Factors affecting heat loss from the body

2

Large and small surface-area to volume ratio

Animals in warm climates frequently have large extremities, such as ears, when compared to related species form cold climates. Fennec foxes have much longer ears than their European counterparts which in turn have longer ears than the Arctic fox. Being well supplied with blood vessels and covered with relatively short hair, ears make especially good radiators of heat. Animals in colder climates have a tendency to be more compact, with smaller extremities, than related species in warm climates. In this way heat loss is reduced.

3 4

Safety

. Be extremely cautious - very hot liquid scalds.

2. Reduce accidents by utilising a large working area and working in pairs.

3. Be Careful when using scissors and don't walk around the room with them.

4. A laboratory coat is not essential for this experiment, it is optional and up to the participating student.

Fair test

. Ensure that I carry out a fair investigation.

2. Control the volume of liquid using accurate measuring instruments.

3. Use similar shaped beakers.

4. Take the water from the same source.

5. Make this a fair experiment by repeating both experiments. Results should be consistent.

Variable 1: Insulation

Planning:

Apparatus

* 5 x 200ml beakers

* 5 x thermometers

* 1 x kettle

* 1 x scissors

* 1 x stop watch

* Cotton wool

Method

. Set up 5 beakers of same size and volume. (Same make if this is possible because some glass beakers have different thicknesses).

2. Prepare insulation using the cotton wool and scissors. (Special care must be ensured).

3. This should be done like so: the first beaker requires no insulation, the second beaker requires 1 layer of insulation including the bottom and the top. The third requires two layers of insulation; fourth - 3 layers, and fifth - 4 layers likewise.

4. After the preparation of insulating the beakers place one thermometer in each of the beakers.

5. Fill and boil the kettle ensuring enough water is available to fill 5 beakers with 200ml of water.

6. Immediately record the initial temperatures start the stop watch and secure the tops of each beaker tightly. (Consider elastic bands to secure the insulating cotton wool).

7. Take the temperature every 5minutes and record these. These results will used for analysis.

8. Continue taking the temperature 5 minutes for the next 30 minutes.

Hypothesis

The addition of insulation will reduce the rate of heat lost. The reason for this is that heat is lost in 4 main ways in these experiments; evapouration, conduction, convection and radiation. By the addition of cotton wool which I am using to simulate the insulation, I am preventing evapouration because the hot air can't escape, and I am preventing convection because there are no currents of air taking away the hot air.

I also predict that the rate at which heat is lost will be reduced by more than half because the ways in which heat can be lost is eliminated by the addition of cotton wool as the insulation.

Results

Time (mins)

0 layers

layers

2 layers

3 layers

4 layers

Initial 0

84°C

84°C

84°C

84°C

84°C

5

73°C

81°C

81°C

82°C

82°C

0

65°C

78°C

78°C

79°C

80°C

5

59°C

72°C

71°C

75°C

78°C

20

55°C

71°C

73°C

74°C

77°C

25

51°C

70°C

72°C

73°C

75°C

30

48°C

65°C

70°C

71°C

73°C

Total temp.

Lost

36°C

9°C

4°C

3°C

1°C

% loss

43

22.5

6.5

5.5

3

Room temperature 21°C

Variable 2: surface area to volume ratio

Planning:

Apparatus

* 3 x beakers (sizes - 200ml, 350ml, & 500ml)

* 3 x thermometers

* 1 x kettle

* 1 x stop watch

Method

. Set up the 3 beakers each with their own thermometer inside.

2. Fill and boil the kettle with enough water to fill the three beakers with 200ml of liquid.

3. Using an appropriate instrument to measure out 200ml of water - fill each beaker with 200ml of boiling water.

4. Immediately after step 4, record the initial temperature in each beaker and promptly start the stop clock.

Hypothesis

For this experiment I hypothesis that the largest beaker will lose heat faster than the other two beakers of similar shape but different volumes and ratio's. The volumes, surface areas and surface area to volume ratios for each of the beakers I will be using are:

Beaker 1: Volume = 200ml Surface area = 181.5cm2 Surface area/volume = 0.91

Beaker 2: Volume = 350ml Surface area = 253.3cm2 Surface area/volume = 0.72

Beaker 3: Volume = 500ml Surface area = 380.5cm2 Surface area/volume = 0.76

To put it more precisely the beaker with the largest surface area will lose more heat than the other two beakers. Because as the size increases the volume increases. The larger the surface area/volume ratio the larger the heat lose. This is because the molecules have less distance to travel and more surface area to lose heat.

