Biology Heat Loss Practical Write Up Essay

, Research Paper

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. 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.