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Notes On Heat

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Published in: Physics
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The PPT will describe about the Heat.

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  1. HEAT Heat and Temperature In this lesson, we shall learn about the fundamental concepts related to heat and temperature. So, let us directly address the obvious questions: What is heat? What is temperature? How is heat different from temperature? At its most basic level, heat is a form of energy that cannot be seen, but can be felt. For example, in winter, when you stand near a burning fire, you experience the heat but you cannot see it. Heat always travels from a region of hotness to a region of coldness and it can travel through either conduction, convection or radiation. Heat energy flows from a body at higher temperature to a body at lower temperature. The transfer of heat from one body to another is because of the difference in their temperatures. There will be no transfer of heat energy if the two bodies are at the same temperature. As the body absorbs the heat, the temperature rises and when the body loses heat, its temperature falls. In case of hot and cold water also, if we mix the two, then the heat would flow from the hot water to the cold water. Suppose we bring in contact two bodies with temperatures 40 oc and 80 0 C, then heat will flow from the body having 80 oc of temperature to the body having 40 oc of temperature. This transfer of heat will continue until both the bodies attain equal temperature. More the heat more will be the warmth of a body. However, at times, there are conditions when the change in flow or transfer of heat cannot be recorded by any temperature rise or fall. In fact, the temperature remains constant. This happens because of the flow of Latent (hidden) Heat. Temperature is the measurement of the degree of hotness or coldness of a body. Units of Heat The Sl unit of heat is joule (J), whereas temperature is measured in either Kelvin (K), Fahrenheit (0 F) or Celsius (0 C). Heat can also be measured in calorie (cal) and kilocalorie (kcal). One calorie of heat is defined as the amount of heat required to raise the temperature of 1 gram of an object by | 0 C. Numerically, 1 cal = 4.186 joules 1 kcal = 4,186 joules (Since, 1 kcal = 1,000 calories) Factors on which Gain or Loss of Heat Depends The heat absorbed or released by a body depends on the following factors: 1. The mass of the body: The amount of heat gained or lost by a body is directly proportional to its mass. More the mass (m) of the body, more is the heat (Q) required and less the mass of the body, lesser is the heat required.
  2. Thus Q m 2. Temperature change: The quantity of heat absorbed or released by a body depends directly on the change in the temperature of the body. Higher the change in temperature (AT) more is the heat absorbed by the body. Thus, Q ocAT 3. Material of the body: The quantity of heat also depends on the material to which it is applied. Some materials absorb more heat than others do. Thus, the following relation can express the heat absorbed or released by a body Q oc m AT Effects of Heat on a Body Heat produces the following three effects on a body: 1. Change in temperature of the body— When a body is heated, its temperature rises because its molecules begin to move faster and the average kinetic energy of the molecules increases. The reverse happens when the body is cooled, that is, the average kinetic energy decreases and so the temperature decreases. Again, the change of temperature depends on: (a) The quantity of heat absorbed or released and (b) The material of the body. 2. Change in state of the body — Due to the application of heat on bodies, the inter-molecular spaces increase and so, there is a change in state from solid to liquid to gas. Cooling leads to reduction of the intermolecular spaces and therefore changes in state from gas to liquid to solid. Sometimes, there is a direct change of state from solid to gas and gas to solid. 3. Change in size of the body — When a body is heated, it undergoes expansion in size, and the phenomenon is known as Thermal Expansion. Cooling leads to contraction of a body. Comparison of Expansion in Solids, Liquids and Gases Thermal expansion of substances can be of three types: linear, surface area or superficial and volume expansion. Out of these, solids can undergo all three types of expansion while fluids only experience volumetric expansion. In linear expansion, the length of solids, particularly rods of different materials experience an increase in length. This increase in length is proportional to the product of the original length and the rise in temperature. Its mathematical expression is: AL = aLoAT where, AL = Increase in length of the solid
