and the mercury in the capillary tube is shaken back into the bulb to reset its indication to the current air temperature. Heat Transfer Temperature & Temperature scale Heat Transfer: Heat transfer is a transfer of the energy from a high temperature object to a lower temperature one. Heat transfer changes the internal energy of both systems involved according to the first law of thermodynamics. Heat can be transferred by three ways: conduction, convection and radiation. Temperature & Temperature scale Heat Radiation: Energy is transferred by electromagnetic radiation. All of the earth's energy is transferred from the Sun by radiation. Our bodies radiate electromagnetic waves in a part of the spectrum that we can't see called the infra-red. However, there are some cameras that can actually see this radiation. Temperature & Temperature scale The color and texture of different surfaces determines how well they absorb the radiation. 1. Black objects absorb more radiation than white objects. 2. Matt and rough surfaces absorb more than shiny and smooth surfaces. If you are ever in the snow, take a black and a white piece of cardboard, both the same size. Lay them down on the snow side by side. Temperature & Temperature scale Over time you will notice that the black cardboard sinks deeper into the snow because it absorbs more heat from the sun and therefore melts more snow underneath it. The relationship governing radiation from hot objects is called the Stefan-Boltzmann Law: Temperature & Temperature scale Where P is the net radiated power measured in Watt. e is the emissivity (= 1 for ideal radiator). 𝐴 is the radiation area in 𝑚2. 𝑇 is the temperature of the radiator in Kelvin. 𝑇𝑆 is the temperature of the surroundings in Kelvin. 𝜎 = 5.67 × 10−8𝑊𝑎𝑡𝑡/(𝑚2𝐾4) is a constant called Stefan-Boltzmann constant. Temperature & Temperature scale Temperature & Temperature scale Example: A student tries to decide what to wear is staying in a room that is at 20°𝐶. If the skin temperature is 37°𝐶, how much heat is lost from the body in 10 minutes? Assume that the emissivity of the body is 0.9 and the surface area of the student is 1.5 𝑚2. Using the Stefan-Boltzmann's law: Temperature & Temperature scale Notice how the units cancel up with each other. The total energy lost during 10 𝑚𝑖𝑛 is Generally, every object at non-absolute-zero temperature radiates radiation at all wavelengths. But the amount of energy radiated at each wavelength depends on the body temperature. A body at temperature 800℃ will look red, but at temperature 3000℃ would look White. Temperature & Temperature scale The wavelength at which the radiation is most intense is given by the Wien Displacement law Where B is a constant and equals to 2.898 × 10−3 𝑚k. Temperature & Temperature scale Example: Sun surface temp is 6000𝐾. What is wavelength of maximum radiation? Solution: Cooling of the Human Body Temperature & Temperature scale Cooling of the Human Body: Hypothermia: Abnormally low body temperature. The condition needs treatment at body temperatures of 35℃ (95 ℉) or below. And hypothermia becomes life threatening below body temperatures of 32.2 ℃ (90 ℉). Temperature & Temperature scale Stage 1 Body temperature drops by 1– 2 °𝐶 below normal temperature (down to 35– 37 °𝐶). Mild to strong shivering occurs. The person is unable to perform complex tasks with the hands; the hands become numb. Breathing becomes quick and shallow. A person may feel sick to their stomach, and very tired. Often, a person will experience a warm sensation, as if they have recovered, but they are in fact heading into Stage 2. Temperature & Temperature scale Another test to see if the person is entering Stage 2 is if they are unable to touch their thumb with their little finger; this is the first stage of muscles not working. They might start to have trouble seeing. Temperature & Temperature scale Stage 2 Body temperature drops by 2–4 °𝐶 below normal temperature (33– 35 °𝐶). Shivering becomes more violent. Muscle miscoordination becomes apparent. Movements are slow, accompanied by a mild confusion, although the person may appear alert. Surface blood vessels contract further as the body focuses its remaining resources on keeping the vital organs warm. The person becomes pale. Lips, ears, fingers and toes may become blue. Temperature & Temperature scale Stage 3 Body temperature drops below approximately 32°𝐶. Shivering usually stops. Difficulty speaking, sluggish thinking, and amnesia (memory loss) start to appear; inability to use hands and stumbling is also usually present. Cellular metabolic processes shut down. Below 30 °𝐶, the exposed skin becomes blue and puffy, muscle coordination becomes very poor, walking becomes almost impossible, and the person exhibits incoherent/irrational behavior including terminal burrowing or even a stupor. Temperature & Temperature scale Stage 3 Pulse and respiration rates decrease significantly, but fast heart rates can occur. Major organs fail. Clinical death occurs. Because of decreased cellular activity in stage 3 hypothermia, the body will actually take longer to undergo brain death. Thermoregulation Temperature & Temperature scale Thermoregulation: Thermoregulation is a process that allows your body to maintain its core internal temperature. All thermoregulation mechanisms are designed to return your body to homeostasis. This is a state of equilibrium. Temperature & Temperature scale Temperature & Temperature scale A healthy internal body temperature falls within a narrow window. The average person has a baseline temperature between 98°F (37°C) and 100°F (37.8°C). Your body has some flexibility with temperature. However, if you get to the extremes of body temperature, it can affect your body’s ability to function. For example, if your body temperature falls to 95°F (35°C) or lower, you have “hypothermia.” This condition can potentially lead to cardiac arrest or brain damage. If your body temperature rises as high as 107.6°F (42 °C), you can suffer brain damage. Many factors can affect your body’s temperature, such as spending time in cold or hot weather conditions. Temperature & Temperature scale Factors that can raise your internal temperature include: 1. Fever. 