Education and Manpower Bureau
Using Datalogger in the Teaching of Physics

Suggested Teaching Activities:

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
 To observe air stratification in our daily lives.

Equipment
 Clamps and stand. Datalogger interface connected to a PC 2 temperature sensors Metre stick BluTack/tape

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.

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

 Datalogger interface connected to a PC 2 temperature sensors 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

 Datalogger interface connected to a PC 2 temperature sensors 2 containers for the water Salt Freezer

Experiment Set-up

 Fill both cups with enough water to cover the temperature probe. Obviously the more water used, the longer the experiment will take. 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. Place cardboard with two cut out holes on top of the container. This stops the water spilling and also provides insulation. 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. 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. Set-up the software to record one reading per minute. 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. Start the datalogger software to collect data and display both channels on a graph. Check that the temperature from both channels begins to decrease. Allow the experiment to continue until both temperatures have levelled off.

Discussion

 1 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.
 Pure Water Start Time Start Temperature Cooling from room temperature towards freezing 0 hours Freezing Cooling to the freezers minimum temperature Stable region at minimum temperature

 2 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. 3 At what temperature did the salt water begin to freeze? Why is this lower than for the pure water? 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). 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? 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. 7 How might the purity of the water effect the shape of the graph? 8 How would the concentration of salt effect the results? 9 What results would you expect if the samples had been boiled rather than frozen? 10 Use the results to estimate the latent heat of freezing (you will need to know the specific heat capacity of water).

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.

2003