Results

Time

200cm3 : 181.5cm2

SA/V = 0.91

350cm3 : 253.3cm2

SA/V = 0.72

500cm3 : 380.5cm2

SA/V = 0.76

Initial 0

89°C

86°C

80°C

5

78°C

71°C

66°C

0

68°C

61°C

54°C

5

62°C

54°C

50°C

20

58°C

50°C

46°C

25

52°C

45°C

41°C

30

48°C

42°C

38°C

Total temp

Lost

42°C

44°C

42°C

% loss

46

51

52.5

Room temperature 23°C

Problems

. The temperature of the room had increase of 2° (21°C - 23°C) during this investigation which caused me be concerned about the accuracy of the results I recorded. This could have been due to the amount of hot air in the room by the amount of students who were partaking in the investigation that day, and people's body heat could have contributed to the rise in temperature.

2. Taking the temperatures from the beakers which were insulated caused me concern also because some heat would have been lost by removing the insulating lid and exposing the liquid to air causing slight evapouration. The graph shows a possible error (2 layers - yellow line) recorded; as the beaker with 2 layers had lost more heat than the beaker with 1 layer. The recorded temperature at 15 minutes for the beaker with 1 layer was 72°C and for the beaker with 2 layers was 71°C. Considering the principle of insulating this should not have been the case and the beaker with 1 layer should not have been at a temperature higher than that of the beaker with 2 layers. The possible explanation for this is simply human error.

3. After comparing the two variables it appears that there is a slight difference between the end results for the two beakers with a capacity of 200ml. The first experiment shows that there was 36°C (43%) temperature loss and the second experiment shows that 41°C (46%) was lost from 0 - 30 minutes duration. This worries me because if this experiment were to be repeated by someone else how accurate would my results be then? However after the 30 minutes was up both had an end temperature of 48°C, therefore it could have been that for the first experiment the thermometer had not reached the optimum value of 89°C and instead I recorded the initial temperature to be 84°C. Or heat could have been lost whilst the thermometer was adjusting to temperature of the liquid.

4. I hoped that I would have been able to start recording my experiments at a temperature of 100°C - this was not achieved and the maximum I was able to reach was 89°C.

Conclusions

When I compared my results to my predictions it would appear that my results support what I had initially predicted; that the beakers with the largest surface were faster conductors of heat. The rate of heat loss also decreases with added insulation. I feel that the results from my first variable; insulators, are more useful to clarify my predictions than my second variable, surface area/volume ratio.

Both graphs from my experiments show curves and some are close to straight line almost to the extent that I could have inserted a line of best fit, due to the varying rate of heat loss. I imagine that the curves would have evened out into a straight line if I had of extended the duration of the experiment showing that heat loss had slowed right down to almost a halt.

The beaker with 4 layers of insulation lost only 13% of the initial temperature and the beaker with no insulation lost on average 41% approx.

First I obtained the result from taking the two results which I recorded for the 200ml beakers with no insulation and calculated the average.

Variable 1

Variable 2

Initial starting temp.

84°C

89°C

Total temp. Lost

36°C

42°C

% Loss

43

46

So, 36 + 42/ 2 = 39°C total temp, lost.

% lost = 43 + 46/2 = 41%

I also hypothesised that with the use of cotton wool as an insulator, heat loss should be reduced by more than half.

For this I was able calculate the percentage of heat loss that the cotton wool was able to prevent.

. With 4 layers heat loss = 11°C

No layers heat loss = 36°C

25°C difference, so, 25/84 x 100 = 29.8% prevention

2. With layers heat loss = 11°C

No layers heat loss = 42°C

31°C difference, so, 31/89 x 100 = 34.8% prevention

3. 29.8 + 34.8/2 = 32.3% prevention from heat loss.

It is therefore correct to say that the rate of heat loss decreases with time.

References

Toole.G & Toole.S. Understanding Biology for Advanced Level. Court Rd, LONDON 1999. Pg 497

2 http://en.wikipedia.org/wiki/Polar_bear

3 http://en.wikipedia.org/wiki/Fennec_Foxes

4 http://en.wikipedia.org/wiki/Arctic_fox

Bibliography

* Insulation -

Toole.G & Toole.S. Understanding Biology for Advanced Level. Court Rd, LONDON 1999.

Google search engine

* Surface area and volume -

Toole.G & Toole.S. Understanding Biology for Advanced Level. Court Rd, LONDON 1999.

Google search engine

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