The aim of my investigation is to determine the specific heat capacity of aluminium.
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The Specific Heat Capacity of Aluminium Aim: The aim of my investigation is to determine the specific heat capacity of aluminium. Theory: Specific heat capacity is the amount of energy required to raise the temperature of 1kg of mass by 1degree Celsius.(1) In order to calculate the specific heat capacity heat capacity (c) of aluminium I can use the equation, H=mc T (2) Therefore c= H m T I can measure the mass of the aluminium, (m), and the change in temperature of the block, ( T), however the energy change, ( H), is hard to measure. Another equation I can use to calculate H is the equation E=ItV (3). I can measure the energy supply using an electrical heater and recording the current the time and the voltage. Using the first law of thermodynamics or conservation of energy (E=(H. Therefore ItV=mc(T. Using this knowledge I can design the basic circuit required to record the necessary measurements. The aluminium block has two holes one containing a heat filament and one containing a thermometer. In order to try and measure the energy from the heating filament entering the block I need to prevent as much heat being lost into the environment as possible. Heat can escape in three ways, by convection, conduction and radiation. 'Convection involves the bulk motion of a fluid (liquid or gas) and is usually caused by hot fluid (being less dense) rising and displacing cold fluids. Radiation involves the emission and absorption of electromagnetic radiation. Conduction takes place in solids and in fluids, regardless of any bulk motion of the fluid.'(4) Conduction, like convection, requires a temperature difference to be established within the material, which conducts. In order to overcome the conduction the conduction and radiation from the aluminium I will lag the block with polystyrene to prevent conduction and convection and also silver foil to prevent radiation. The polystyrene contains air, which is a very poor conductor, the air is trapped and convection can therefore not occur.
Studying results table three which was when the aluminium was heated for 10 minutes and timed for a further 10 minutes in which the heating source was switched off, it can be seen that the specific heat capacity of the aluminium block was 922 Jkg-1 oC-1. This value is very near the text book values for the specific heat capacity. I was surprised to find that this set of readings were nearer to the actual value than the readings shown in results table 1, which was when the aluminium was only heated for five minutes. I would have predicted that these results would have been the nearest to the actual reading as the final temperature reached would have been lower than that of any of the other sets of results, the temperature gradient would therefore be smaller between the block and the air, therefore convection would be less likely to occur and heat would not be lost producing more accurate results. This was not the case, however it can be seen that for results number 1 there is in fact no drop in temperature once the heating source was switched off which was what I would have predicted. Therefore the inaccuracy of these results was not due to the loss of heat and is more likely to be the result of a time delay ie the time taken for the heat from the heating filament to be transferred into the aluminium block and then into the thermometer. That is why the maximum temperature recorded is after the heating filament is switched off. Only 10 readings were taken in total, where as for results 3, 20 readings were taken which is double that of results1. Fewer readings were taken recording results one than for results three, this may have resulted in less reliable results. However the results are likely to be more accurate as a lower final temperature is reached and therefore the heat gradient is less and less heat will therefore be lost.
This explains why results 1 were not effected by heat loss to the environment as the final temperature was lower as the initial temperature was lower and the experiment was not undertaken for as long. The temperature gradient was less therefore no heat was lost. A way to eliminate the need for a cooling correction altogether is to cool the aluminium to about 5 degrees below room temperature and the heat steadily to about 5 degrees above, the heat gained by the aluminium from the surrounding during its initial heating will be almost identical to the heat lost by the aluminium into the surrounding in the second half of the experiment. The cooling correction is therefore not necessary. In this experiment the position of the thermometer is very important as the temperature varies throughout the block, the centre of the block being hotter than that of the edge of the block. However there was only one possible place in which the thermometer could be placed. An improvement to the experiment would be to vary the place which the thermometer was placed within the aluminium block and see how the results are affected. This could not be done in school, as the equipment was not available. Another improvement could be to use light gates instead of using stop clocks as this means that human reaction times are not involved and the results will therefore be more accurate. To reduce the error caused by the time taken to read the stop clock thermometer, the voltmeter and the ammeter you could be ready looking at the thermometer as the required time is approaching and the read the temperature at the exact time as this is the result which changes most frequently and has the most influence on the results. The reading on the ammeter and voltmeter remain almost constant with slight fluctuation, however an average reading of these can be taken so a slight delay in reading these values will have little effect on the results.
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