Transfer Of Thermal Energy
| The Concept of Conduction of Heat |
| Explain the concept of conduction of Heat |
| is the transfer of heat ener gy through solids, for example, metals. Generally solid |
| substances contain particles which are close together. Each particle vibrates at one position but |
| cannot move to another position. |
| Solid materials differ greatly in their ability to conduct HEAT. |
| Good and Bad Conductors of Heat |
| Identify good and bad Conductors of Heat |
| Solid materials differ greatly in their ability to conduct HEAT. |
| These are the substances which allows the passage of heat ener gy easily example all metals. |
| Metals contain tiny particles called electrons (particles that carry electricity through metals) |
| which are free to move inside the metal and carry energy from hotter places to colder places. |
| These are materials which does not allow the passage of heat and electricity e.g Non – metals, |
| GOOD CONDUCTOR BAD CONDUCTOR |
| How to Minimize Heat Losses due to Conduction |
| Explain how to minimise Heat losses due to Conduction |
| There are some simple ways to reduce heat loss, including fitting carpets, curtains and draught |
| Heat loss through windows can be reduced using double glazing. The gap between the two panes |
| of glass is filled with air. Heat loss through conduction is reduced, as air is a poor conductor of |
| heat. Heat transfer by convection currents is also reduced by making the gap is very narrow. |
| Heat loss through walls can be reduced using cavity wall insulation. This involves blowing |
| insulating an material into the gap between the brick and the inside wall, which reduces the heat |
| loss by conduction. The material also prevents air circulating inside the cavity, therefore |
| reducing heat loss by convection. |
| Knowledge of Conduction in Daily Life |
| Apply knowledge of conduction in daily life |
| The difference in conductivity of various materials can be demonstrated using Edser s apparatus |
| The apparatus consists of copper can with identical rods of aluminum, copper, lead and iron |
| fixed to the bottom of the can. |
| The can is supported by a metal ring which is clamped to a retort stand. When hot water is |
| poured inside the copper can, heat will be passed along the rods by conduction. |
| After some time, it will be observed that wax coatedon the rods will melt and move down the |
| rods. Note how far along the rods the wax has melted when the apparatus reaches a steady state. |
| This indicates that the materials from which the rods are made have different thermal |
| conductivities. Of the four metal rods, the copper rod is observed to conduct heat more quickly |
| Conduction of Heat Energy through Liquids |
| All liquids expect mercury and gases are poor conductors of heat. |
| Gases are far worse conductors of heat than liquids. |
| Fluids are bad conductors of heat. They transfer heat by means of convection. |
| The Concept of Convection of Heat |
| Explain the concept of convection of heat |
| Convection is the transfer of Heat through the fluids (Liquids or Gases) |
| Convection in Fluids in Terms of Kinetic Theory of Matter |
| Explain convection in fluids in terms of kinetic theory of matter |
| Convection currents are the curr ents of a liquid that move from the bottom to the top of the liquid |
| container when the liquid is heated. |
| The heated liquid expands and becomes less denser and so can float upwards and replaced by |
| colder denser liquids that sinks. |
| Convection air current occurs due to the unequal Heating of the Earth s atmosphere by the sum. |
| (Thus current called strong convection current). |
| How to Minimize Heat Losses due to Convection in Daily Life |
| Explain how to minimise heat losses due to convection to daily life |
| When you understand the effects of cold water on the body, and how the body responds, you are |
| far more prepared to make life-saving decisions, either for yourself or in a rescue situation. |
| It s actually quite simple: the body attempts to maintain a constant core temperature |
| ) through a balance of heat loss and heat gain. Body heat is normally gained through |
| activities such as exercise and shivering, and also with the application of external heat sources |
| Convection is the process of air or water flowing by the skin and carr ying away body heat. It s |
| convective heat loss that you try to prevent by staying as still as possible in the water. Staying |
| still, the boundary layer of water next to the skin is heated by the body and remains undisturbed. |
| If you move around in the water, you disrupt that boundary layer of warmer water, and that |
| Once a body has been in cold water for an extended period of time, most of the skin is cool with |
| little blood flow. However, there are critical areas that are lighter (warmer) than the surrounding |