  3. Lo = Actual length of the solid AT = Increase in Temperature a = Coefficient of Linear Expansion The Coefficient of Linear Expansion (a) can be defined as the increase in length per unit length -1 per degree rise in temperature and its unit is (oc) Superficial Expansion of solids signifies an increase in the surface area of the solid due to heating. This is mainly evident in cases of flat metallic plates. Mathematically, Superficial Expansion can be expressed as: AA = ßAoAT where, AA = Increase in surface area of the solid Ao = Actual surface area of the solid AT = Increase in Temperature ß = Constant (Coefficient of Superficial Expansion) The Coefficient of Superficial Expansion is the ratio of increase in area to its original area for every degree increase in temperature. The unit of the Coefficient of Superficial Expansion is Cc) 1. The volume expansion is observed by all three states of matter and is experienced greatly in gases and liquids more than in solids. Its mathematical representation is as: AV = yVoAT where, AV = Increase in volume of the substance (solid, liquid or gas) Vo = Actual volume of the substance (solid, liquid or gas) AT = Increase in Temperature y = Constant (Coefficient of Volumetric Expansion) The Coefficient of Cubical Expansion, also known as the Coefficient of Volumetric Expansion means the ratio of the increase in volume to its original volume per degree rise in temperature. Once again, the unit is (OC)-I. Another definition of the Coefficient of Cubical or Volumetric Expansion is the increase in volume per unit original volume per Kelvin rise in temperature. The three coefficients can be interrelated as, Thermal expansion has numerous applications in daily life.
  4. 1. Blacksmiths use the principles of Superficial Thermal Expansion to fix iron rims to wooden wheels. The wooden wheels are generally larger in comparison to the rims. As soon as the iron rim cools down, it fixes tightly on the wheel. 2. Keeping in mind the Linear Thermal Expansion of iron, gaps are kept between the railway fishplates to allow for the linear expansion of the iron plates in summers as well as due to the heat generation because of friction between the iron rails and the iron wheels of trains. 3. Iron bridges have rollers in one end to allow for their expansion in summers. Water displays some unique characteristics in relation to thermal expansion and contraction. Water, at OOC contracts in volume when heated and this contraction in volume continues up to 40C. Thereafter, its volume increases steadily up to 1000C. This phenomenon is called the Anomalous Expansion of Water. Latent Heat At the beginning of the lesson, we learnt about the concept of change of state of bodies due to the absorption and release of heat. Closely related to that is the concept of Latent or Hidden heat that causes the change in shape at a given temperature. So, what is Latent Heat? The heat energy absorbed or released by a substance during a change in its physical state (phase) that occurs, without changing its temperature, is known as latent heat. The latent heat per unit mass is known as specific latent heat. Specific latent heat is denoted as L. If Q is the heat required to change the state of a substance of mass m at a constant temperature, then the specific latent heat of the substance is defined as: Therefore, L = Q/m The unit of measurement of specific latent heat is J/kg. There are two different types of latent heat: latent heat of fusion and latent heat of vapourisation. The latent heat associated with melting a solid or freezing a liquid is called the latent heat of fusion. The latent heat associated with vapourising a liquid or a solid or condensing a vapour is called the heat of vapourisation. Latent Heat of Fusion — The amount of heat required to change the state of unit mass of a solid to liquid at constant temperature at its melting point or vice versa, is known as latent heat capacity of fusion (L ). Specific latent heat of fusion is also defined as the heat energy required to melt 1 kg mass of a solid substance into a liquid. Latent heat of fusion of ice — The specific latent heat of ice is 334 J/g, which means that 336 joules of heat is required when 1 gram of ice turns into liquid. This is a very high value. This also means that 336 joules of heat energy must be withdrawn from water to convert it into ice. This is the only reason why water does not easily convert into ice. When ice changes into
