HEAT

1. HEAT

2. Wesawearlierthat the HEATof a body indicates the total kinetcenergyofall the moleculesthatconstituteit ; therefore, heatisenergy and forthisreasonitisalsocalledthermalenergy. We have also seen that if we put in contact two bodies A and B with different temperatures, t1 and t2, with t2 > t1, (being able to contain different amount of heat), there will be a transfer of heat from the body at the higher temperature to that at the lower temperature, up to when the two bodies reach the same temperature ( thermal equilibrium). t2 t1 t2 > t1 Therefore we can say that heat is a form of energy in transit that is trasmitted from a body with a higher temperature to one with a lower temperature. This energy transfer is completed only when, after a given time, the two bodies in contact reach the same final temperature, t called equilibriumtemperature. Evidently : t t1 < t < t2 When this occurs, we say that an equlibrium condiction has been reached. Therefore heat is no more than energy transfer between two bodies in contact; this process occurs spontaneously when two bodies are put in contact, each one with a different temperature, and it is completed when the thermal equilibrium is reached. It is quite clear that by supplying heat to a body we raise its temperature, while subtracting (tacking away) heat we decrease its temperature.

3. Heat is a scalar quantity, represented by the symbol Q and is measured in calories . In the S.I we use the Kcal : 1 Kcal= 1000 = 103 cal 1 Kcal represents the quantity (amount) of heat required for 1 Kg of distilled water in order to raise the temperature by 1°C (one degree Celsius), from 14,5°C a 15,5°C , at atmospheric pressure. We have realized tha heat is energy, , therefore it can be meausured in Joule. James Prescott Joule (1818-1889) demonstrated the thermal energy-mechanical energy equivalence) In formulas: W=J•Q that say you can get heat from energy or work and viceversa. Heat-work equivalence 4186 J ofmechanicalenergy are neededtoincreaseby 1 K (1 degree) the temperature of 1 Kg of water Mechanical equivalent of heat: Thermal equivalent of water : Example: Represent the heat Q of 100 Kcal in Joule: Represent the thermal energy of 196 Joule in Kcal (kilo calories): N. B. When we rub hands, shortly after we experience a feeling of warmth . It is proof that some work has been transformed into heat.

4. We can directly heat a body, for example a mass of water, placing it in contact with a heat source, for example a gas cooker. We will now perform three easy experiments in order to formulate the relationship between heat and temperature: Experiment 2 Experiment 3 Experiment 1 In twoequalcontainersplaceequalmassesofbodiesconsisting in differentsubstances (water and iron) In two equal containers place different amount of water In two equal containers place the same amount of water The required heat in order to have the same temperature increase (for examople 10°C ) is directly proportinal to the mass. The necessary heat to increase the temperature of a substance, of fixed mass, is directly proportional to temperaturechange. The mass and the heat being equal, temperature increase depends on typeof substance. As B contains a greater amount of water, in order to increase its temperature by 10°C, a grater amount of heat is required compared to A. The same amount of heat supplied to a body with the same mass results, in different bodies, in a different temperature increase. With the same mass of water, if we want to increase the temperature, more heat is required. . This information can be summarized by the basic formula for calorimetry:

5. WhereQ stands for the heat amount transferred to the body, m is its mass, the temperature change, where (t1 is the initial temperature and t2 is the final temperature. The quantity c, called specific heat, is introduced to take into account the type of substance. We can infer that: by substituting, we have: Then, if m = 1Kg and That is the specific heat of a substance expresses the necessary heat amount to be supplied to a unit mass (m=1Kg) of such substance to increase by 1°C its temperature. Theunit of measurement of the specifc heat, in the S.I., is: We are now reporting a table with the specific heat of some substances (at room/environment temperature and atmospheric pressure) N.B. The basic formula for calorimetry is valid also in the event the body colls down; in this case, it is a matter of heat flowing from the body and the negative temperature change shows a decrease in temperature.

