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School-college Physics Notes: Thermal energy 5.1 Density

Density & particle theory: 5.1 What is density? Formula units for density? Why is density important?

Doc Brown's Physics exam study revision notes

5.1a What is density? What is the formula for density? Why is density important? Examples

Density is a measure of how compact a material is - it indicates how much space or volume a given mass occupies.

The greater the mass of material in a given volume, the greater the density of the material.

The density of a material depends on what it is made up of (atoms and their arrangement) and its physical state.

The more spread out the particles, the lower the material's density - which is why gases have a very low density.

The more closely the particles are packed together, the greater the density - which is why solids have the highest density.

See the particle model of the states of matter

The density for a given material is the same whatever its shape or size for a given physical state.

The scientific symbol for density is the Greek letter rho  (ρ)

The formula for density is: ρ = m ÷ v

DENSITY (kg/m3) = MASS (kg) ÷ VOLUME (m3)

Density units in physics are usually kg/m3.

However, in chemistry, density data is often quoted in g/cm3 because most quantitative measurements in a school/college chemistry laboratory are usually quoted in grams (g) and ml (cm3), so I've sometimes quoted both sets of units,

so don't get them muddled! and note that:   g/cm3 = kg/m3 ÷ 1000  (can you work out why?)

Its advisable to be able to convert mass and volume units e.g.

mass: 1 kilogram = 1000 grams, so:  g ÷ 1000 = kg   and   kg x 1000 = g

volume: ml = cm3, 1 cubic metre = 1 million cm3,  so:  cm3 ÷ 106 = m3   and   m3 x 106 = cm3

5.1b Why is density data important? Examples of densities

Density is very important property to know about a material

e.g. if the density of an object is less than that of water (~1000 kg/m3) it floats.

If the density of an object is more than that of water it sinks!

In general: if the object has a density < fluid it floats and if density of object is > fluid it sinks.

However, although shape doesn't affect density, shape does affect flotation on non-flotation, otherwise, how can a steel ship float on water!?

Examples of density in kg/m3 (at ~room temperature, 20oC)

The table lists the densities of many common materials, all of them are useful materials for some application or other.

Note that gases are so much less dense than liquids or solids. - refer to particle model.

 Material Density Comments hydrogen 0.09 The element H, least dense material, 'floats' in less dense air helium 0.18 The element He, next least dense material, 'floats' in less dense air - balloons air 1.3 Mainly nitrogen N2 and oxygen O2 molecules. carbon dioxide 1.9 cork 240 wood 380 - 700 Important construction material solid paraffin wax 720 petrol 710 - 770 Important fuel crude oil 840 - 970 Variable composition of hydrocarbons, floats on water - polluting oil spills ice 920 Floats on water, less dense than water. water 1000 Useful solvent, transferring thermal energy in central heating systems seawater 1030 More dense than pure water, you float more easily! rubber 1520 Useful material for flexible joints or shock absorbers. brick 1920 Important construction material concrete 2370 Important construction material. glass 2580 Important construction material marble rock 2560 Mainly calcium carbonate CaCO3, useful for sculptures, kitchen work tops quartz rock 2640 Mainly silicon dioxide, silica, SiO2, useful for kitchen worktops aluminium 2640 Important metal, used for light alloys - aircraft construction bromine 3120 One of only two liquid elements at room temperature iron 7500 The element Fe, cast iron has many uses - we experience as 'heavy' objects! steel 7900 Mainly Fe, with added elements, important construction material. copper 8800 Copper wiring and piping. lead 11340 'Heavy' metallic element, used in lead roofing mercury 13600 One of only two liquid elements at room temperature gold 19300 Very dense important metal in jewellery osmium 2260 The most dense element in the periodic table

All is explained in the section in Forces section 7. 'floating and sinking'

Keywords, phrases and learning objectives for density

What is density? What is the formula units for density? Why is density important?

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