Units of measurement• In the UK we use two main systems ofmeasurement - imperial and metric• The standard international system isknown as SI units
Mass and weight• Mass is constant everywhere in theuniverse. Weight changes from place toplace• A nail will keep the same mass wherever itis: on a workbench or on the moon
Density of solids• Solid materials that have the same shapeand size can have a different mass• This lightness/heaviness is known asdensity• Density = mass ÷ volume
Density of liquids• Liquids and gases also have differentdensities• It is important to understand the density ofwater• Water is less dense when heated
WaterAt 4˚C water is said to have a density of 1.0Amount Temp. Mass1m³ 4˚C 1000 kg1m³ 82˚C 967 kg
Relative density(specific gravity)• Measures the density of a subject compared towater• 1m³ of water has a mass of 1000 kg• 1m³ of mild steel has a mass of 7700 kg• Mild steel’s relative density (density comparedto water) is 7.7
Density of gases• With gases, air is given the relativedensity of 1.0• Is helium more or less dense than air?
PressureForce applied by unit area• (N/m²) also known as (Pa)Other terms: bar, lbs/in²• 1bar = 100,000 N/m²• 1 lbs/in² = 6894 N/m²
• Pressure is exerted by solidobjects in downwards only• Liquid exerts pressure downwardand outward
Pressure exerted on all sides not just the base
• You will need to understand the effectspressure has on pipes and fittings• The internal pressure is affected bywhat is being carried (water/gas)
Atmospheric pressure• The pressure exerted by the earth’satmosphere changes as you get higherabove sea level• At sea level it is 1 bar = 14.7 psi• psi = lbs/in² (pounds per inches squared)
Properties of water• Water is a chemical compound made fromhydrogen and oxygen (H2O)• Water can dissolve lots of gases and solidsto produce solutions
Hard waterWhen water falls on ground that hascalcium carbonates or sulphates (chalk,limestone or gypsum) in it, they dissolveinto a solution called hard water.
Soft waterHas no dissolved salts like calciumcarbonates/sulphates.
pH value and corrosion• pH is the level of acidity or alkalinity of asubstance.• Acids and alkalis can cause corrosion,metals are more at risk
Atmospheric corrosion• Pure air or water alone are not very corrosive• Together as moist air they attack ferrousmetals (rusting)• Other things in the atmosphere increase thecorrosive effect, eg salt, carbon dioxide,sulphur
Non ferrous metals• Copper, lead, aluminium• These have protective barriers (usuallysulphates) which prevent furthercorrosion. This happens afteroxidisation.• This is known as patina
Corrosion by water• Ferrous metals are more likely to becorroded by water(think of black sludge in radiators (ferrousoxide))• Ferrous oxide and rust combine to makehydrogen
The difference between heatand temperature• Heat is a unit of energy measured inJoules (J). It is the amount of heat energyin a substance.• Temperature is measured in centigradeand is the degree of hotness of asubstance.
• The wire has a temperature of 350˚C• The water has a temperature of 70˚C• The wire is far hotter, but has less energy
Thermometers• A bi-metallic strip is used to convert atemperature change into movement• Liquid thermometers use mainly alcoholand mercury
Specific heat capacity• The amount of heat needed to raise 1kgof material by 1˚C.• It takes 4.186 kJ to raise the temperatureof water by 1˚C, but only 0.385kJ to raisethe temperature of copper by 1˚C.
Specific heat capacity• Work out the amount of heat needed toincrease the temperature of 250g of waterfrom 20oC to 56oC.
Specific heat capacityq = m x Cg x (Tf - Ti)m = 250gCg= 4.18 J oC-1g-1(from table above)Tf = 56oCTi = 20oC
Specific heat capacityq = 250 x 4.18 x (56 - 20)q = 250 x 4.18 x 36q = 37 620 J = 38 kJ
Thermal expansion andcontractionThese are known as coefficient valuesLead has a higher coefficient value than,say, copper ie it will move more whenwarm.
Plumbing materials• Plastic has coefficient value of0.00018• So if we had a 6m long guttering wecan work out how much it will expandat 19°c!• To do this we need three values
1 Length (meters)2 Temperature rise(°c)(19 degrees is used asstandard temp rise)3 Coefficient value
SoSo 66 (m) X (Temp rise)(m) X (Temp rise)1919°c°c XX(coefficient value)(coefficient value) 0.00018 =0.00018 =6 x 19 x 0.00018 = 0.02052m or6 x 19 x 0.00018 = 0.02052m or20.52mm20.52mm