| tissue. This is because blood is flowing through major blood vessels, which are near the skin |
| surface. These areas in the neck, armpits and groin are areas of high heat transfer. That means |
| that these areas have high heat loss in the cold but allow heat gain in the heat. This is why, in a |
| rescue scenario, the most effective rewarming often consists ofapplying external heat directly to |
| the armpits as well as the chest. |
| As a final note, it s important to realize that the activity of swimming (which is naturally thought |
| of as producing a heat GAIN), in cold water conditions will result in increasing the blood flow to |
| blood vessels close to the skin, and because of conduction and convection, it can actually |
| increase the rate of heat LOSS and expedite the onset of hypothermia. |
| Knowledge of Convection to Daily Life |
| Apply knowledge of convection to daily life |
| Domestic hot water system |
| Convection currents are used to circulate hot water from a boiler in a domestic hot water |
| system. The system consists of aboiler B, a hot water stor age tank, H and cold water supply tank |
| (cistern) C all connected by pipes. |
| When water is heated (electrically or by fire) at the button of the boiler, it expands and |
| become less dense, and so rises to the top. |
| The hot water in the boiler passes through the outlets at the top of the boiler into the |
| upper part of the hot water storage tank. |
| The lower portion of the storage tank is filled with cold water from the cistern, which is |
| high enough to drive the hot water out when the hot water tap T is open. |
| The cistern is fitted with a ball-cock which maintains the level of water in the cistern by |
| allowing water in when the level falls. |
| Explain the concept of radiation |
| Radiation is transfer of heat energy fr om one point to another without the requirement of any |
| The stars including the sum illuminate the world by radiation. |
| Radiant energy from the sun Reaches the Earth through the Vast empty space ?(vacuum) existing |
| between the atmosphere and the sun. |
| This energy travels with the speed of light and has similar properties to light i.e. Radiant energy |
| can be reflected absorbed and Transmitted. |
| The body which absorbs radiant ener gy becomes heated up and its temperature rises. |
| Good Absorbers and Emitters of Radiant Heat |
| Identify good absorbers and emitters of radiant heat |
| Radiant energy can be detected by means of a thermopile. |
| Thermopile is an instrument which convents radiant energy ( radiant heat energy) into electrical |
| If the terminals of the thermocouple are connect to a galvanometer by connecting wires, a current |
| flows in the galvanometer G when the thermopile is directed towards a hot body, such as an |
| An increase in deflection of Galvanometer G is observed when the current thought the electric |
| lamp is increased. Comparison of Radiant energy |
| The amount of Heat energy radiated by a body depends on: |
| The Temperature of the body. |
| The Nature of surface the body. |
| The surface area for the body |
| To demonstrate the fact that the amount of Heat energy radiated from a body depends on the |
| nature and area of its surface (Leslie s cube) can be used. |
| The figure below shows Comparison son of Radiant energy from different – substance. |
| Leshe s cube is a cube – shaped metal Box which has Three of its sides painted with |
| different colours e.g Green, Black and Grey. |
| One side is highly polished serve as a reflecting surface. |
| The cube is placed on a Turn table R and Maintained Hot by Running steam into it. |
| Thermopile, T connected to a galvanometer G is placed at a fixed distance from the cube |
| by Turning the Turn table. |
| The black side of the cube will produce the largest deflection of the Galvanometer G, |
| While the polished surface will produces the leats deflection. |
| The alternative demonstration of the absorption of radiant Heat by a surface can be per |
| formed by using two tiny plates and Ban sern burner. |
| Heat Losses due to Radiation |
| Minimize heat losses due to radiation |
| The vacuum flask was designed by sir James Dewar for purpose of stoning condenser air in the |
| Now days used for keeping liquids hot over a period of Time. It would also keep liquids Cold for |
| The vacuum flask consists of the double walled glass vessed with a vacuum between the walls. |
| The walls are silvered on the vacuum side. The flask controls convection, conduction and |
| radiation of Heat energy. |
| Convection is prevented by the vacuum space between the walls and by closing the flask at the |
| Conduction is reduced by having the container made of glass, which is a bad conductor of heat. |
| The stopper is made of a bad conductor e.g. cork or rubber. |
| The vacuum is also a non – conducting space. The outer glass wall is supported by a pad of felt |
