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Published byBernadette Newman Modified over 9 years ago
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Materials : Density Which is heavier, 1kg of fluffy feathers or 1kg of lead?
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1kg of anything has exactly the same mass : 1kg
1kg of anything has exactly the same mass : 1kg... The classic trick question! But in everyday language we would all say that “lead is heavier than feathers”.... We need a new word to describe the difference... Density
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Learning Objectives To understand qualitatively the concept of density and how it affects us To be able to successfully tackle density problems including alloys To use density investigations as a vehicle for teaching “How Science Works” and “the scientific method” Book Reference : Pages
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Density: Concepts Density: is a measure of how much mass is contained in a given unit volume Meaning: how heavy (in g or kg) one unit volume (in cm3 or m3) of the material is. Something we consider as "heavier" has a higher density, (is more dense). Something we consider as "lighter" has a lower density (is less dense)
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Density: Calculations
Density = mass Volume Or if you insist on squiggly Greek letters... = m / V (pronounced roh/roe/ro) Units : Mass : kg (preferred) or g Volume : m3 (preferred) or cm3 Density : kg / m3 (preferred) (or g/cm3)
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Density: Warning on Units 1
In keeping with the “SI” (Le Système International d'Unités) units used throughout Physics, kg/m3 are the preferred unit for density However, for many everyday values and also in the field of chemistry g/cm3 are often used Take real care converting for square and cubic units!
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Density: Warning on Units 2
For example... While there are 100cm in 1m, for 1m3 there are 100x100x100 = 106 cm3 in each m3 Standard prefixes for the SI units of measure:
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Density: Warning on Units 3
Litres, (not a correct SI unit) are also a minefield..... As the name suggests there are 1000ml in a litre. Also 1ml 1cm3 And there are 1000 litres in 1m3 Which adds up since we know that there are 106 cm3 (or ml) in 1 m3
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Density: Measuring Volumes 1
To be able to calculate the density we need to know the volume.... Regular solids.... Cubes, cuboids, cylinders and spheres etc. We measure, (with a ruler, callipers or micrometer and calculate the volume using the appropriate formula: Sphere : V = 4/3r3 Cylinder : V = r2h
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Density: Measuring Volumes 2
Liquids : Simply measure the liquid in a measuring cylinder... But... HSW : Reading a meniscus Many liquids form either convex or concave meniscus within vessels. Care must be taken to read at the correct level to avoid parallax errors
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Density: Measuring Volumes 3
Irregular solids : We cannot simply measure/calculate the volume of such solids, However, we can Archimedes's principle and use a displacement can... Carefully lower the irregular object into the “brimmed” can The volume of the displaced water is equal to the volume of the object
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Density: Alloys Alloys are mixtures of two metals. For example brass consists of copper (60%) and zinc (40%) If we have a volume V of an alloy which is made of two metals A and B then.... If VA is the volume of A then the mass is given by AVA. Likewise for B So the total mass of the alloy m = AVA + BVB Hence the density = m/v = AVA + BVB V
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