6. Examples Let’s calculate the amount of heat absorbed by 500 gr of water to pass from 12°C to 18°C. In simboli: How much heat is required to warm up 100 gr of gold to pass from 13°C to 1063°C, temperature at which gold melts ? In simboli:

7. divided by we obtain: By the equation We define thermal capacity the phisical quantity: Its unit of measurement is : The thermal capacity of a body tells us whether it absorbs a great amount of heat or little heat to increase its temperature by 1°C. Water has a high thermal capacity, this means that, compared to other substances, it takes more time to heat up (in fact it has to absorb a greater amount of heat), but it also takes more time to cool down (it has to release a greater amount of heat) . PHISICS around us Thermal capacity and coastal climate The heatcapacityof water hasconsequences on the climate; indeed, at the samelatitude, nearbylakes or the sea the temperature isnottoo high, in summer , and nottoo low, in winter (mild or temperate climate), reducing the termaldifferencethatisfound, instead, in the continentalareas (farawayfrom the sea). Becauseofitsgreaterthermalcapacity, water warms up more slowlythan the ground; in summer, then, when the temperature of the groundis high, the temperature of water islower and ithelpstokeepcoastalareascool; on the contrary in winter, the temperature of the groundislower; water cools down more slowly and helpstokeepsurroundingareas temperate (mild) . Lakes and sea work astheywere big thermostats (temperature regulators)

8. The high specific heat , and so the high thermal capacity of water, allows us to realize some phisical phenomena. On a summer day, both the beach and the sea receive the same amount of solar heat. However , around 11 a.m. if we walk barefoot on the sand, we get burned; this does not happen on the shoreline. Why? hot sand = sabbia “bollente” Foreshore/shoreline = Bagnasciuga The high specif heat of water means that a lot of heat is required to increase by 1°C its temperature, while the low specific heat of the sand causes a small amount of heat to increase a lot its temperature. That’s why sea and sand, while still getting the same amount of heat, warm up differently ( a little the water and great deal the sand). The device for measuring the heat of a body is the calorimeter.

9. The heat capacity of a substance is equal to the amount of heat energy (Q) transferred to it divided by its temperature change (ΔT). [J/K] The specific heat capacity of a substance is the amount of heat required to raise the temperature of one kilogram of that substance by one kelvin. [J/(kgK)]

10. Heat is energy transferred from one body to another (or from a body to the environment) that are in thermal contact with each other, due to a temperature difference between them. Together with all other forms of energy, heat is measured in joules. Historically, heat was measured in calories. Joule’s machine : it is a thermally insulated container full of water where a vertical rod, which has got some welded paddles, rotates inside it. Two small weight hanging down through a system of pulleys and wires so the paddles can rotate and let the water into the container be mixed. An inside thermometer measures the water temperature both before and after the experiment. W=J•Q 4186 J = 1 Kcal W (oppure Q) = 4186J •5Kcal = 20930 J To express the heat of 5Kcal in Joule To express the work of di 20930J in Kcal Q = W/J=20930J/4186= 5Kcal The calorie (or gram-calorie) is the amount of energy required to raise the temperature of one gram of water from 14.5 °C to 15.5 °C. In SI units, the calorie equals 4.184 joules.

11. Heat trasmission • Heat can be transferred to bodies in three different ways ( processes): • Conduction (related to solids) • Convection (related to fluids = liquid and aeriform substance) • Radiation (related to vacuum (of space) Conduction is the propagation that occurs in solid bodies and consists of a trasmission of thermal energy or heat without displacement (motion) of matter . Conduction is the transfer of heat energy within a substance without displacement of matter . Suppose to heat a metal bar with a section S and length d, at the A end, rising the temperature t2. How long after the B end, which at the start had a temperature t1 (t1< t2) warms up to get to a temperature t2? The problem has found a solution with the mathematician J. Fourier (1768 – 1830) , who formulated the thermal conduction law that is a relation between heat, time and temperature. thermal conduction law = Fourier’s law Where Q stands for the heat amount flowing from a homogeneous slab surface area S, called Ato another (B), is the difference in temperature between the two surfaces in contact with the heat source, d is the distance between them and is the elapsed time interval. The constant K , called thermal conduction, depends on the substance examined.

12. TABLE Thermal conduction coefficients N.B. An high value of thermal conduction coefficient K means that is high the amount of heat per unit of time, considering the same surface, length and temperature range. N.B. Good thermal conducts (through which a fast trasmission of heat occurs) have a high thermal conduction coefficient). Thermal insulators (through which the trasmission of heat is slow), instead, have a low thermal conduction coefficient).