| or cork attached to a plastic case. |
| Radiation is minimized by the of silvered surfaces. The silvered surface reflects any Radiant heat |
| energy coming f rom the outside or inside the flank. |
| MEASUREMENT OF THERMAL ENERGY |
| required to raise the temperature of an object or substance |
| by one degree. The temperature change is the difference between the final temperatur e ( T |
| the initial temperature ( T |
| The Factors which Determine Heat Quality of a Substance |
| Explain the factors which determine heat quality of a substance |
| Heat is a form of energy transferred between bodies due to difference in temperature between |
| them. The energy possessed by the body due to its temperature is called the internal thermal |
| energy. The heat content is due to the random motion of the particles that make up the body. The |
| heat content is determined by its mass, temperature change and the specific heat capacity of the |
| Determine the heat capacity |
| Heat capacity is the quantity of heat required to raise the temperature of a substance by one |
| Heat capacity = mass of the substance X specific heat capacity |
| Find the heat capacity of a lump of copper of mass 50kg. The specific heat capacity of copper is |
| The specific heat capacity of copper, C = 420J/KgºC |
| Required: To calculate heat capacity, H.C. |
| Calculating a quantity of heat |
| The quantity of heat required to change the temperature of a body with mass, M Kg by Q |
| degree Celsius is MCQ joules. |
| In order to raise the temperature of a body, heat must be supplied to it. |
| In order to lower its temperature, heat must be removed from it. |
| The Heat Equation is therefore written: |
| Heat Gained or Heat Lost = Mass X specific heat capacity X change in temperature |
| Q= change (Rise or fall) In Temperature of the body. |
| Water of mass 3kg is heated from 26ºc to 96ºC. Find the amount of heat supplied to the water |
| given that the specific heat capacity of water is 4.2 x 10 |
| Specific Heat capacity, C = 4.2 X 10 |
| Initial temperature, Qi= 26 ºC |
| Final Temperature, Qf = 96ºC |
| The Specific Heat Capacity |
| Determine the specific heat capacity |
| Specific heat capacity is the quantity of heat required to raise the temperature of a unit mass of a |
| substance by one degree Celsius. |
| The quantity of heat supplied to or taken away from a body depends on: |
| The temperature different, T |
| The thermal properties of the body. |
| Heat energy tends to flow from High temper atures to Low temperatures |
| If you pick up a warm object, heat energy transfers from the object to your hands and your hands |
| feel warm. If you pick up a cool object, heat energy tr ansfers from hands to the object and your |
| Determining specific Heat capacity |
| Calorimeter – Is the special instrument or vessel used for measurement of Heat. |
| Calorimeter is highly polished metal can usually made of copper or aluminium. |
| It is flitted with an insulating cover in which there are two holes. |
| Two holes allow a thermometer and a stir rer to be inserted. |
| The stirrer is made of the same metal as that of the calorimeter. |
| Demonstration of the specific Heat capacity of a solid |
| Determining specific Heat capacity by Method of Calculation. |
| Heat lost by solid, Hs = Ms x Cs (Qs – Qf) |
| Heat Gained by Calorimeterand stirrer, Hc = Mc x Cc (Qf – Qi) |
| Heat Gained by Water, HW = Mw x Cw (Qf – Qi) |
| But the heat lost by the solid is equal to heat gained by the calorimeter and stirrer plus the heat |
| gained by the water in the calorimeter. |
| Heat gained by a calorimeter and content equal to heat lost by the solid. |
| Mc Cc (Qf – Qi) + Mi Ci ( Qf-Qi) = Ms Cs ( Qs-Qs) |
| A piece of metal with a mass of 200g at a temperature of 100ºC is quickly transferred into 50g of |
| water at 20ºC find the final temperature of the system ( specific Heat capacity of water Cw = |
| 4200J/ Kg ºC specific Heat capacity of the metal Cm = 400J/KgºC. |
| Ms Cs (Qs-Qf) = Mc Cc (Qf-Q) Mm Cw (Qf-Ql) |
| Cs. Is the specific Heat capacity of the solids. |
| Determining the specific heat capacity of liquid, Cl |
| Heat Gained by calorimeter and stirrer |
| Q be the final Temperature of the system |
| If there are no heat Losses to the surroundings, then. |
| (Heat gained by water) = (Heat lost by metal) |
| Change of state is the transformation of the condition of matter from one (state) to another |
| caused by the change In temperature. |
| The Behaviour of Particles of Matter by Applying Kinetic Theory |
| Explain the behaviour of particles of matter by applying kinetic theory |
| The kinetic theory of matter (particle theory) says that all matter consists of many, ver y small |
| particles which are constantly moving or in a continual state of motion. The degree to which the |
| particles move is determined by the amount of energy they have and their relationship to other |
