Density
| If a forest is 'dense' the trees are closely packed together. Dense objects have their particles packed tightly together. The more you can pack into a smaller space the greater the density will be! Density therefore involves how much 'stuff' you have (mass) and how much space it takes up (volume). |
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Take a look at the two boxes on the right. Each box has the same volume. If each ball has the same mass, which box would weigh more? Why?
The box that has more balls has more mass per unit of volume would weigh the most. This property of matter is called density.
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The density of a material helps to distinguish it from other materials. We can list the mass of a cubic metre of lots of materials and compare them. Small items made of the densest materials would still be quite ‘heavy’ whereas large items made of the least dense materials would still be quite light!
THE GENERAL EQUATION
Density tells you how much
mass has been packed into a given volume
of space.
At KS3 this is abbreviated
to:
but at A level we use the symbol 'ro' for density ( 
) making the equation:
UNITS
It is usually expressed in
how much mass (in kilogrammes) you can pack into one cubic metre:
Values for densities are listed
in tables using this unit, so that people can look them up and use them.
In exams and tests you will
sometimes find it expressed in how much mass (in grammes) you can pack
into one cubic centimetre:
Work in whatever units you
are given.... don't change from one to another unless you are told to.
However there is a relationship between the two units.
A kilogramme is a thousand
grammes and a cubic metre is a million cubic centimetres, therefore the
density in kg/m3 is always a thousand times bigger than the
density expressed in g/cm3
PERFORMING CALCULATIONS
Always lay your calculations
out carefully.
- Extract
information from the question and list it on the right hand side
of the page
- Ensure
that you write in the units for the values too!
- Write
out the basic equation
- If
necessary work out values required from the information given
- Substitute
the numbers into the equation
- Calculate
the answer
- Put
in the correct units
The density of an object determines
whether it will float or sink. See the Archimedes
Principle.
TO FIND THE
DENSITY OF AN OBJECT
1. Use a set of scales
to find its mass (in grammes or kilogrammes)
2. Find its volume.
- If it is regular
you can work it out from measurements.
- If it is irregular
you will need to use a displacement can or Eureka can (together
with a measuring
cylinder) to measure the volume.
3. Calculate its density
(as shown above).
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