Heat loss from polystyrene cups SAMPLE ACCOUNT

AIM OF THE EXPERIMENT

(IMPORTANT)
In my practical coursework I have decided to investigate how the loss of heat from a polystyrene cup is affected by the thickness of the sides of the cup.

WHAT I AM GOING TO DO

(IMPORTANT)
To do this I will collect a polystyrene cup, fill it with hot water and measure the drop in temperature of the water during a set time.
I will then repeat the experiment but put another cup round the first to double the thickness of the sides. I will repeat the temperature measurements. I will go on doing this for up to a thickness of eight cups.

WHAT AM I GOING TO CHANGE (my variable)

(IMPORTANT)
The thickness of the cup sides (in this experiment there is only one variable)

WHAT AM I GOING TO KEEP CONSTANT

(IMPORTANT)
The starting temperature of the hot water and the time of cooling.

SAFETY

(you should always mention this somewhere)
I will be heating the water with a bunsen burner so I will wear goggles for this. I will also be careful when pouring the hot water into the cups.

PREDICTION

(you may not always need to do this)
I predict that as the thickness of the cup sides gets bigger the temperature of water in the cup will not go down so much.

PRELIMINARY EXPERIMENT

(this may not be needed)
I will do a preliminary experiment to get a rough idea of how long I should time for. This is necessary because if the time is too short there will not be enough cooling to measure and if the time is too long I may not be able to finish the experiment.

SCIENTIFIC THEORY SAMPLE

(there are lots of different levels possible here)

The bigger the temperature difference between the water and room temperature the quicker the temperature of the water will go down. This is why I am going to always start timing when the hot water is at the same temperature. I will try to start at 80oC each time.

The thicker the wall of the cups the more difficult it will be for heat to travel through it. So by putting one cup inside another I will be making the wall thicker. I would expect that it would be twice as difficult for the heat to travel through two cup walls than through one and so for two cups the temperature should drop twice as slowly. This means that in 5 minutes the drop in temperature for two cups will be half what it was for one cup.

I have looked up some advanced theory and I have found that for a particular material the temperature drop is inversely proportional to the thickness of the walls. In other words double the thickness and you will halve the drop, three times the thickness and you will reduce the drop to one third.
However I have got to remember that the water is not surrounded by polystyrene. The top of the water is open to the air so I am going to make a paper lid to fit over the cups. This lid will always be the same thickness and so my results may not quite agree with my theory.

WHAT I WILL NEED

(important if other people are to check your experiment)
Eight polystyrene cups of the same size
Paper to make a lid
Scissors
Thermometer
Bunsen, tripod, beaker, heat resistant mat, gauze, cloth for holding beaker
Safety goggles
Stop watch

DIAGRAM OF MY APPARATUS

(notice that it is clearly labelled)


Warning and suggestion: only draw your diagrams using the computer if you are really sure that you can make them look the way that you want. Like typing the account it can waste you a lot of time and it may not turn out right in the end.

WHAT I DID

(A simple account of the experiment) SAMPLE

I heated the water in a beaker to nearly boiling then I careful poured it into one of the cups. I covered it with the paper lid and put the thermometer in. When the temperature had fallen to 80 ^oC I started the stop watch. After 5 minutes I took the reading of the stop watch.
I recorded the start and end temperatures in a table and then worked out the drop in temperature.
I repeated the same procedure for two cups, then three up to eight. Each time putting the cups one inside the other.
Before taking each thermometer reading I stirred the water gently to be sure that it was all at the same temperature. (You should always do this).

I repeated some of the results ads a check.
(This is a very good idea – sometimes you can plot the graph before you repeat results. Looking at the trend line on the graph will show you which ones to do again).

OBSERVATIONS AND RESULTS SAMPLE

(This section has not been completely filled it. It is shown as an example of how you can present a good results table)



The temperature was taken after five minutes
The same starting temperature was used each time.
The mass of water in the cup was the same each time (350 g)

A note on results tables
Each column clearly headed with the quantity and the right units – in this case ^oC.
Numbers written in neatly
Table ruled out – yes, using a ruler!
Record all your results – if you average the readings still record both values.

Repeated results SAMPLE

I check a number of the results to make sure that I got an accurate graph.
One of them (for 5 cups) in the first trial did not fit the line.
(We have just guessed that one of them is out. We call this an anomalous result and it should be checked).

CALCULATIONS, ANALYSIS AND CONCLUSIONS SAMPLE

I worked out the temperature drop for each experiment and plotted a graph to show the change. The graph shows the temperature drop on the Y axis and the number of cups on the X axis.

You can see that my prediction was about right, the line goes down which shows a smaller drop in temperature for more cups. However the drop does not quite halve when I doubled the number of cups. This is due to some inaccuracy in my readings, loss of heat from the lid and a change in the temperature of the room.

CONCLUSION (always put one it so that it can be seen clearly)

The temperature drop is less when the thickness of the walls of the cups is bigger. Putting one cup inside the other showed this. The drop in temperature almost halves when the thickness of the cup walls is doubled.

This agrees with my theory (see planning section). (you don't need to write out the theory twice)

GRAPH

Graphs must have:
a title
labelled axes with a scales that the final graph fills the graph paper
correctly plotted points
a best fit line – sometimes called a trend line
you should plot all the points, even the ones that do not seem to fit your line
they may not always need to show the zero point on either axis
try and make them fill the graph paper

EVALUATION, IMPROVEMENTS AND ALTERNATIVES SAMPLE

I think that the experiment went fairly well and that the results give a firm conclusion to support my prediction. I found one anomalous point on the graph but to check this I repeated that result (as well as one or two others).

To improve my experiment I would take the following precautions:
I may need to shield the cups and thermometer to avoid draughts.
What about the air gaps between the polystyrene cups?
Could I have used polystyrene lids?

I would try to use a more accurate thermometer

Alternative experiments could be done with other cups and different starting temperatures.