I believe a graph showing the relationship between the 2 factors stated would show that the heated water started to lose heat as the water heated past the temperature of the atmosphere it was being tested in. A graph may not show an immediate temperature change, as instruments will be slow to convey change and human error may also contribute to the late noticing of minute changes on inaccurate instruments.
I believe that a graph will demonstrate a curve whereby temperature rise is proportional to time taken until a point, (around 5 degrees Celsius above the original temperature of the water) where the rate of temperature rise will slowly slow down. This difference will only be minor to begin with, however as the temperature rise continues to increase the it will take a considerable amount more time for the same rise to be observed.
I believe that when the time the water heater is allowed to heat the water increases more of the above graph will be exposed. By this I mean that to start with as the time of heating increases there will be a close to proportional temperature increase difference, however as the time continuous to increase the temperature rise difference will seem to slow. This is because as the water gets to a higher temperature it will lose more heat, thus meaning that time taken to heat the water is no longer directly proportional to increase in temperature. As the time gets to a certain length there will be little or no increase, as the heater has heated the water to a temperature at which the amount of heat energy lost by the water is the same as the amount of heat energy provided by the heater at the set voltage.
I also believe that when the amount of water being heated is doubled the temperature rise will be halved. This is because the heater will need to provide the water with twice as much energy to raise it by the same amount as when there is half as much water, due to their being double the amount of water to heat. Equally when twice as much power is used, (double the voltage multiplied by current), the same volume of water will rise by twice as many degrees as it would when half the power was used.
Pilot Study
I intent to investigate how the mass of water heated effects the temperature rise when time is constant, and the power of the heater remains the same. I intend to investigate this, as I believe that in the eventuality that results are inconclusive the time taken in one specific water mass can be examined and used to draw conclusions on how the time heated for affects the temperature rise of the water.
Apparatus
Polystyrene cup, water, thermometer, voltmeter, ammeter variable power pack, 3 circuit wires, stop clock, electronic balance, electric heater, glass beaker.
Method
A polystyrene beaker should be taken and placed on the electronic balance. The weight should then be reset to zero. The appropriate amount of water should be poured in and then subtracted to until the mass of the water is equal to the set amount being tested, around 28g (minimum water safe to use), or around 56g (double the previous measurement). Both measurements will need to be tested so that a suitable set of results can be analysed to see the effects of heating time on the temperature rise. It is best to do one at a time to ensure safety, (so that each set of heated water is being watched), and for ease in recording results to an accurate level.
A thermometer should be placed in the water and left to acclimatise to the water, so that an accurate start temperature is ascertained. The heater should be set up in the circuit, (as shown above). When ready to start, the power pack should be set to 6 Volts, (according to the voltmeters reading) and the stop clock should be started and a reading of the temperature should be taken every 30 seconds. This should give an accurate description of the temperature increases over the 10-minute period of time that the water is heated for. Results for shorter times can then be extrapolated by simply examining parts of the results, (for example the first 3 minutes of heating could be examined showing a lower temperature loss than if the experiment is continued).
Results
Apparatus
Variable power pack, voltmeter, ammeter, electrical heater, thermometer, 3 circuit wires, 2 polystyrene cups, distilled water, stop clock, electronic balance and a glass beaker.
Method
Two polystyrene cups should be taken, and cut to fit so that they meet the specifications of the insulated cup shown above. The cup to contain water should then be placed on the electronic balance. The balance should be reset and then water can be measured out until the appropriate amount is left in the cup. The cup should then be placed in a glass beaker. This will serve to trap a pocket of air, providing further insulation, as well as ensuring that if the polystyrene cup should break the hot water will not scald anyone, as it will simply drip into the glass beaker.
To ensure that repeats taken provide a constant and reliable field of results it is important to exactly replicate the conditions of each experiment. This means that if at all possible the same polystyrene cups and electric heater should be used. A further precaution is that the water being heated must be stirred with the thermometer, or another tool that is the same temperature as the water. This will ensure that minimal heat is removed from the area that the thermometer is testing by convection currents. A sufficient amount of water must also be used. This must be enough that the whole of the bulb of the thermometer is covered, (for accuracy), and that the whole of the heater is covered in water as it will superheat if not completely submerged in water, (this is because water carries heat away from the heater at around 20 times the speed that air does).
When ready to begin the power supply should be turned to provide 6 Volts, (read from the voltmeter for accuracy, any more will ruin the electric heater and cause anomalies in the results). The stop clock should be started. Readings should be taken every 30 seconds and recorded immediately, to ensure results are not forgotten. Temperatures should also be taken carefully before every 30 seconds so that errors do not occur in reading the temperature after the exact time slot, (when the temperature has risen to a further extent).
The experiment should be repeated a minimum of 3 times so that a mean average can be taken from the results. I will then examine my results and extract sections to show how the temperature rise is affected by the time the water is heated for.
