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FILTER HOLES
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Objectives
- To compare the hole sizes in several
common filters, and the number of holes in a given area.
- To measure hole sizes and calculate
the number of holes in a given area
- To compare the number of holes in common
filters with those used in the filtration of pigments.
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Curriculum links
English National Curriculum
Design & Technology: 5a
Mathematics: 3: 1a, 1c, 1e, 1h
| Gateway story
Filters are used in the colour industry in many ways, including
the separation of a solid (powder) from a liquid, and the separation
of solids (powders) according to particle size. An example of the
first kind of filtration is required when a pigment is first made,
as it appears as part of a 'slurry'. The slurry has to be separated
into the pigment and the waste liquid. An example of the second
kind of filtration occurs to ensure the quality of a pigment, which
requires the removal of particles that are considered 'too large'.
Gateway elements
- Animated graphics of a filter press
- Chameleon looking at large particles of pigment.
Animated graphics of the filter press: The slurry enters
the press from the left. It passes under pressure (i.e. it is 'squeezed')
through the filter membrane or cloth. The solid pigment remains
on one side of the filter, and the liquid passes through. When the
press is full, it is opened, and the filter cake ('squashed, damp
powder') removed.
Gateway discussion
Whilst looking at the gateway, ask the children the following in
addition to the main gateway question:
- How is the filter press working?
- What is it separating?
- Could small and big particles ('bits') be separated in a similar
way?
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Approximate time required: 30
minutes
Resources needed
Per group:
Colander
Sieve
Tea-strainer
2-3 hand lenses
2 blue pigment samples of different particle size, in transparent containers
(see List of
Suppliers).
Suggested organisation
Children work individually.
Carrying out the activity
Children are first asked to measure the size
of holes in the three filters.
(Note: A colander can have holes of various sizes, often getting smaller
towards the centre. Children could measure different holes and take an
average, or choose which holes to measure).
They should discover that it is difficult to
measure the holes accurately. Tell the children that another means by
which the filters can be compared is by counting the number of holes in
a given area. This could be left as an open-ended problem to solve, or
children can be given a number of the following suggestions (depending
on the level of differentiation required):
- a rubbing can be taken of a small area of
each filter (the sieve and tea-strainer can be placed over a rounded
object, such as a ball, to aid this process)
- a square centimetre is drawn onto the rubbing
- the 'dots' from the tea-strainer and sieve
rubbings can be joined to make squares
- squares that are whole or greater than a
half are counted within the square centimetre.
A set of results may look like this (but will
of course vary with the individual filters used):
| Filter |
Number of
holes/cm2 |
| colander |
4 |
| sieve |
19 |
| tea-strainer |
49 |
| filter
for pigments |
49500 |
To help children appreciate the fineness of
the filters used in the colour industry, the final figure is added to
the children's charts when they have completed their calculations. The
filter used to separate the fine particles of pigments (show children
the samples) has approximately a thousand times more holes per square
centimetre than the tea-strainer. The holes are so small that they are
not visible to the human eye.
The only difference between the two samples
of blue-coloured pigment is the particle size of each of the powders.
Extensions / links
Mathematics
Some of the most able Year 6 children may be
able to take this problem a stage further, by being asked to calculate
the approximate hole size from the number of holes per square centimetre.
They can then compare this answer with the area of each hole they originally
measured. For example, a typical example for a tea-strainer might be:
1 hole measures 2.4 x 2.4 mm2 = 0.24
x 0.24 cm2 = 0.0576 cm2
If there are 19 holes in 1 cm2 of
tea-strainer, each hole measures 1/19 = 0.0526 cm2.
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