Mammals are warm blooded animals, which means that their body temperature is held more or less constant (37.5oC for humans) by balancing the heat absorbed from metabolic processes with heat lost through external cooling.
Generally, most heat is generated by internal metabolic processes, and generally heat is lost to a cooler environment. In very cold environments humans apply layers of clothing to help reduce heat lost, whereas mammals use layers of fat and fur to help reduce heat loss.
Several mechanisms are used to regulate the body's internal temperature.
Have the students hold their hand in each of the environments (i.e. in the ice water and the air) for 15-30 seconds . Describe how cold the water feels compared to the cold air.
Put on a loose fitting waterproof glove (for example, a slightly oversized dish washing glove) on one hand only ... on the other hand put on a tight fitting latex or vinyl surgical glove.
Simultaneously place both hands wrist deep in the pail of ice water for 15-30 seconds.
In which case does the water seem to be cooler?
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Convection
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Convection, especially in liquids is very efficient mechanism whereby heat energy is transported away from warm objects.
Convection also works well in gases where the flow of the gas (such as air in room) is unrestricted. Two methods are used to prevent heat loss due to convection One is to limit the ability of the gas or liquid to flow and the second is to prevent gases and liquids from coming into contact with the warm object. Fiberglass insulation is an example of the former method and vacuum thermos bottles are an examples of the latter method. Explain these examples |
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Conduction
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The rate at which heat is conducted depends upon the chemical composition of the conduction material.
In general, materials with "free" electrons (metals) conduct both heat and electricity well. Copper and aluminum are examples of
materials that conduct both efficient conductors of heat and electricity.
Non-metallic materials, such as sulfur, some ceramics and certain plastics are generally poor conductors heat and electricity. Porous ceramics have been produced which are remarkable for their inability to conduct heat, such materials are called thermal insulators. The heat shield tiles on the Space Shuttle Orbiter have properties that limit both conduction and convection and are therefore exceptionally good thermal insulators. |
Radiation
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All warm objects emit electromagnetic radiation. The power of the radiation emitted (watts per square metre) depends
only on the temperature of the object (The Stefan-Boltzmann Law).
The human has a body temperature of 37.5oC. At that temperature we emit infrared radiation at a rate of 170W/m2. In a warm environment, heat is also radiated back (from warm objects in the environment) and the net heat loss is usually relatively small. However, if the surrounding environment is cold... the human body radiates thermal energy which is not replaced, and body cooling occurs very rapidly. Materials which reflect infrared radiation back to the hot object help reduce heat loss. Aluminized "space blankets" accomplish this. |
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To demonstrate this, begin by having two students sit quietly for five minutes (to establish their resting pulse rate).
After five minutes have elapsed determine each participant's pulse rate (beats per minute) and record the result.
The participants must remain sitting (resting) for the duration of the experiment. Next, have one of the participants drink a glass of room temperature water while other participant drinks a glass of ice water. The ice water should be consumed carefully and reasonably quickly, but not so quickly that there is a risk of a "brain freeze"..ouch! Take the pulse rate of each student before and after drinking the glass of water. Continue to measure each participant's pulse rate at five minute intervals for 15 minutes after drinking the glass of water. Record your results. |
Answer: The water may be 38 degrees warmer in terms of temperature, but water under certain conditions is approximately 40 times more effective in extracting heat from an object than air. Thus the net heat loss from the seal?s body is 40 times greater in water then in air. In fact it is easier for these mammals to maintain their body temperature in air as opposed to being submerged in water.
Weddell seals have been observed to hyperventilate just before they dive. Suggest a reason that they might do this.
Answer: By depleting excess carbon dioxide from their bodies they are able to hold their breath longer while diving underwater.
Answer: Warm air around dish washing glove asks as an insulator and well as the fact that the rubber glove is slightly thicker than the stretched latex glove therefore providing some extra insulation.
Answer: Oddly enough, both water and air a good insulators; however, the density of water is very much greater than that of air and it can absorb much more heat per unit volume than air. Consider the reverse situation, and raw egg will become hard boiled much more quickly when placed in water at 100oC than in air at 100oC. This effect has a great deal to do with the relative density of air with respect to water and therefore their heat content (energy) per unit volume.
Both water and air can be used as insulating materials if they can be prevented from flowing next to the warm surface. Consider a diver's wet suit. It is filled with tiny water filled cells that prevent water from flowing next to the divers skin and therefore it acts as an insulator.
For both water and air, the major source of heat transfer away from warm a object is due to convection or equivalently, air or water flowing over the warm surface. The moving fluid is warmed slightly ... but it is quickly replaced by fresh cold water (or cold air).
To summarize, water absorbs much more heat per unit volume than air, partly because of its higher density; and both air and water remove heat be flowing over the warm surface and supply a constant source of a colder medium which continually absorbs more heat energy.
Answer: The body must offset the effects of the cold water which is ingested. In response to the cooling effect of the cold water on the body's core temperature the heart rate increases. In an energy limited environment like Antarctica you don?t want to drink cold water because it will take a lot of energy and heat to warm up cold water inside your body.