13. The thermal conduction law is particularly applied to the thermal conduction through a flat wall (problem of the wall) with a thickess d. PHISICS around us Double glazing Wall with thermal insulator Heat dispersion (dissipation) through a wall Heat dissipation through the wall is shown by means of thermography, with a red colour, which highlights how the dispersion is higher through doors and windows. In order to reduce dispersion some devices (contrivances) are used: double glazing or panels of thermal insulators . In the building industry, to achieve a good thermal insulation of buildings, double walls are built between which thermal insulators, such as polystyrene, are placed.

14. Example How much heat is dissipated in one second by a concrete wall (with no openings) of surface S = 9 m2 , 36 cm thick with a conduction coefficient K = 0,69 W/m • °C , if the difference between the indoor and outdoor temperature is 25°C? Speed at which heat is transferred = Heat transferred in the unit of time HEAT SPEED DIFFUSION= How much heat is dissipated in a day? 1g = 24h = 24*3600 sec Further example A rectangular wall of oak wood (with no windows), surface S = 5 m2 , 15 cm thick, in a chalet, divides the inside environment, where the temperature is kept at 19°C (is about 19°C), from the outside environment , where the temperature is at -11° C . Knowing that wood thermal coefficient is K = 0,20 W/m • °C , how much is heat dissipated during an hour ?

15. CONVENZIONE Suppose we place a container full of water in contact with a heat source. We notice that, warming up the water pot, after few minutes, the layers close to the flame heat up and consequently expand; increasing their volume, density decreases and they move upward, forcing the surface colder layers to move downward. In this way the convection currents produced facilitate the liquid to heat up. Convection is the transfer of heat within a substance (liquid or fluid) through the displacent of matter (particles within the fluid). These displacements are known as convection currents. The convection currents: red stands for hotter water upward movement , blue stands for the cold water downward movemente. PHISICS around us Air convection currents influence the climate and are exploited to heat our homes. The convenction phenomenon is used in radiator heating plants. The water boiler C, heated by the burner B, moves upward through the pipe M up to the radiator R, then back again to the boiler through pipe N.

16. PHISICS around us Near a radiator there is always hot air going up and rolling away. Cold air, going down, gets closer and it is in turn heated and goes up again; so a closed cycle occurs. radiator BREEZE sea breeze = brezza di mare Land breeze = brezza di terra The sun gives the same amount of heat to the sea and the land; however, the sun warms up earth (ground) more quickly than the sea. Therefore during the day the air above the earth expands, becoming less dense and so rising. The air above the sea takes its place; so a closed cycle is achieved: hot air above, fresh sea air under: so, sea breeze, is fresh sea air blowing in from the sea to land. During the night, earth cools more quickly than the sea and the elements are inverted: hotter air, going up, is above the sea and it is replaced by colder air above the earth; so land breeze is fresh air blowing in from land to sea.

17. Radiation The sun gives off heat that reaches the Earth propagating through interplanetary space that is empty . Radiation is the third way by which heat transfer occurs without any other material means, neither solid, nor liquid, nor gaseous. Radiation is the transfer of heat even through empty space; in this case it isn’t necessary the presence of any body. PHISICS around us A bottle with water, in summer, after a short time, becomes hort, while in winter, cold; this happens because of the heat exchange between internal and external environment. Thermos (or Dewar bottle) are containers that keep a liquid or solid substance at almost constant temperature for a long time; they consist of double walls between which a vacuum is provided (the best thermal insulator), reducing heat dissipation by conduction . The inside is silver coated, in order to reflect rays and reduce heat dissipation by radiation. THERMAL INSULATOR = containers /substances preventing/reducing heat exchange with the external environment .

18. EFFETTO SERRA THE GREENHOUSE EFFECT The sun releases radiation that hit the earth: one part of which is absorbed and reflected by air, another part by the ground in the form of infrared rays (IR). The lowerr layers of the atmosphere, rich in water vapor and carbon dioxide (exhaust gases), prevent the infrared rays , emitted from the ground, to pass and to go away, so they return down furtherly heating the earth surface. The phenomenon is similar to what happens in a greenhouse: the glass walls let the sunlight in, but trap the infrared rays, resulting in an increase in temperature compared to the external environment. Consequences of such effect are desertificaztionand excessive melting of glaciers, leading to heavy effects on the earth climate (rising sea levels and coastal cities submerged by the seas).

