Heat and Temperature Lecture Notes (PDF)

COLLEGE OF MEDICINE AND ALLIED HEALTH SCIENCES-COMAHS UNIVERSITY OF SIERRA LEONE FACULTY OF BASIC MEDICAL SCIENCES DEPARTMENT OF PHYSICS PREMED/PRE-PHARMACY PROGRAM  PHYSICS LECTURE NOTES FOR PRE-MED 1 & PRE-PHARMACY  TEMPERATURE AND HEAT Heat is something familiar t...

COLLEGE OF MEDICINE AND ALLIED HEALTH SCIENCES-COMAHS UNIVERSITY OF SIERRA LEONE FACULTY OF BASIC MEDICAL SCIENCES DEPARTMENT OF PHYSICS PREMED/PRE-PHARMACY PROGRAM  PHYSICS LECTURE NOTES FOR PRE-MED 1 & PRE-PHARMACY  TEMPERATURE AND HEAT Heat is something familiar to all of us. We can feel heat entering our bodies from the sun or from hot coffee or tea. We can also feel heat leaving our bodies as we feel the chill of night or the cooling effect of sweat after exercise. What is heat? How do we define it and how is it related to temperature? What are the effects of heat and how does it flow from place to place? However, the two  Heat is defined as the transfer of energy across the boundary of a system due to a temperature difference between the system and its surroundings.  There are many common phrases that use the word “heat” incorrectly. Heat, internal energy, and temperature are all different quantities  Temperature The concept of temperature has evolved from the common concepts of hot and cold. Our senses provide us with a qualitative indication of temperature. Temperature can be thought of as the property that determines whether an object is in thermal equilibrium with other objects. Two objects in thermal equilibrium with each other are at the same temperature.  Thermometers and Temperature Scales A thermometer is a device that is used to measure the temperature of a system. Thermometers are based on the principle that some physical property of a system changes as the system’s temperature changes.  Types of thermometers Thermometer Thermometric Range/℃ Physical property substance Liquid-in-glass Mercury or alcohol −39 𝑡𝑜 500 Change in length of liquid with temperature Constant-volume gas Gas −270 𝑡𝑜 1500 Change of gas pressure at constant volume with temperature Platinum resistance Resistance wire −200 𝑡𝑜 1200 Change in electrical resistance with temperature thermocouple Two dissimilar metals −250 𝑡𝑜 1500 Change in electric potential or current between two metal junctions at different temperatures  Fixed points Some property of matter is selected whose value varies with temperature Two standard degrees are chosen, called fixed points and numbers are assigned to them. Lower fixed point- the temperature of pure ice in equilibrium with air- saturated water at standard atmospheric pressure and is designated as 0℃ Upper fixed point- the temperature at which steam and pure boiling water are in equilibrium at standard atmospheric pressure and is taken as 100℃  Fixed points The difference between the upper fixed point and the lower fixed point gives the fundamental interval There are three main scales in current use: the Celsius scale, the Fahrenheit scale and the Absolute zero (Kelvin ) scale The values 𝑋100 and 𝑋0 in the figure below are represent the steam and ice point respectively. If 𝑋𝜃 is the value of the property at some temperature 𝜃 which we wish to know the value of 𝜃 in ℃ is given by the equation.  𝜃 𝑋𝜃 − 𝑋0 = 100 𝑋100 − 𝑋0 This eqn. defines the temperature 𝜃 in ℃.  Relationship Between Scales The relationships between the three common temperature scales are shown in equations below. Temperatures on these scales can be converted using the equations. 9 Celsius to Fahrenheit 𝑇𝐹 = (𝑇 +32) Celsius to Kelvin 𝑇𝐾 = 𝑇𝐶 + 273 5 𝐶 5 Fahrenheit to Celsius 𝑇𝐶 = 𝑇𝐹 − 32 Kelvin to Celsius 𝑇𝐶 = 𝑇𝐾 − 273 9 Example 1 Example 2  Liquid-in-glass Thermometer A common type of thermometer is a liquid-in-glass. The material in the capillary tube expands as it is heated. The liquid is usually mercury or alcohol  Comparison between Mercury and Alcohol mercury alcohol Has higher conductivity thus Has low conductivity thus responds to changes in responds slowly to temperature temperature quickly It is plain, hence cannot be It is opaque, hence can be clearly clearly seen seen It does wet glass due to its It does not wet glass concave meniscus It does not vaporize easily It can vaporize easily It boils at 357 ℃ It boils at 78 ℃  Clinical Thermometers There are several kinds of thermometers that can be considered as clinical thermometer such as Liquid-in-glass thermometer, digital thermometer etc. For example, Mercury-in-glass thermometer is used in hospitals for measuring the temperature of human body. The normal temperature of the human body is 37°C but it may rise to about 41°C in cases of high fever. The temperature range of the clinical thermometers is therefore between 35°C to 43°C. The thermometer has a small bulb made of thin glass. The bulb has a narrow bore through which a fine mercury thread moves. The fine bore of the tube makes it possible for small temperature changes to cause large changes in the length of the mercury column, making the thermometer very sensitive to temperature changes. The most prominent feature of the thermometer is the kink or constriction of bore near the bulb. The kink allows the expanding mercury to flow past it when the thermometer is placed in contact with the human body. The mercury threads however remains in position in the upper part of the capillary tube when the thermometer is taken away from the patient. The body temperature can thus be read at leisure. Mercury can be returned to the bulb past the kink by vigorously shaking the thermometer. Some clinical thermometer has a thick stem which acts as a cylindrical lens, magnifying the thread of mercury for easy viewing. It is advisable to sterilize the clinical thermometer in boiling water at normal pressure because the thermometer has a small temperature range of which does not extend up to 100°C. at such a high temperature, the glass will break due to excessive expansion of the mercury.  Constant-volume gas thermometer If the volume of a fixed mass of a gas is kept constant, its pressure changes appreciably when the temperature changes. 𝜃 𝑃𝜃 −𝑃0 Hence it is defined by the eqn. = 100 𝑃100 −𝑃0 Where 𝑃0 , 𝑃𝜃 , and 𝑃100 are the pressures at the ice point, the required temperature, and steam point.  Resistance thermometer The electrical resistance of a pure platinum wire increases with temperature and since resistance can be found very accurately it is a good property on which to base a temperature scale. 𝜃 𝑅𝜃 −𝑅0 It is given by the eqn. = 100 𝑅100 −𝑅0 Where 𝑅0 and 𝑅100 are the resistances at ice and steam points respectively and 𝑅𝜃 is the resistance at the required temperature.  Thermocouple thermometer Such thermometers are used to measured very high temperatures. When two different metals (e.g. copper and constantan) are joined at the end and one end (the hot junction heated) is heated, while the other end (the cold junction) is kept constant in melting ice, an electric current flows along the metals. This is the thermoelectric effect and the set up is called a thermocouple. Solved examples  Practice problems 1. While traveling outside Sierra Leone, you feel sick. A companion gets you a thermometer, which says your temperature is 39. What scale is that on? What is your Fahrenheit temperature? Should you seek medical help? 2. What is the change in length of a 3 cm long column of mercury if its temperature changes from 37 °C to 40 °C , assuming the mercury is constrained to a cylinder but unconstrained in length? Your answer will show why thermometers contain bulbs at the bottom instead of simple columns of liquid.  Practice problems 3. A person taking a reading of the temperature in a freezer in Celsius makes two mistakes: first omitting the negative sign and then thinking the temperature is Fahrenheit. That is, the person reads – x °C as x °F. Oddly enough, the result is the correct Fahrenheit temperature. What is the original Celsius reading? Round your answer to three significant figures.