| particles. The particles might be atoms, molecules or ions. Use of the general term 'particle' |
| means the precise nature of the particles does not have to be specified. |
| Particle theory helps to explain properties and behaviour of materials by providing a model |
| which enable us to visualise what is happening on a very small scale inside those materials. As a |
| model, it is useful because it appears to explain many phenomena but as with all models it does |
| have little attraction between them |
| are held tightly and packed |
| are fairly close together with |
| fairly close together - they are |
| some attraction between them |
| are free to move in all directions and |
| strongly attracted to each other |
| are able to move around in all |
| collide with each other and with the walls of a |
| o are in fixed positions but |
| directions but movement is limited by |
| container and are widely spaced out |
| attractions between particles |
| Solids, liquids and gases |
| The model can be used to help explain: |
| what happens during physical changes such as melting, boiling and evaporating |
| do not have a definite shape |
| do not have a definite shape |
| flow and fill the bottom of a |
| expand to fill any container |
| are difficult to compress as |
| container. They maintain the same volume |
| the particles are already packed |
| unless the temperature changes |
| because there are only a few particles |
| are difficult to compress because |
| are often dense as there are |
| there are quite a lot of particles in a small |
| are often low density as there |
| many particles packed closely together volume |
| are not many particles in a large space |
| are often dense because there are |
| quite a lot of particles in a small volume |
| The graph of temperature versus temperature for a Heated. |
| The Melting Point of a Substance from its Cooling Curve |
| Determine experimentally the melting point of a substance from its cooling curve |
| is the process of change of the state of matter from solid into liquid e.g ice into water. |
| Melting point (M.P): Itis the temperature at which solid substance tends to change into liquid. |
| Freezing: It is the process of change of the state of matter from liquid to solid e.g water into ice. |
| Freezing point: Is the temperature at which liquid change into solid. E.g water change into ice at |
| Evaporation:Is the process of change liquid substance into vapour (gas) |
| Sublimation: It is the change of state of matter from solid to gas and vice versa without passing |
| through the liquid phase.e.g. ammonium Chlonde ( NH |
| CL) and Iodine tends to sublime. |
| Sublimation point is the temperature at which a solid tends to change into gas and vice versa |
| without passing through liquid state. |
| Condensation:Is the change of state of gaseous state of matter into liquid state.e.g steam into |
| Deposition: Is the change of the state matter from gas into solid. e.g. Ammonium chloride vapour |
| and Iodine vapour into solid (NH |
| Demonstration of cooling and melting curves for (octadecanoic acid). |
| Melting point (m.p) table |
| Substance Melting point (ºC) |
| The Effect of Impurities on the Freezing Point and the Boiling Point of a |
| Demonstrate the effect of impurities on the freezing point and the boiling point of a substance |
| The effect of dissolved substances on the boiling point and melting point (M.P) means that the |
| additional of impurities will result in increased (B.P) and (M.P). |
| Effect of impurities on Boiling Point |
| When an impurity is added to a substance its boiling point is elevated, i.e., its boiling point is |
| The elevation in boiling point increases with increase in concentration of the solute because |
| when adding the solute vapour pressure of the solution becomes lower than pure solvent. Thus |
| the solution has to be heated more to make the vapour pressure equal to atmospheric pressure. |
| Thus the boiling point gets elevated. |
| For example boiling point of water is 100 |
| C under normal atmospheric pressure. If we add sugar |
| or salt to this water its vapour pressure becomes lower and boiling point increases. |
| Generally, when 1 mole of any non electrolyte is dissolved in 1 litre of water the elevation of |
| Effect of impurities on freezing point |
| When an impurity is added its freezing point is lowered i.e. its freezing point decreases. |
| The depression in freezing point increases with the increase in concentration of the solute |
| because on adding the solute the vapour pressure of solution becomes lower than that of pure |
| solvent. Since freezing point is the temperature at which vapour pressure of liquid and solid |
| phase are equal, therefore, for the solution, this will occur at a lower -temperatur e. |
| For example the freezing point of water is O |
| C under normal atmospheric pressure. If we add |