19. Remember • Sun rays pass through the atmosphere and heat the earth surface. • From the erath surface heat radiates in the atmosphere in the form of infrared radiatio. • About 30% (per cent) of the infrared radiation is lost in space. • In natural conditions, about 70% (per cent) of the infrared radiation is absorbed by green house gases in the atmosphere; in turn the gases reflect it again on the earth surface. N.B. Earth radiation (infrared rays) Solar radiation (visible and ultraviolet light) Low atmosphere

20. Heat transfer mechanisms can be grouped into three categories: conduction, convection and radiation. Heat is transferred throughout our environment all of the time.

21. 1. If one end of a metal rod is placed over a fire, that end will absorb the energy from the flame. 2. The molecules at this end of the rod gain energy and begin to vibrate faster. 3. These molecules transfer energy to neighbouring atoms or molecules. The heat is transferred from the warm end to the cold end.

22. Thermalconductivity, denotedas λ, is a measureof a material’s abilitytoconductheat. A body with sectional area A and length L is placed in contact with two heat sources at different temperatures: T1 and T2. It can be observed experimentally that the quantity of heat Q transferred in the time interval Δt is:• directly proportional to the difference in the temperatures measured at the two sides of the body; • directly proportional to the sectional area A; • inversely proportional to the length of the body. Fourier law of heat conduction

23. Convection is the transfer of heat by the mass movement of a fluid. As the temperature of a fluid in contact with a hot object increases, the fluid expands, its density decreases, and it rises according to Archimedes’ law (convective circulation). This type of heat transfer takes place in liquids and gases. Convection occurs naturally in the Earth’s atmosphere. example: the Sun warms the ground and convective air currents transfer heat from the surface to the atmosphere.

24. Radiation is the mechanism by which heat is transferred through electromagnetic waves. The energy carried by the wave is related to the wavelength (measured from crest to crest). Shorter wavelengths carry more energy than longer wavelengths. Electromagnetic waves are so called because they have both electric and magnetic field components.

25. The specific heat of gold is over 30 times smaller than that of water.Therefore, for the same amount transferred heat, a kilogram of gold will heat up from 20 °C to 90 °C (∆T = 70 °C), whilst water will only heat up from 20 °C to 22 °C (∆T = 2 °C).

26. Heat sources It is said thermal source any body able to provide heat to one or more bodies. We have two kinds of thermal source: a) Combustible (fuel) b) Solar heat • A combustible (fuel) is a substance which, combining with oxygen, develops heat (thermal energy). We have / there are: - solid fuels (combustibles) ( wood, coal, fossil etc… ) • - liquid fuels (petrol oil, alcohol, etc…) • gaseous fuels (methane, acetylene, etc…) For all pratical purposes, it is useful to know the calorific value of a fuel, that is the heat amount, expressed in kcal, produced by 1 Kg of fuel fully burning , Q = m•potere calorico Calorificvalueof some fuels Q = m•calorificvalue Exercise How much heat can we produced from 2Kg of charcoal? Q = m•calorificvalue = 2Kg •7500 Kcal/Kg = 15.000 Kcal

27. Example We want to heat by 50° degrees Celsius a quantity of water of 200 Kg using a wood stove. How much wood to be burned do we need ? In simboli: Relazione fondamentale della calorimetria: Basic relationship for calorimetry Q = m•potere calorico Assuming, however, that only 40% of the heat produced during the combustion of the wood remains at water, haw much wood to be burned do we need, this time? Q =0,40 • Qcombustione Qcombustione = Q/0,40 =25000Kcal

28. b) Solar heat The Sun is the main source of heat, for our planet. When we use the heat from burning wood or oil, we do nothing but draw from deposits of solar energy that were formed thanks to plants and/or animal organism. When we warm ourselves by means of an electric heather, heat is produced by a different phisical phenomenon which will be studied/discussed next year. Energy sources An Energy source is any source producing energy. There are two types of energy sources: sustainable and notsustainable (or exhaustible) • Sustainable energy sources are: the sun (thermal energy), water (hydroelecritic power), wind (wind • energy/power); geothermal energy . They are unlimited in time, that is, they are endless. • Exhaustible energy sources sono: coal, oil (petrol, gasoline), methane (natural gas) and uranium. They are limited, in time, this means that sooner or later they will deplete.