  5. water, it requires energy to overcome the inter-atomic forces of attraction to convert into liquid. Consequences of High Latent Heat of Fusion of Ice — Due to high latent heat of fusion of ice, the lakes and ponds in cold countries do not quickly freeze. They freeze slowly keeping the marine life and the surroundings at moderate temperatures. The currents developed in oceans and seas carry the icebergs from one place to another due to high latent heat of fusion of water. Poultry and fish products are kept in ice than in water at the same temperature for storage. Latent Heat of Vapourisation — The amount of heat required to change the state of a unit mass of a liquid at constant temperature at its boiling point to the gaseous state or vice versa, is known as the latent heat capacity of vapourisation. Latent heat of vapourisation of water — The specific latent heat of vapourisation of water has a very high value of 2,260 J/g, which means that 2,260 joule of heat is required when 1 gram of water turns into steam. This also means that 2,260 joule of heat energy must be withdrawn from steam to convert into water again. When water converts into steam, it requires 2,260 J of energy to overcome the interatomic forces of attraction. Consequences of Latent Heat of Vapourisation of Water/Steam — In cold countries, the steam is run through the heat pipes to keep the house warm, as steam releases heat energy of 2,260 A lot of heat is required to evaporate water; this is the only reason that the water bodies do not evaporate easily. The thermal energy of steam is used to run a steam engine. Heat Capacity The heat capacity C of an object is the proportionality constant between the heat Q that the object absorbs or loses and the resulting temperature change AT of the object; that is, Q = C AT = C (Tf— Ti) where Ti and Tf are the initial and final temperatures of the object. Heat capacity C has the unit of energy per degree or energy per kelvin. The heat capacity C of, say, a marble slab used in a bun warmer might be 179 cal/0C, which we can also write as 179 cal/K or as 749 J/K. The word "capacity" in this context is misleading in that it suggests analogy with the capacity of a bucket to hold water. That analogy is false, and you should not think of the object as "containing" heat or being limited in its ability to absorb heat. Heat transfer can proceed without limit as long as the necessary temperature difference is maintained. The object may, of course, melt or vapourise during the process. Specific Heat Capacity Two objects made of the same material—say, marble—will have heat capacities proportional to their masses. It is therefore convenient to define a "heat capacity per unit mass" or specific
  6. heat c that refers not to an object but to a unit mass of the material of which the object is made. Therefore, the previous equation becomes, Q = cmAT = cm (Tf- Ti) Through experiment we would find that although the heat capacity of a particular marble slab might be 179 cal/C0 (or 749 J/K), the specific heat of marble itself (in that slab or in any other marble object) is 0.21 cal/g 9C- (or 880 J/kg 9K). In determining and then using the specific heat of any substance, we need to know the conditions under which energy is transferred as heat. For solids and liquids, we usually assume that the sample is under constant pressure (usually atmospheric) during the transfer. It is also conceivable that the sample is held at constant volume while the heat is absorbed. This means that thermal expansion of the sample is prevented by applying external pressure. For solids and liquids, this is very hard to arrange experimentally, but the effect can be calculated, and it turns out that the specific heats under constant pressure and constant volume for any solid or liquid differ usually by no more than a few percent. Gases, as you will see, have quite different values for their specific heats under constant-pressure conditions and under constant-volume conditions. Advantages of High Specific Heat of Water 1. 2. 3. 4. 5. Due to its high specific heat, 1 kilogram of water requires 4,200 J of energy to raise its temperature by | 0 C. For this reason, it acts as a coolant and can absorb large amount of heat. As water has high specific heat capacity, it can absorb large amount of heat and takes a longer time to cool down. Thus, it is used in water pads and hot bags for fomentation purposes. High specific heat of water keeps the soil moist and does not let it dry easily. It also leads to the formation of sea and land breeze. The specific heat of water is higher than land, so the land gets heated up faster in comparison to sea in the daytime. This sets up a breeze moving from the sea towards the land. Similarly, at night, land cools down faster than sea and this sets in the breeze from land to sea at night. In some cold countries, wine bottles and juices are put under water, so that they do not freeze quickly. This is because water has a high specific heat capacity. Principle of Calorimetry A calorimeter is a device that is used to determine the amount of heat evolved or released in a process or for measuring specific heat. Amount of heat is measured by using the principle of calorimetry. We already know that amount of heat energy absorbed or released by a body depends on its temperature, mass and the material of the body. The principle of calorimeter states that, when a hot body is placed in contact with a cold body, heat flows from hotter body to the colder body until they reach a common final temperature. The amount of heat lost by the hot body will be equal to the amount of heat gained by the cold body at thermal equilibrium.