Once the water heating has been monitored for at least 10 minutes the results should be tabulated and then plotted on a graph showing the relationship between temperature rise and time taken to heat the water. This graph can then be easily compared to the temperature rise that would occur if no heat were lost, (the ideal case), thus showing how the length of time the water is heated for affects the temperature rise of the water, and also the amount of heat that is lost by the water at any time during the experiment.
Results
Analysis
The results collected were plotted on a graph, (time against temperature rise). This graph shows temperature rise and time are directly proportional, however as time taken carries on increasing temperature rise starts to slow down, causing less of a heat rise for subsequent times. This shows that heat is being lost, as the water’s temperature is no longer increasing proportionally to time.
I have drawn a line to show the ideal case on my graph to supplement the line of best fit. The ideal case line shows how the water should react if no heat is being lost. From this it is clear to see that as the temperature becomes greater and greater above that of the room more and more heat escapes, (difference between ideal case and line best fit grows larger as the temperature increases). This means that as the temperature increases the rate of reaction begins to slow down.
The results recorded from the experiments performed prove my hypothesis, (that temperature rise is proportional to time taken up until a point where the water begins to lose heat), is accurate. If the two factors were completely proportional then there would have to be no heat loss from the water at all.
My results show that there is only a noticeable heat loss from the water after 3 minutes, when the temperature has increased by 29 degree Celsius. From here onwards as the water heats up even more the heat loss, (difference between ideal case and line of best fit), becomes even greater.
From my graph results for other heating times can also be drawn. For example it can be seen that when water is electrically heated for 3 minutes heat loss is almost unnoticeable, whereas after ten minutes there is a large degree of heat loss.
If the experiment were to be continued there would come a point at which the heat loss of the water would be the same as the amount of heat being provided to the water. At this point the water would no longer rise in temperature, but would stay the same, (until water had evaporated forcing the experiment to be stopped due to the heater not being completely covered by water). My results indicate that this situation occurs when the water has increased by around 64 degrees Celsius to a temperature equal to approximately 80 degrees Celsius. It is possible that the water will still increase in temperature, however this will happen at a slower rate until a point where there is no noticeable increase in temperature, (preliminary work suggests that this point is around 96 degrees Celsius, as the temperature did not rise for ten minutes after it reached 96 degrees Celsius).
I believe that if less insulation were used in the experiment, (no glass beaker, and no lid) then heat would be lost at a faster rate from a temperature lower than 29 degrees Celsius, producing a flatter heating curve that will not reach such a high temperature.
In conclusion I believe that temperature rise and time taken to heat water are proportional, until a point where the water temperature exceeds that of the surrounding environment. At this point heat will be lost meaning that subsequent heating time will yield less of a rise in the water’s temperature. The water will then eventually reach a point at which heat loss is equal to heat gained from the heater, at this point the water will no longer increase in temperature.
Evaluation
I believe that the results collected were sufficiently accurate to base my conclusions on. Despite there being anomalies in my results I believe that my conclusions are still accurate. A series of 3 anomalous results did occur in the third experiment. These showed a complete stop in temperature rise at around 50 degrees Celsius. This is because the thermometer became partially removed from the water; meaning that though it still read the temperature increase it was being cooled at the same rate. Once the error was realised the thermometer was placed back in the water and monitored. It then proceeded to indicate a regular increase in temperature that resembled the results from the other experiments.
A mean average was taken from the 3 sets of results. This limited the effect of the anomalies upon the overall average results of the experiment. I have still labelled the 3 affected readings as anomalies.
To draw my conclusions I used results from the other 2 experiments, as well as the area of the graph not affected by the anomalies. I believe this still provided an accurate overview of my findings, as all of the results gathered obeyed the same trends.
I believe that there were only minor inaccuracies in the recording of results due to human error, (in reading the thermometer), as special care was taken to check the reading before the given time interval so that the results recorded were accurate for the exact time of the reading. Because of measures like this in recording results to the highest possible degree of accuracy, along with the compensation for anomalous results, I believe that my results were accurate and reliable enough to base my conclusions on.
Even though results were accurate I believe that certain measures could be employed to increase the accuracy of results in the future.
I believe that a see-through plastic lid, (similar to those found on soft drinks) could be used instead of another modified polystyrene cup. This would provide better insulation due to the more precise fit, as well as helping to irradiate anomalies caused by the thermometer coming out of the water.
To increase accuracy I also propose that an electric thermometer may be more accurate in measuring the temperature of the water, (as well as being easier to read). More accurate voltmeters and ammeters could also be used to ascertain the power of the heater to a more accurate degree, (if this were to be investigated).
Further experiments could be performed to supplement conclusions drawn from this experiment by other affecting factors in the heating of water being investigated. Investigations into the mass of water and the power of the heater would prove useful in drawing further conclusions as to the effect of factors involved in the electrical heating of water. This could be achieved by the mass of water, or power of the heater being varied, then the same experiments where time is varied to gauge the affect on temperature rise could be repeated to see if the same conclusions could be drawn when the mass of water, or power of heater were constant at a new value. This could then be compared to the original results, and the effect of the new variable could be seen in relation to how the time taken to heat water affects the temperature rise.
