The idea of density
Taking up space
Which is heavier, a tonne of lead or a
tonne of feathers?
Everyone knows that lead is heavier than
feathers, but that is not the right answer.
Tonne of anything weighs the same.
1t=1000kg
One tonne of feathers occupies much more
space than one tonne of lead.
Comparing materials
• We say that lead has a greater density than
feathers.
• The density of material tells us about the
amount of a substance in one cubic
centimetre of the materials.
• For each material we find the mass of 1cm3.
Explaining density
• Air has very low density ,ρ=0.0013g/cm3
• Particles are spread out, far apart from each
other.
• Lead is a solid.
• Particles are packed closely together.
• The particle model tells us why lead has a
greater density than air.
Question
• When water boils, it becomes steam. Its
particles spread out and occupy a greater
volume.
• Has its density increased or decreased?
Measuring density
• Density=mass/volume
• Mass in grams
• Volume in centimeters cubed
Mass is measured using a balance.
Measuring volume (rectangular solid block):
Volume=length x width x height
Or volume of liquid with measuring cylinder, read
the volume from the scale.
Irregular solid- put in the water in the measuring
cylinder. Calculate the increase in volume.
Density calculations
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P:124
Density=mass/volume
Mass=density x volume
Volume= mass/density
(Units
1 l= 1dm3
1m3=1000 l)
Q
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1. ρ=78kg/m3 ρ=….g/cm3 (7.8g/cm3)
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2.V=4,2cm3, ρ=2500kg/m3, m=? (glass spheres-marbles) (m=10,5g)
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3. m=3300kg, ρ=2200kg/m3, V=? (1.5m3)-concrete block
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4. one object m1=5kg,V1=0.001m3;other object m2=150g, V230cm3. Which
of these objects has a greater density?
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5. m=?, a=10cm, ρ=800kg/m3 (wood block)
Floating and sinking
• When a piece of wood is placed in water the
wood floats.
• This is due to the difference in the densities of
the wood and the water.
• When two substances, such as a solid and a
liquid or a liquid and a liquid, are put together
the less dense substance floats above the
denser substance.
Archimedes and the crown
• Archimedes is a Greek mathematician, lived in
Syracuse (Sicily).
• He made important discoveries, about the crown
from King of Syracuse.
• Archimedes put the crown in container of water
and measured the rise in level.
• Then he did the same with an equal mass of pure
gold. The rise in level was different. So the crown
could not have been pure gold.
• He measured the volume of irregular solid.
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To do:
Read
Fined some example
To repeat