Heat Loss


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Biology Coursework Practical – Heat Loss

The aim of this practical is to find to what extent does the surface area to volume ratio of an object affect the rate of heat loss from the object

Hypothesis: As the ratio of surface area to volume of an object decreases the rate of heat loss from the object will also decrease. Objects with the same surface area to volume ratios loose heat at the same rate so long as there are not other variables involved.

Background Knowledge:

     The surface area to volume ratio of an object is determined by dividing the surface area by the volume and putting it into a ratio of one.
e.g. A flask of volume 200cm3 and a surface area of 160cm2 will have a surface area to volume ratio of:
     160
     ---- : 1     =     1.25 : 1
     200
     Objects that are not the same size but have the same surface area to volume ratios loose heat at the same rate. So a flask, with a volume of 200cm3 with a surface area of 160cm2 and a surface area to volume ratio of 1.25:1, will loose heat at the same rate as a similar flask of volume 625 and a surface area of 500 which also has a surface area to volume ratio of 1.25:1. However, generally when you increase the size of an object the surface area to volume ratio decreases so in this example it is very likely that the two flasks in question are different shapes.
     In this experiment the two flasks which will be used will different surface area to volume ratios as follows:

100 cm3 flask: Volume = 100, Surface Area = 115. Surface Area to Volume Ratio =

     115
     ---- : 1     =     1.15 : 1
     100
500 cm3 flask: Volume = 500, Surface area = 330. Surface Area to Volume Ratio =

     330
     ---- : 1     =     0.66 : 1
     500
     As it is seen the ratio is lower in the 500 cm3 flask. This means that the rate of heat loss should be less than the 100 cm3 flask.
     Heat is lost by three different processes:
* Conduction
* Convection
* Radiation..
Conduction is the process by which heat is transferred from on solid to another. When a solid is heated the molecules inside, which are normally almost static, start to vibrate. When another solid is brought into contact with the heated solid the energy from the vibrating molecules at the edge of the heated solid is transferred to the outer molecules of the other solid.

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This energy then spreads through the second solid until it is also heated. This process is particularly effective in metals.
Convection is the way by which liquids and gases transfer heat. When gases and liquids heat up they rise and so when air is heated it rises. However as it rises it looses energy to its cooler surroundings. Eventually it cools to a point where it begins to fall. Once it is near the source of heat it begins to rise again and takes heat up with it. These cycles are known as convection currents and it is the way by which gases and liquids transfer heat. Convection currents become more efficient when they have a larger volume of air so many insulators work on the principle of restricting the movement of air. For example cavity wall insulation reduces the space in which air has to move and so reduces the efficiency of convection currents.
Radiation is the way that heat is transferred from a solid to a liquid or gas. It is a mixture of conduction and convection as, like conduction, heat is transferred by contact but then it is also like convection because the heat pushes the molecules away bringing in colder molecules.
When heat is transferred from one medium to another, as with radiation, there is a degree of inefficiency which results with not all the heat being transferred. As in this experiment we will be using water in a glass container the effect of the heat having to change from a liquid to a solid to a gas will have a slight effect on the readings taken however this probably will not have a very significant effect.

Method:

     For this experiment two flasks will be set up. They will have volumes of 100 and 500 cm3. These will be attached to a clamp stand and then filled with warm water. Warm water will be used as then less time will be required to heat the water to the desired temperature of 90oC. One the flasks have been filled they will be heated to get the water to 90oC. Once the flasks are both at 90oC bungs will be placed in the top which have thermometers running through them. Using these thermometers the temperature will be recorded every 30 seconds for 20 minutes. This will then be repeated. The experiment is repeated to improve the accuracy of the data.
     In order to ensure a fair test the experiment will be started at the same temperature each time and will also be allowed to run for the same length of time. Also the depth of the thermometers in each flask will remain constant for both tests. The volume of water will be as equal as possible in both experiments however due to human inaccuracy it is unlikely that the volumes will be exactly the same. Hopefully any variations between experiments will be so small as to be of no consequence.
     At the end of the experiment the results will be plotted in a table and the average results will be calculated and plotted on a comparative graph so that it is possible to see any variations between the two sets of data.

Results:

Minutes     Temp 100cm3 - 1     Temp 100cm3 - 2     Temp 500cm3 – 1     Temp 500cm3 - 2     Average – 100cm3     Average – 500cm3
0     90     90     90     90     90     90
1     88     88.5     89.5     88.5     88.25     89
2     86     87     88.5     87.5     86.5     88
3     84.5     85     88     87     84.75     87.5
4     83     84     87     86     83.5     86.5
5     81.5     82     86     85.5     81.75     85.75
6     80     80.5     85.5     84.5     80.25     85
7     78     79     84.5     83.5     78.5     84
8     77     78     84     83     77.5     83.5
9     75.5     76     83     82     75.75     82.5
10     74     75     82     81     74.5     81.5
11     73     74     81.5     80.5     72.5     81
12     72     72.5     81     80     72.25     80.5
13     70     71     80     79     70.5     79.5
14     69.5     70     79.5     78.5     69.5     79
15     68     69     78.5     78     68.5     78.25
16     67     68     78     77     67.5     77.5
17     66     67     77     76     66.5     76.5
18     64.5     65.5     76.5     75.5     65     76
19     64     65     76     75     64.5     75.5
20     63     64     75     74.5     63.5     74.75