| sugar or salt to this water its vapour pressure lowers and freezing point decreases. |
| Generally, when 1 mole of any non-electrolyte is dissolved in 1 litre of water the depression in |
| freezing point of water is 1.86 |
| The impurities present in a liquid pull its two fixed points away from each other i.e. the |
| freezing point is lowered while the boiling point is raised. |
| The depression in freezing point and the elevation in boiling point increases with increase |
| in the concentration of the solute or impurity i.e. these are the colligative properties that depends |
| only on the no. of moles of the solute. They are independent of the nature of the solute. |
| The Effect of Pressure on the Boiling Point and Freezing Point of a Substance |
| Demonstrate the effect of pressure on the boiling point and freezing point of a substance |
| If a substance expands on solidifying, e.g., water, then the application of pressure lowers its |
| If a substance contr acts on freezing, the pressure raises its melting point, e.g., paraffin wax. |
| The freezing point of water is lowered by 0 .007 ºC per atmosphere increase in pressure, whereas |
| that of paraffin wax increases by 0.04 ºC per atmosphere increase in pressure. |
| When a is liquid heated, its temperature rises and eventually remains constant. |
| Boiling is the process of forming bubbles of vapour inside the body of a liquid. It rises to the |
| surface of liquid. The process usually depends onexternal pressure above the liquid. |
| The Phenomenon of Regelation |
| Explain the phenomenon of regulation |
| Regelation is the Refreezing process which takes place when copper wire is passed through the |
| Regelation is the Refreezing process which takes place when the wire is observed to Cuts right |
| through the ice block and falls on the floor. |
| The Concept of Boiling and Evaporation in Respect to the Kinetic Theory of |
| Give the concept of boiling and evaporation in respect to the kinetic theory of matter |
| If a liquid is heated, the particles are given more energy and move faster and faster expanding the |
| liquid. The most energetic particles at the surface escape from the surface of the liquid as a |
| vapour as it gets warmer. Liquids evaporate faster as they heat up and more particles have |
| enough energy to break away. The particles need energy to overcome the attractions between |
| them. As the liquid gets warmer more particles have sufficient energy to escape from the liquid. |
| Eventually, even particles in the middle of the liquid form bubbles of gas in the liquid. At this |
| point the liquid is boiling and turning to gas. The particles in the gas are the same as they were in |
| the liquid except that theyhave more energy. At normal atmospheric pressure, all materials have |
| a specific temperature at which boiling occurs. This is called the "boiling point" or boiling |
| temperature. As with the melting point, the boiling point of materials vary widely, e.g., nitrogen - |
| 210°C, alcohol 78°C, and aluminium 459°C. |
| Any material with a boiling temperature below 20°C is likely to be a gas at room temperature. |
| When liquids boil the particles must have sufficient energy to break away from the liquid and to |
| diffuse through the surrounding air particles. As these particles cool down and lose energy they |
| will condense and turn back to liquid. When steam is formed by water boiling at 100°C the |
| particles quickly condense as the surrounding air temperature is likely to be much less that |
| 100°C so the particles cool rapidly. In fact the "steam" coming out of a boiling kettle can only be |
| seen because some of the gas particles have condensed to form small droplets of water. |
| Within a liquid some particles have more energy than others. These "more energetic particles" |
| may have sufficient energy to escape from the surface of the liquid as gas or vapour. This |
| process is called evaporation and the result of evaporation is commonly observed when puddles |
| or clothes dry. Evaporation takes place at room temperature which is often well below the |
| boiling point of the liquid. Evaporation happens from the surface of the liquid. As the |
| temperature increases, the rate of evaporation increases. Evaporation is also assisted by windy |
| conditions which help to remove the vapour particles from the liquid so that more escape. |
| Evaporation is a complex idea for children for a number of reasons. The process involves the |
| apparent disappearance of a liquid which makes the process difficult for them to understand. It is |
| not easy to see the water particles in the air. Also, evaporation occurs in a number of quite |
| differing situations - such as from a puddle or bowl of water where the amount of liquid |
| obviously changes, to situations where the liquid is less obvious - such as clothes dr ying or even |
| those where there is no obvious liquid at all to start with - such as bread drying out. A further |
