Sarahisme Posted August 7, 2005 Share Posted August 7, 2005 well heres a small problem, do you think i have a reasonable esimate/answer ? Cheers -Sarah Link to comment Share on other sites More sharing options...
Sarahisme Posted August 7, 2005 Author Share Posted August 7, 2005 my solution: Link to comment Share on other sites More sharing options...
swansont Posted August 7, 2005 Share Posted August 7, 2005 The Stefan-Boltzmann law also takes into account the temperature of the reservoir (surroundings) The question is a bit strange in its wording; if the room is at the same temperature, there is no net radiation - the body is in thermal equilibrium. Was there an additional part to the question that changes the context? Anyway, you need to use (T4 - T4r) The answer for a body in a room at "room temperature" should be around 100-150 W, depending on your estimated values Link to comment Share on other sites More sharing options...
Mokele Posted August 7, 2005 Share Posted August 7, 2005 Quick and dirty answer from the biological end: Your average human runs on about 2000 calories (actually kcal) a day, and, being a mammal, 90% of those go into heat production. So, given that your core body temperature doesn't really vary much (and if it does, that's a big, BIG problem), you have a back-of-the-envelope estimate of about 1800 kcal per 24 hours, which is 87 W. In cold rooms, it might increase, but in warm rooms, it won't decrease very much (instead, we sweat and thereby attempt to cool evaporatively). Mokele Link to comment Share on other sites More sharing options...
swansont Posted August 7, 2005 Share Posted August 7, 2005 Quick and dirty answer from the biological end: Your average human runs on about 2000 calories (actually kcal) a day' date=' and, being a mammal, 90% of those go into heat production. So, given that your core body temperature doesn't really vary much (and if it does, that's a big, BIG problem), you have a back-of-the-envelope estimate of about 1800 kcal per 24 hours, which is 87 W. In cold rooms, it might increase, but in warm rooms, it won't decrease very much (instead, we sweat and thereby attempt to cool evaporatively). Mokele[/quote'] Which is why, if you want to be comfortable in a ~72F/22C room, you need to put some clothes on - otherwise you radiate more energy than you generate, and you feel cold. Link to comment Share on other sites More sharing options...
MetaFrizzics Posted August 7, 2005 Share Posted August 7, 2005 Lets say the average person walks at most a mile a day, and functions essentially as a 100 watt heat lamp. What about an olympic athelete or weight-trainer, who works out semi-daily in a gym? I am guessing the heat-output is seriously cranked up on such a person. Is Arnold Schwartznegger equivalent to say a bank of four food-warmer infra-reds? (1) What is the maximum sustained heat output possible for a healthy adult? (2) If you have a standard window air-conditioner cooling a normal livingroom or office, what is the additional load five warm bodies put on the system? (3) What is the actual temperature required for comfortable nudity and moderate exertion (like you know what)...? Link to comment Share on other sites More sharing options...
Sarahisme Posted August 8, 2005 Author Share Posted August 8, 2005 The Stefan-Boltzmann law also takes into account the temperature of the reservoir (surroundings) The question is a bit strange in its wording; if the room is at the same temperature' date=' there is no net radiation - the body is in thermal equilibrium. Was there an additional part to the question that changes the context?[/quote'] unforunately there wasnt, beacuse thats what i thought at first too... Link to comment Share on other sites More sharing options...
Sarahisme Posted August 8, 2005 Author Share Posted August 8, 2005 or do you think when it says a room at the same temperature, that it means the room is at a constant temperature thoughtout it? Link to comment Share on other sites More sharing options...
Sarahisme Posted August 8, 2005 Author Share Posted August 8, 2005 The answer for a body in a room at "room temperature" should be around 100-150 W' date=' depending on your estimated values[/quote'] i assume you mean 100-150 W/sec? Link to comment Share on other sites More sharing options...
Sarahisme Posted August 8, 2005 Author Share Posted August 8, 2005 ok so here is my updated answer... this the 'net' heat radiated by the person isn't it? also the question could be a trick one (so that the heat radiated = heat absorbed)???? Link to comment Share on other sites More sharing options...
swansont Posted August 8, 2005 Share Posted August 8, 2005 i assume you mean 100-150 W/sec? No, a 1 W = 1 J/s Watt is a unit of power = energy/time Link to comment Share on other sites More sharing options...
Sarahisme Posted August 8, 2005 Author Share Posted August 8, 2005 oops sorry yeah, lol just me rushing things so what did you think of my updated answer (in post #10) ? Link to comment Share on other sites More sharing options...
Sarahisme Posted August 9, 2005 Author Share Posted August 9, 2005 i think it is correct this time, although the orginal question itself(as has already been mentioned) was quite dodgey Link to comment Share on other sites More sharing options...
swansont Posted August 9, 2005 Share Posted August 9, 2005 i think it is correct this time, although the orginal question itself(as has already been mentioned) was quite dodgey Looks good. Link to comment Share on other sites More sharing options...
swansont Posted August 9, 2005 Share Posted August 9, 2005 Lets say the average person walks at most a mile a day' date=' and functions essentially as a 100 watt heat lamp. What about an olympic athelete or weight-trainer, who works out semi-daily in a gym? I am guessing the heat-output is seriously cranked up on such a person. Is Arnold Schwartznegger equivalent to say a bank of four food-warmer infra-reds? (1) What is the maximum sustained heat output possible for a healthy adult? (2) If you have a standard window air-conditioner cooling a normal livingroom or office, what is the additional load five warm bodies put on the system? [/quote'] I don't know about olypic athletes, but if you believe the readout on the stair machine, I can generate ~ 300 W or so of mechanical energy (about twice what I get on the exercise bike), assuming the human body is 20-25% efficient, so that means ~ 1 kW of waste heat. Which explains all the sweating that Mokele mentioned. (Raising your skin temp to 38C gets you to only ~250 W radiated in Sarah's example) Ah-nold doesn't do aerobic exercise much if he's lifting, but I bet Lance Armstrong is a human food warmer. The extra load of 5 people is going to be about 400-500 W. I would think that was obvious from the discussion. Link to comment Share on other sites More sharing options...
Sarahisme Posted August 9, 2005 Author Share Posted August 9, 2005 thanks for all your help guys (especially swansont of course ) Link to comment Share on other sites More sharing options...
MetaFrizzics Posted August 9, 2005 Share Posted August 9, 2005 Yes thanks Swansont. I guess I could have worded my questions better. I was thinking that *any* activity by a human being could just be counted as 'heat generation' in the sense that even organized transport of weights is always imparting energy to the environment in the form of kinetic translational (or potential) energy. So I can't get my head around the idea that 'heat production' will be only 20% efficient! (Is the other 80% wasted as heat? LOL) I think too I hadn't really thought through the air-conditioner idea, but I suspected (still suspect) complications because the heat cannot be 'radiated' out of the room via the six walls, since the temperature differential is opposing it. (i.e., if anything heat from outside is leaking in). But now thinking about it you are right I think: the load is directly transferred to the air-conditioner and not diluted by any mechanism I can forsee. Link to comment Share on other sites More sharing options...
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