Book - School of Science and Technology

2 FundamentalsBefore any part of this subject is pursued in further detail, however, it would seemappropriate to discuss some of the quantities and units that are relevant, with particularreference to the meaning and use applied to them in the present context.Units and quantitiesThe notes included here are not intended to be a comprehensive glossary of termsincluded in the SysteÁme International d'UniteÂs but, rather, an aide meÂmoire covering thosewhich are specific to the subject matter of this book but are not necessarily in everydayuse. The four basic units are the kilogram (kg) for mass, the metre (m) for length, thesecond (s) for time and the kelvin (K) for thermodynamic temperature, all with theirmultiples and sub-multiples. From these, the following secondary units are derived:Force. The unit here is the newton (N), which is the force necessary to accelerate a massof one kilogram to a velocity of one metre per second in one second, that is,1Nˆ 1 kg m/s 2 . When a mass of 1 kg is subjected to acceleration due to gravity, theforce then exerted is 9.81 N.Heat. The unit of energy, including heat energy, is the joule (J), which is equal to a force ofone newton acting through one metre.Heat Flow. The rate of heat flow is represented by the watt (W), which is equal to onejoule produced or expended in one second, that is, 1 W ˆ 1 J/s ˆ 1 Nm/s.Pressure. The standard unit is the newton per square metre (N/m 2 ), also known, moreconveniently, as the pascal (Pa). The bar continues to be used in some circumstancesand 1 bar ˆ 100 k Pa.Specific Heat Capacity. This is the quantity of heat required to raise the temperature ofone kilogram of a substance through one kelvin, the units being kJ/kg K. Where heatflow in unit time is involved, the unit becomes kJ/s kg K ˆ kW/kg K.Specific Density. The unit for this quantity is the kilogram per cubic metre (kg/m 3 ).Volume. The cubic metre is the preferred unit but the litre (1 dm 3 ) is in general use since itis much more convenient in terms of a comprehensible size. To avoid printing confusionbetween the figure 1 and the letter l, this book will spell out the world litre in full.Properties of materialsTable 1.1 lists a variety of materials and provides details of some relevant physicalproperties. These serve as a source of reference against the following notes:Latent heat. When the temperature of water at atmospheric pressure, is raised fromfreezing to boiling point, i.e. through 100 C, the heat added is 420 kJ/kg (4:2 100). Toconvert this hot water to steam, however, still at atmospheric pressure and still at 100 C,will require the addition of a significantly greater quality of heat, i.e. 2257 kJ/kg.*This value is the latent heat of evaporation of water and its magnitude, 5.4 times thatneeded to raise water through 100 C shows its importance. A similar phenomenon occurswhen water at 0 C becomes ice at the same temperature; this change of state releasing330 kJ/kg, the latent heat of fusion of water.* If water is evaporated at 20 C, the latent heat of evaporation is 2450 kJ/kg.