| complication is that evaporation may be of a solvent from a solution e.g. water evaporating from |
| salt water to leave salt. These situations are quite different yet all involve evaporation. |
| Evaporation may also involve liquids other than water e.g. perfume, petrol, air fresheners. The |
| particle model can be used to explain how it is possible to detect smells some distance away |
| Latent Heat of Fusion and Vaporisation |
| Demonstrate latent heat of fusion and vaporisation |
| Latent Heat is the energy when is supplied in form of heat required to change the state of the |
| Matter from one form into another. |
| Latent heat is not determined (detected) by using a thermometer. So latent heat is also called |
| Specific latent Heat is the energy supplied to a unit Mass and change Its state from one state of |
| Matter to another state of matter. |
| Latent heat of Vaporization is the heat required to change a liquid into a gaseous state at constant |
| Mass of Beaker + Water = M |
| Time taken to Boil away = t |
| Specific latent heat of = L J / kg Vapor |
| Heat gained by steam = (M |
| In this experiment , the Heat gained by the Beaker may be Neglected. |
| Latent heat of fusion is the amount of heat r equired to change a substance from solid to liquid at |
| Calculate the amount of Heat required to melts 800g of Ice at 0ºC The specific Latent of fusion |
| Mass of Ice , M = 800g (0.8kg) |
| Specific Heat of fusion, L = 33400 J/kg |
| Determination of the specific Latent Heat of fusion of Ice. |
| Mass of Calorimeter + stirrer = M |
| Mass of calorimeter +Water =M |
| Mass of Calorimeter +Water = M |
| Initial Temperatur e of Water = Q |
| Final temperature of Water =Q |
| The Ice melts and forms Water at 0ºC .The Water formed warm up to Temperature Qf.Heat |
| gained by ice during melting at 0ºC = (M |
| ) L where L is the specific latent Heat of fussion. |
| Heat gained by the water formed = (M |
| CW is the specific heat capacity of water. |
| Heat lost by the original water in the calorimeter = (M |
| heat lost by calorimeter and stirrer = M1 C |
| Cc is the specific heat capacity of the material of the calorimeter. |
| Applying the heat equation: |
| (Heat gained by ice in Melting + Heat gained by the Water formed) =(Heat lost by calorimeter |
| and stirrer + Heat lost by original Water) |
| ) L + (M3- M2) CW QF= M1 C |
| Specific Latent heat of Vaporisation is the amount of heat required to change a unit Mass of |
| liquid into gaseous state ( Vapour) at constant temperature. |
| Specific latent Heat of fusion is the amount of heat required to change a unit Mass of solid |
| substance into liquid at constant temperature |
| SUBSTANCE SPECIFIC LATENT HEATOF FUSSION J/ kg |
| 0.6 kg of ice at - 10ºC is dropped into 2kg of Water 49ºC contained in a Copper calorimeter of |
| mass 0. 15kg . If the final temperature of the Mixture is 20ºC fin d the specific latent Heat of |
| Specific Heat capacity of ice = 2.1 x 103 J/ KgºC |
| Specific Heat capacity of copper = 420 J/ Kg ºC |
| Specific Heat Capacity of Water = 4200 j/ Kg º C |
| Heat gained by ice during warming up form - 10 ºC to 0ºC |
| = ( 0 . 6 ×2 . 1 X 103 ×10) |
| Heat gained when ice at 0ºC changes to water at 0ºC = 0.6L; where L is the latent heat of fusion |
| Heat gained by cold Water in warming up f rom 0ºC to 20ºC |
| =( 0 . 6 ×4 . 2 × 103 × 20) |
| Heat lost by Water during cooling from 49ºC to 20ºC |
| Total Heat gained = Total Heat lost |
| 12600 + 0.6 L + 50400 = 243600 + 1827 |
| The Mechanism of Refrigeration |
| Describe the mechanism of refrigeration |
| Refrigerator is a machine which can enable Heat to flow from a cold Region to a Hot region.The |
| Basic principle used in Refrigeration is Cooling by absorption of latent Heat |
| A volatile liquid such as freon, evaporates inside the copper coils |
| cabinet or the refrigeration. |
| The latent heat of Vaporization comes from the air surrounding the coil i.e. from the |
| inside of the freezing g cabinet |
| An eclectically driven pump |
| and force it into the heat |
| hich is made of copper coils. |
| The coils of the heat exchanger are filled with cool fins |
| In the heat exchanger, vapor is compressed by the pump and condensed back to liquid. |
| The conversion of vapour into liquid in (c) gives out the latent heat of vaporization, |
| which is conducted away by the fins. |
| The condensed liquid is then returned to the evaporator coil (A) through avalve (V) (in |
| this way a continuous circulation of vapour and liquid is set up). |
| The rate of evapor ation and the degree of cooling is controlled by a thermostat, which |
| switches the pumps motor on and off at intervals. |
| The thermostat can be adjusted to give the desir ed low temperature inside the freezing |
| cabinet where food is preserved. |
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