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Observing
Air Stratification
Introduction
This
experiment works best in a room with a very high ceiling.
It also works best when there is a mixture of warm and
cold air in the room. This might occur late in the day
during the summer, when there is still hot air in the
room but the air conditioner has been just turned on to cool
the air inside the room.
Objective
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To observe air stratification in our daily lives.
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Equipment
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Clamps
and stand. |
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Datalogger
interface connected to a PC |
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2
temperature sensors |
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Metre
stick |
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BluTack/tape
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Experiment
Setup
Set
one of the external temperature sensors hanging down 3-5 cm
above the floor. Don't let it touch the floor, because
the floor itself might be warmer or cooler than the air
right near the floor. Use BluTack to hold it in place
against the table, but make sure it does not touch the
wall either. There should be air all around the sensor.
Place the other temperature sensor about 2 metres from
the floor, but don't put it right above the computer monitor.
The monitor gives off heat, and this will affect the
sensor readings. Use a bit of BluTack to hold the sensor.
Connect
the 2 temperature sensors to the datalogger interface.
Place the interface in the middle of the 2 sensors. Use
the datalogger software to plot the temperature collected
from the 2 sensors in the same graph against time.
Results
and Answers
At
first the two lines will be very close together. Give
the sensor a few minutes to register their new temperatures.
Now look at the graph. Do the 2 lines show different
temperatures? If so, why do you think this is ? And if not, why
do you think this is?
Conclusion
"Air
stratification" is the result of hot air rising and cold
air sinking. If you see a large difference, then you can
see where all of your heat has gone! It is up near the
ceiling, because warm air is less dense and more buoyant
than cold air, causing it to rise. Cold air is denser and
less buoyant, so it settles down near the floor. What
if you do not see a difference? It could be that your
ceiling is relatively low, so there is not enough room
for the air to stratify (split into warm and cold areas).
It could also be because all of the air in the room is
the same temperature. Since it is the difference in temperature
that causes stratification to occur, air that is the same
temperature does not stratify. Also, if there is noticeable
air movement (like a draught or a fan) in the room, the
air will be mixed together and there will be less stratification.
How
does heat travel through glass?
Objective
To observe heat transfer through glass.
Equipment
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Datalogger
interface connected to a PC |
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2 temperature sensors |
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Tape/sticker |
Experiment
Set-up
Set-up two temperature sensors up to measure the temperature
inside and outside a window. You may find that over
a long period of time it is sensible to record your
data remotely. This means that your datalogger stores
the data and does not need to be connected to the computer
whilst it is collecting data. You will need to connect
the datalogger to the computer to see the data when
you have finished the experiment.
Analysis
| 1. |
How
does the temperature vary each day? |
| 2. |
Which
sensor shows the greatest range of temperatures? |
| 3. |
How
do you think the heat travels through the glass? |
| 4. |
How
do the weather conditions affect the results? |
| 5. |
Try
to use a light sensor at the same time to monitor
the light level on the window. |
Freezing
of Pure and Salt Water
Introduction
This experiment looks at the temperature
of two cups of water as they are frozen. One cup contains
pure water whilst the other contains salt water. The
experiment is ideal for demonstrating how salt depresses
the freezing point of water. It is also a good introduction
to the theories of latent heat.
Equipment
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Datalogger
interface connected to a PC |
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2
temperature sensors |
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2 containers for the water |
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Salt |
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Freezer |
Experiment
Set-up
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Fill
both cups with enough water to cover the temperature
probe. Obviously the more water used, the longer
the experiment will take. |
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Add
salt a little at a time to one of the cups of water
and stir until it dissolves. Stop adding salt
when the solution is saturated (i.e. the salt will not
dissolve). Mark the cup that has salt water so that
it can be identified later. |
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Place cardboard with two cut
out holes on top of the container.
This stops the water spilling and also
provides insulation. |
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Put
one temperature sensor in each cup of water and
secure in place with a rubber band around the cup.
Ideally each probe should be in the middle of the
cup and not touching the edges. |
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Plug
the sensors to the datalogger interface and run
the datalogger software. Name one channel "Pure
Water" and the other "Salt Water". Check that the
temperature readings look sensible. |
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Set-up the software to record one reading per minute.
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Place
both cups into the freezer and shut the door, being
careful to feed the cables for the temperature sensor
out the edge of the door without pulling the sensors
out of the cups. |
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Start the datalogger software to collect data
and display both channels on a graph. Check that
the temperature from both channels begins to decrease. |
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Allow
the experiment to continue until both temperatures
have levelled off. |
Discussion
| 1.
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Look
at the trace showing pure water. By looking at the
changes in the gradient of the graph, see if you can
identify the regions of the graph described below.
For each region, fill in the time and the temperature
at the start of each region.
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| Pure
Water |
Start
Time |
Start
Temperature |
| Cooling
from room temperature towards freezing |
0
hours |
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| Freezing |
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| Cooling
to the freezers minimum temperature |
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| Stable
region at minimum temperature |
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| 2.
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Before
the pure water begins to freeze, the temperature
drops rapidly. Once it begins to freeze the temperature
remains fairly constant. Explain why this is.
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| 3. |
At what temperature did the salt water begin to
freeze? Why is this lower than for the pure water?
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| 4. |
Identify the sections of the graph that show the
freezing of the pure and salt waters. The pure water
shows a fairly flat region whilst the salt water
has a gentle slope. Explain why. (Hint - more salt
can be dissolved in warm water).
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| 5. |
If you look carefully at the region where freezing
starts you may notice a small slip where the temperature
actually increased for a short period. Why did this
happen?
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| 6. |
Think about what happens to the temperature of the
two samples as they warm back up to room temperature.
Sketch a graph of temperature against time showing
what you think will happen.
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| 7. |
How might the purity of the water effect the shape
of the graph?
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| 8. |
How would the concentration of salt effect the results?
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| 9. |
What results would you expect if the samples had
been boiled rather than frozen?
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| 10. |
Use
the results to estimate the latent heat of freezing
(you will need to know the specific heat capacity
of water).
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Teacher's notes
The experiment does not take long to prepare, but will
take a few hours to carry out. For this reason some
planning is required. Either start the experiment first
thing in the morning, or alternatively start in the
afternoon and leave to run overnight (this will require
the PC to be left running).
It
is possible that the purity of the water may have an
effect on the results. Bottled water may have better
results than tap water.
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