Analysing the Results:

     These results agree with the prediction that was made. As the graph shows the 100 cm3 flask looses heat much faster than the 500 cm3 flask. So much so that the final result for the 500 cm3 flak is 11.25oC higher than the final result for the 100 cm3flask. All four sets of temperature readings, as well as the averages, show the same trend. As time passes heat is lost at a steady rate. This may not be true for lower temperature readings, however within the limits of this experiment it is impossible to predict any great variation away from the stated trend. The graphs show fair results, however places where error occurred are obvious as the graphs gradient changes because the decrease in rate was less than what was the norm for that flask. Generally for the 100 cm3 flask the decrease in rate was around 1 degree per minute, with some exceptions. For the 500 cm3 flask the rate was generally 0.5 degrees per minute, again with some exceptions. This is what we expected as the surface area to volume ratio of the 500 cm3 flask is roughly half of the surface area to volume ratio of the 100 cm3 flask. This is shown in the readings that, on the whole, the rate for the 500 cm3 flask was half that of the 100 cm3 flask.
1.15 / 2 = 0.575     Therefore 1.15 / 2 * 0.66
     The reason for this is because of two things. In the small flask and the large flask the water molecules are moving at an equal speed at the start of the experiment. However as time passes the molecules start to slow down. The reason that they don’t slow down at a faster rate, as they would if they were on their own, is because all the molecules in the flask are colliding with each other, thus maintaining the momentum. The difference between the small flask and the large flask is that in the large flask there is a greater number of molecules to collide, this means that the odds of a collision occurring are much higher. So although the number of collisions is not enough to maintain the heat, the proportionally higher number of collisions in the higher flask means that the heat decreases at a slower rate as the collisions maintain the heat. The second reason for the slower heat loss in the larger flask is that, while the proportionally higher number of collisions in the centre maintains heat there, the number of molecules hitting the sides is equal. This means that the heat loss from the collisions between water molecules and the glass of the larger flask is proportionally equal to the small flask. While heat at the perimeter is lost at the same rate the heat at the centre goes down at a slower rate and this maintains the heat for longer in the large flask hence the slower rate of heat loss.
     There may be other factors such as the different thickness of glass in the two flasks. The large flask has thicker glass as it has to support more weight. Also there are other small factors such as the thickness of the bung and exact location of the thermometer. However these are so insignificant to be of no consequence.

Evaluating the results:

These results have shown what we predicted and as they were obtained from a well regulated and controlled experiment we can assume then to be accurate. The results do have some minor flaws, however these are not very serious and do not compromise the accuracy of the experiment. These slight experimental errors are, almost entirely, human errors. Any errors that were not caused by human error when measuring are not known to be. If there are any other errors apart from human error then they are most likely to be unstoppable, for example the ambient room temperature which is not controllable under the circumstances.
There are ways in which these results could be improved or the experiment changed to give better and more useful data. The factors that were entirely out of our control were: the room temperature and the movement of air in the room. The only way that these factors could be countered would be in a commercial laboratory where the experiment could be conducted under strictly controlled environmental conditions. However it is expected that these factors would have little affect on the overall change in temperature. The factors that also may have affected the results were: the purity of the water; the accuracy of the measuring instruments; the accuracy of timing and the accuracy of the volume of water. All these factors could be improved upon. Distilling the water would remove any impurities, which could alter the rate that the water itself lost heat. By using electronic thermometers and timing equipment the accuracy of the readings could be slightly improved. However it was considered that the readings were probably accurate enough as they are. The volume could be more accurately measured using a more accurate measuring instrument than the eye. Equipment such as a pipette or similar could be used to this end. Other than this there are no major factors which might need changing.
The experiment could be changed to investigate the heat lost from flasks of the same size covered in various materials. These could be substances like: black paint; cling-film; kitchen foil; silver paint; lard and foam plastic. These provide a wide range of different insulator properties. The black paint absorbs and emits more heat radiation because of it colour whereas the foam traps air and so prevents efficient convection currents from being set up. This insulates by a different means but could, in principle, be equally good or better. The lard is suggested because it is close to blubber, it would be interesting to consider this as this is the way that animals naturally insulate themselves. For example animals like the seal, walrus and sea lion all have blubber layers as they live in the arctic where it is cold. On consideration that would have to be made with this experiment is the thickness of the insulatory layer.
To conclude, the results showed the trend which was predicted at the start of the experiment. Although there were some discrepancies these were not serious enough to be considered as invalid. On the whole the experiment achieved its aims and proved what was expected.


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