  7. Principle of Calorimetry A calorimeter is a device that is used to determine the amount of heat evolved or released in a process or for measuring specific heat. Amount of heat is measured by using the principle of calorimetry. We already know that amount of heat energy absorbed or released by a body depends on its temperature, mass and the material of the body. The principle of calorimeter states that, when a hot body is placed in contact with a cold body, heat flows from hotter body to the colder body until they reach a common final temperature. The amount of heat lost by the hot body will be equal to the amount of heat gained by the cold body at thermal equilibrium. At thermal equilibrium, Heat lost by hot body = Heat gained by cold body Transfer of Heat Heat is transferred from one body to another through Conduction, Convection and Radiation. Conduction — This is the process of transfer of heat in solids. Heated molecules vibrate vigorously about their mean positions and transfer heat from one to the other until they reach an equilibrium when all the molecules are heated to the same degree. Thus, in transfer of heat through conduction, a material medium is required. In conduction, heat can be transferred from the hot region to the cold region in any direction. — In cases of fluids (liquids and gases), heat is transferred in the form of Convection convection, which requires a material medium. In convection, heat is transferred from a hot region to a cold region vertically. As substances gain heat, molecules expand and they become lighter, causing them to rise up. The colder molecules, being heavier, replace the lighter molecules, setting up a convectional current. This phenomenon is evident in nature in the Water Cycle. The same principle is also utilised in refrigerators. Radiation — This form of heat transfer does not require any material medium as heat travels in the form of electro-magnetic waves. Thus, in radiation, heat can be transferred through vacuum as well. The heat from the sun reaches us through radiation. Dark-coloured substances attract more radiant heat than light-coloured substances. Conductors and Insulators of Heat The materials, which allow heat to pass through them, are called good conductors of heat. Silver, aluminium, iron and copper are examples of good conductors of heat. Materials, which do not allow heat to pass easily through them, are called poor conductors of heat or insulators. Plastic, wood and cotton are examples of insulators. Water and air are also poor conductors of heat. Heat transfers very slowly through water and air.
  8. The specific heat capacity of good conductors is lower than that of the bad conductors of heat. Let us discuss now few applications of conductivity of heat, which are as follows: • Utensils are made up of metals, which are good conductors of heat. • The handles of the cooking utensils are made up of bad conductors, so that we can hold them when they are hot. • Mercury and alcohol are used as thermometric liquid in making thermometers, as they are good conductors of heat. • Fur and animal hair are bad conductors of heat. They trap air in between their fibres. Change in State of a Substance When heat energy is added to a substance, it results in many changes. One of the effects of heat is the change in the state of a substance. Some of the changes in the state have been discussed below. Fusion/Melting— It is a change of state from solid to liquid, due to addition of heat to a system. When a solid is heated, the atoms gain energy and start vibrating vigorously due to which their intact structure breaks and the change of state takes place. Upon heating, ice melts into water at 0 0 C. The process in which a solid changes into a liquid on heating at constant temperature is called melting. The temperature, at which a solid melts into a liquid by absorbing heat, is called the melting point of the solid. Freezing — This process is just the opposite of melting. When heat is withdrawn from a liquid, its atoms and molecules come closer and it converts into a solid. This process is called freezing. For example, water turns into ice at 0 0 C. The temperature, at which a solid turns into a liquid by withdrawing heat, is called freezing point. The freezing point or the melting point temperatures are the same. Vapourisation/Boiling — It is the change of state from liquid into gas by absorption of heat. As the temperature of the liquid is increased, the molecules gain kinetic energy and start moving farther apart. For example, water turns into steam at 100 0 C. This process is called boiling or vapourisation. The constant temperature, at which a liquid converts into a gas, is known as boiling point of that liquid. Boiling point of water is 100 0 C. Condensation — The change from gas to liquid by releasing heat, at a constant temperature, is called condensation. Condensation takes place when the atoms of a gas come in contact with a surface at a very low temperature. The atoms lose their energy and their arrangement changes. For example, the water droplets formed on the glass filled with cold water. Evaporation — The change from a liquid state into vapour state at any temperature, below boiling point upon heating is known as evaporation. For example, water vapours are always present in the atmosphere at temperatures which are very less than the boiling point. Sublimation — The process in which a solid substance directly converts to gaseous substance, is known as sublimation. For example, camphor, naphthalene, etc., are some substances which show sublimation. Solidification — It is the process in which the gas condenses directly to form solid. For examples, carbon dioxide converts directly into solid dry ice. Differences between Boiling or Vapourisation and Evaporation