2. Exercise. 3. Digestion. Factors that can lower your internal temperature include: 1. Drug use. 2. Alcohol use. 3. Metabolic conditions, such as an under-functioning thyroid gland. Temperature & Temperature scale Thermoregulation in humans: : There are four ways of heat loss: evaporation, convection, conduction and radiation. 1. Evaporation – Body heat turns sweat into vapor. 2. Convection – Heat loss by air or water moving across the skin surface. 3. Conduction – Direct contact with an object. 4. Radiation – The body radiates (like a fire - you can feel heat without being inside the fire). Temperature & Temperature scale Your hypothalamus is a section of your brain that controls thermoregulation. When it senses your internal temperature becoming too low or high, it sends signals to your muscles, organs, glands, and nervous system. They respond in a variety of ways to help return your temperature to normal. Temperature & Temperature scale How does thermoregulation work? When your internal temperature changes, sensors in your central nervous system (CNS) send messages to your hypothalamus. In response, it sends signals to various organs and systems in your body. They respond with a variety of mechanisms. Temperature & Temperature scale If your body needs to cool down, these mechanisms include: 1. Sweating: Your sweat glands release sweat, which cools your skin as it evaporates. This helps lower your internal temperature. 2. Vasodilatation: The blood vessels under your skin get wider. This increases blood flow to your skin where it is cooler — away from your warm inner body. This lets your body release heat through heat radiation. Temperature & Temperature scale If your body needs to warm up, these mechanisms include: 1. Vasoconstriction: The blood vessels under your skin become narrower. This decreases blood flow to your skin, retaining heat near the warm inner body. 2. Thermogenesis: Your body’s muscles, organs, and brain produce heat in a variety of ways. For example, muscles can produce heat by shivering. 3. Hormonal thermogenesis: Your thyroid gland releases hormones to increase your metabolism. This increases the energy your body creates and the amount of heat it produces. Temperature scales Temperature & Temperature scale Temperature scales: Thermometers measure temperature according to well-defined scales of measurement. The three most common temperature scales are Fahrenheit, Celsius, and Kelvin. Temperature scales are created by identifying two reproducible temperatures. The freezing and boiling temperatures of water at standard atmospheric pressure are commonly used. Temperature & Temperature scale Temperature & Temperature scale On the Celsius scale, the freezing point of water is 0°C and the boiling point is 100°C. The unit of temperature on this scale is the degree Celsius (°C). The Fahrenheit scale has the freezing point of water at 32°F and the boiling point at 212°F. Its unit is the degree Fahrenheit (°F). You can see that 100 Celsius degrees span the same range as 180 Fahrenheit degrees. Thus, a temperature difference of one degree on the Celsius scale is 1.8 times as large as a difference of one degree on the Fahrenheit scale, or ΔTF= 9/5 ΔTC. Temperature & Temperature scale The definition of temperature in terms of molecular motion suggests that there should be a lowest possible temperature, where the average kinetic energy of molecules is zero (or the minimum allowed by quantum mechanics). Experiments confirm the existence of such a temperature, called absolute zero. An absolute temperature scale is one whose zero point is absolute zero. Such scales are convenient in science because several physical quantities, such as the volume of an ideal gas, are directly related to absolute temperature. Temperature & Temperature scale The Kelvin scale is the absolute temperature scale that is commonly used in science. The SI temperature unit is the kelvin, which is abbreviated K (not accompanied by a degree sign). Thus, 0 K is absolute zero. The freezing and boiling points of water are 273.15 K and 373.15 K, respectively. Therefore, temperature differences are the same in units of kelvins and degrees Celsius, or ΔTC=ΔTK. Temperature & Temperature scale The relationships between the three common temperature scales are shown in the figure below. Temperatures on these scales can be converted using the equations in the table below. Temperature & Temperature scale Temperature & Temperature scale Temperature & Temperature scale The Kelvin scale is part of the SI system of units, so its actual definition is more complicated than the one given above. First, it is not defined in terms of the freezing and boiling points of water, but in terms of the triple point. The triple point is the unique combination of temperature and pressure at which ice, liquid water, and water vapor can coexist stably. The triple-point temperature is defined as 273.16 K.