29. Tutti gli esseri viventi, animali e vegetali, hanno bisogno di energia per i propri processi vitali. Le piante verdi la ottengono dal sole attraverso la fotosintesi clorofilliana. Gli animali, e gli esseri umani, hanno bisogno di alimenti , dai quali, attraverso la digestione, ricavano l’energia necessaria. Per metabolismo si intende l’insieme dei processi biochimici tramiti i quali gli alimenti vengono trasformati, dall’uomo, in energia. La maggior parte di questa energia è trasformata in calore (temperatura corporea di circa 36°C) e l’altra per svolgere varie attività. Ad ogni alimento compete un determinato apporto energetico. Ad esempio 100 gr di pane forniscono 1150KJ di energia, mentre 100 gr di zucchero ne forniscono 1640 KJ. Meno energetica è la frutta: 100 gr di uva forniscono 260KJ. In genere, l’apporto energetico degli alimenti è riportato sulle confezioni: viene fornito sia in Kcal che in KJ per 100 gr di sostanza. Fabbisogno energetico L’uomo, in base all’età, al sesso, al peso e all’attività lavorativa svolta, ha bisogno di un determinato apporto energetico giornaliero. Se incamera più energia di quella consumata, l’energia in eccesso viene conservata, sotto forma di grasso, principalmente : Energia (media) fornita da un grammo di alimento Carboidrati = 4,1 Kcal/g Proteine = 4,2 Kcal/g Grassi = 4,3 Kcal/g fianchi e cosce, (donne) pance, (uomini)

30. Passaggi di stato Natural things are made up of countless tiny clusters, called molecules. A molecule , in turn, is made up of one or more smaller particles, called atoms, stiking togeher by cohesion forces. Different substances are formed by different molecules (water, hydrogen, carbon dioxide, glass, etc..) . Molecules Atoms Substances The same substance may turn up with a different “consistency”, depending on external conditions (pressure and /or temperature). A common example is the one related to water, normaly liquid, can be in the form of ice or gas.

31. We can say that substances may occur in different aggregation (custer) states: • solid state (it has its own volume and its own shape) • liquid state (it has its own volume but takes the shape of the container) • gas(eous) state (it has the volume and the shape of the container) • (aeriform) The different aggregation states, in which a substance may appear, depend on the attractive forces between its molecules. If for some reason, by acting on temperature and /or pressure, the molecles are moved away from their initial positions, overcoming the attractive forces, a change of state will occur: it is the case of ice, when we provide heat, it becomes liquid; or water, when we provide heat , it becomes gas. The transition from an aggregation state to another is called phase transition. In order to have the transition from an aggregation state to another a change of temperature is required (providing (giving) or subtracting heat) and /or it is required a change of pressure. PHISICS around us vaporization sublimation In Summer, to say the icecream is liquefying, is, therefore, incorrect. We must say the icecream is melting. melting/fusion gaseous state solid state liquid state solidification condensation/liquefaction congelamento / freezing sublimation /frost

32. 1. The calorie is the amount of energy required to raise the temperature of one kilogram of water from 0 °C to 10 °C. T F 2. Heat due to radiation is transferred through electromagnetic waves.T F 3. A degree on the Celsius scale and a degree on the Kelvin scale both measure the same physical quantity. T F j j j j j

33. 1. Indicate the type of heat transfer beneath each picture. a b c j j

34. A calorimeter is a device used to measure the specific heat capacity of a substance. example: the calorimeter is thermally insulated, i.e. does not allow heat transfers with the environment. It is filled with a known mass m1 of water at a known temperature T1. If a known mass m2 made of a pure substance at a known temperature T2 is placed in the calorimeter, the specific heat of the substance c2 can be calculated by measuring the temperature of thermal equilibrium Te between the water and the substance. c1 being the specific heat of water.