  9. Evaporation is the change of state of a substance from the liquid stage to the gaseous stage at any temperature below the boiling point of that substance. So, this process does not require a heat source. There are certain factors that affect the process of evaporation: 1. 2. 3. 4. 5. 6. The larger is the exposed surface area of the liquid the greater is its evaporation. Clothes are spread out for drying to provide a larger surface area for evaporation of water from them. The rate of evaporation is also influenced by the temperature of the surroundings. The higher the temperature, the greater is the rate of evaporation. Clothes dry faster on hotter days than on colder days because of the heat in the surrounding atmosphere. Temperature of the liquid also plays in important part in the rate of its evaporation. Higher the initial temperature, greater is the rate of evaporation. The sweat in our bodies evaporates because of the already high temperature of the body and creates a soothing effect. If the water vapour content in the surrounding atmosphere is greater, then the rate of evaporation is lesser. The rate of evaporation undergoes significant increase in windy conditions because the strong winds blow away the water vapour just above the liquid's surface. There are some liquids that evaporate much faster in comparison with some other liquids. For example, petrol and kerosene evaporate faster than water. Boiling or Vapourisation is the process of change of state of a substance from the liquid state to the gaseous state on the application of a heat source. The Boiling Point can be directly influenced by the Atmospheric Pressure and the presence of impurities. A higher pressure significantly increases boiling point while the presence of impurities also increases the boiling point of liquids. The differences between evaporation and boiling or vapourisation can be easier to understand from the Table given below: Factors Pace of the Process Region of action Temperature Factors Affecting it Whether there is bubble formation or not? Boiling/Vapourisation Fast Throughout the volume of the liquid Boiling Point. Also, the temperature of the liquid substance remains constant throughout the process. Atmospheric Pressure and presence of impurities Yes Evaporation Slow Only at the surface of the liquid All temperatures below the Boiling point. The temperature of the liquid substance decreases significantly. Area of the exposed surface, wind conditions, temperature of the liquid and of the surroundings, amount of water vapour in the atmosphere and nature of the liquid No
  10. A heat source is required for the Energy source purpose Factors Effecting Rate of Evaporation The source of energy is obtained from the vicinity There are certain factors which affect the rate of evaporation. These are: 1. 2. 3. 4. 5. 6. Temperature of liquid: If the temperature of a liquid rises, the rate of evaporation also increases. Nature of liquid: Some materials evaporate at a faster rate such as petrol, diesel, alcohol, etc. Surface area of liquid: If the surface area of a liquid is more, then the evaporation will take place at a faster rate. Air above the liquid: If the layer of air, which is just above the liquid, is flowing at a fast rate, then the evaporation will take place at a faster rate. Surrounding temperature: If the temperature of the surrounding area is high, then the rate of evaporation will also be high. Dryness of air: If the surrounding air is more dry in nature, then the evaporation will take place at a faster rate. Applications of Evaporation 1. When a person is suffering from a high fever, then cold wet handkerchief is put on the forehead, so that the latent heat gets transferred in the handkerchief and the temperature of the body comes down. 2. To cool down hot liquids, we often put in a saucer or pans with wide surface areas, so that the liquid cools down faster. 3. Evaporation of sweat helps in maintaining the body temperature. The sweat, in order to evaporate, absorbs the heat from the human body and then uses it for evaporation, thereby, bringing down the body temperature. Temperature Now, we come to the discussion of temperature. As stated earlier, temperature is the degree of measurement of the hotness or coldness of a substance. This measurement is done in three scales: the Kelvin scale (K), the Celsius or Centigrade scale (oc) and the Fahrenheit scale (OF). Temperature is measured with an instrument called the thermometer. Kelvin scale — William Thompson created the Kelvin system, which is now in common use in physics, in the 19th century—he later became Lord Kelvin. The Kelvin system has become so central to physics that the Fahrenheit and Celsius systems are defined in terms of the Kelvin system—a system based on the concept of absolute zero. Temperature is really a measure of molecular movement — how fast and how much the molecules of whatever object you're measuring are moving. The molecules move more and more slowly as the temperature lowers. At absolute zero, the molecules stop, which means
