Chapter 1Temperature, Heat, and Heat TransferNur AklisMechanical Engineering Muhammadiyah University of Surakarta

TemperatureTemperature is defined as a measure of how hot something is when we touch it.Because related with the humans feelings and senses that they are not reliable and influenced by psychological factors, temperature measurement should be defined exactly by certain scale.Based on the zeroth law of the thermodynamics that if body A is in thermal equilibrium with body C, and body B is in thermal equilibrium with body C, then A is in thermal equilibrium with B, the thermometer is made.

Temperature (cont)Two objects are in the thermal equilibrium mean that they are at the same temperature.Suppose we want to measure the temperature of two beaker of liquid, A and B. We use a thermometer as body C. Firstly we place it in contact with A and record a reading, say 20oC. Then we place it in contact with B and again obtain the reading 20oC. Thus we know that A and B are at the same temperature and are in thermal equilibrium.

Temperature ScaleTo measure temperature we can useCelsiusKelvin (absolute temperature)Fahrenheit The relation between celcius, Kelvin, Fahrenheit;K = 273, 16 + oCF = 32 + 9/5 oCC= 5/9(F-32)

Thermal ExpansionThermal expansion is phenomena occurred when the temperature of solid or liquid is changed. If at an initial temperature (To) an object has length Lo along some dimension, its length will change by an amount L = L-Lo, and From the experimental was be found that; Where Is the coefficient of linier expansion oC-1 and

Area and volume also change with temperature.For area

For volume

Thermal Expansion (cont)

Thermal Expansion (cont)

ExampleAluminum rivets used in airplane construction are made slightly larger than the holes into which they fit and then cooled in dry ice to -78 oC before being inserted. When they then warm up to room temperature (23oC) they fit very tightly. If a rivet at -78oC is to be inserted into a hole of diameter 3.20 mm, what should the diameter of the rivets be at 23oC. A 100 cm3 beaker made of pyrex glass (=3.2x10-6C-1) is filled to the brim with water at 12oC. What volume of water will overflow when temperature is raised to 60oC. ( water=210x10-6C-1)

Example (cont)A pair of eyeglass frames is made of epoxy plastic. At room temperature (20.0C), the frames have circular lens holes 2.20 cm in radius. To what temperature must the frames be heated if lenses 2.21 cm in radius are to be inserted in them? The average coefficient of linear expansion for epoxy is 1.30 x10-4 (C)-1.

Heat and Thermal Energy Thermal Energy (=internal energy) of system is collective kinetic and potential energy associated with the random motion of the atoms and molecules comprising the system.Heat is the energy transferred between a system and its surroundings because of their temperature difference.

SubstanceSubstanceInternal energyTemperatureInternal energyTemperatureHEAT

Heat and Thermal Energy (cont)Heat flow is positive when energy flows into a system and negative when heat flows out.

Unit of HeatHeat is energy, so the unit of heat can be stated in joule, But for the thermal the unit of energy is calorie (cal).1 calorie is defined as the amount of energy transfer necessary to raise the temperature of 1 g of water from 14.5C to 15.5C.From the mechanical equivalent of heat experiment conducted by joule (see figure below) it is can be found that;1 cal = 4.186 JThe other unit is Btu (British Thermal Unit), whereas 1 Btu = 252 cal = 1054 J

The mechanical equivalent of heat ExperimentthermometerEnergy measurement

Heat Capacity and Latent HeatWhen heat is added to a substance the temperature will rise or it changes from one phase (for example, solid, liquid or gas) to another phase.The resulting rise in temperature (T) depends onThe mass of substanceThe heat addedThe kinds of material

Heat Capacity and Latent Heat (cont)Property of material that influence the change of temperature when the heat added to the substance is a specific heat (c), where specific heat can be defined as the amount of heat required to raise 1 kg of a substance by 1oC, while Heat capacity is the amount of heat required to raise a substance by 1oC.

Heat Capacity and Latent Heat (cont)The relationship between specific heat, amount of heat and changes of temperature can be written as;

WhereQ = heat (joule or calorie)m = massC = specific heat (j/kg.oC or cal/kg.oC T= changes of temperature

Heat Capacity and Latent Heat (cont)Solid, liquid and gas (vapor) are called phases of matter. To change it from some phase to the other, energy must be added or removed. For example to transform substance from solid to liquid (melting process) or from liquid to gas (boiling process) energy must be supplied to the substance. Conversely, energy must be removed to change from gas to liquid (condensing process) or from liquid to solid (freezing process).

Heat Capacity and Latent Heat (cont)The energy that must be added (or removed) to cause the solid-liquid transition in 1kg of a given material is called the latent heat of fusion (Lf). The energy that must be added (or removed) to cause the liquid-gas transition in 1kg of a given material is called the latent heat of vaporization (Lv).From the definition of latent heat, and again choosing heat as our energy transfer mechanism, we find that the energy required to change the phase of a given mass m of a pure substance isL is Lf or Lv

liquidgassolidaddedremovedLvLf

Heat Capacity and Latent Heat (cont)To explain completely, looking for example if -30oC ice is heated (gained heat) continuously, the condition can be seen at graph below

Part A temperature increases, phase is still ice.Part B, after temperature reaches 0oC (melting point of water), temperature still in 0oC and the phase will be changed from ice (solid) to water (liquid).Part C temperature increases from 0o to 100oC.Part D after temperature reaches 100oC, (boiling point of water) temperature constant and phase will be changed to steam or vapor.Part E, After all of water changed to vapor temperature will increase.

Hint Solution problem of heat capacity and heat latent If two substance or more are blended, then the substance that has higher temperature will loose of the energy and the other will gain the energy.In a system, It is can be written as;Qlost = Qgained

Example80 grams of brass (c=0.092 cal/goC) at 292 oC is added to 200 g of water (c=1 cal/goC) at 14 oC in insulated container of negligible heat capacity. What is the final temperature of the system.To 160 g of water at 10 oC is added 200 g of iron (c=0.11 cal/g.oC) at 80 oC and 80 g of marble (c=0.21 cal/g oC) at 20 oC. What is the final temperature of the mixture.Determine the amount of energy that must be lost by freezer to make 1.5 kg of ice at -12 oC from water at 20 oC. En espresso stand prepares steamed milk by bubbling steam at 140 oC through a cup of milk at 30 oC, raising the temperature of milk and its container to 50 oC. What mass of steam is required to heat 220 g of milk (essentially water in a cup of mass 100 g given the data . C milk = 1 kcal/kgoC, C cup = 0.20 kcal/kgoC, C steam =0.48 kcal/kgoC, Lv water = 540 Kcal/kg boiling temperature of water 100 oC

**************************