  11. that you can't cool them any more. No refrigeration system in the world — or in the entire — can go any lower. So the absolute zero temperature is the lowest attainable universe temperature in the universe. The Kelvin system is not measured in degrees but in Kelvins. A temperature of 1000 in the Celsius system is 1000C, but a temperature of 100 is 100 kelvins in the Kelvin scale. This system has become so widely adopted that the official meters-kilograms-seconds (MKS) unit of temperature is the kelvin (in practice, you see oc used more often in introductory physics). Each kelvin is the same size as a Celsius degree, which makes converting between Celsius degrees and kelvins easy. On the Celsius scale, absolute zero is —273.150C. This temperature corresponds to 0 kelvins, which you also write as 0K (not, please note, 00K). So, to convert between the Celsius and Kelvin scales, all you have to use is the following formula: K -C + 273.15 C = K-273.15 And to convert from Kelvins to Fahrenheit, you can use this formula: F = 273.15) + 32 = (9/5)K- 459.67 The Celsius and Fahrenheit Scales So far, we have discussed only the Kelvin scale, used in basic scientific work. In nearly all countries of the world, the Celsius scale (formerly called the centigrade scale) is the scale of choice for popular and commercial use and much scientific use. Celsius temperatures are measured in degrees, and the Celsius degree has the same size as the Kelvin. However, the zero of the Celsius scale is shifted to a more convenient value than absolute zero. If Tc represents a Celsius temperature and T a Kelvin temperature, then 273.15 oc In expressing temperatures on the Celsius scale, the degree symbol is commonly used. Thus, we write 20.00 oc for a Celsius reading but 293.15 K for a Kelvin reading. The Fahrenheit scale, used in the United States, employs a smaller degree than the Celsius scale and a different zero of temperature. You can easily verify both these differences by examining an ordinary room thermometer on which both scales are marked. The relation between the Celsius and Fahrenheit scales is TF = (9/5)Tc + 320 where TF is Fahrenheit temperature. Converting between these two scales can be done easily by remembering a few corresponding points, such as the freezing and boiling points of water. The figure given below compares the Kelvin, Celsius, and Fahrenheit scales.
  12. Trip E point of omec mr.eÉ The Kelvin, Celsius and Fahrenheit temperature scales compared. -m.S4C_-S9BPF5 We use the letters C and F to distinguish measurements and degrees on the two scales. Thus, OOC = 32 OF means that 00 on the Celsius scale measures the same temperature as 320 on the Fahrenheit scale, whereas means that a temperature difference of 5 Celsius degrees (note the degree symbol appears after C) is equivalent to a temperature difference of 9 Fahrenheit degrees. The Triple Point of Water To set up a temperature scale, we pick some reproducible thermal phenomenon and, quite arbitrarily, assign a certain Kelvin temperature to its environment; that is, we select a standard fixed point and give it a standard fixed-point temperature. We could, for example, select the freezing point or the boiling point of water but, for technical reasons, we select instead the triple point of water. Liquid water, solid ice, and water vapour (gaseous water) can coexist, in thermal equilibrium, at only one set of values of pressure and temperature. The figure below shows a triple-point cell, in which this so-called triple point of water can be achieved in the laboratory. By international agreement, the triple point of water has been assigned a value of 273.16 K as the standard fixed-point temperature for the calibration of thermometers; that is, T3 = 273.16 K (triple-point temperature), in which the subscript 3 means 'triple point'. This agreement also sets the size of the Kelvin as 1/273.16 of the difference between the triple-point temperature of water and absolute zero.
  13. GU &.emotneer bulb A triple-point cell, in which solid ice, liquid water, and water vapour coexist in thermal equilibrium. By international agreement, the temperature of this mixture has been defined to be 273.16 K. The bulb of a constant-volume gas thermometer is shown inserted into the well of the cell.

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