February 19, 2017

Homework Help: Physics

Posted by Andrew on Saturday, March 27, 2010 at 6:51pm.

I'm lost on this physics problem can anyone tell me which equations i'm suppose to use in these questions
a) A skier inhales a breath of air at Temp-air = 3.9 ¢XC and exhales it as Volume-air = 1.92 liters of air at Temp-body = 37 ¢XC and atmospheric pressure of P = 95.9 kPa, evaporating mass = 24.8 mg of water from the lungs and breathing passages with each breath. At body temperature the latent heat of vaporization of water is Lv = 2255 x 103 J kg−1. Given that air has a density of £l = 1.13 kg m−3 and a heat capacity of c = 1.02 kJ kg-1 K−1 under the relatively constant pressure conditions of breathing:
(i) What is the magnitude of heat lost in each breath from the evaporation process?
(ii)What is the magnitude of heat lost in each breath from just warming the air?
(iii) If the skier is breathing at the moderate rate of 18.0 per minute, what is the magnitude of the rate of heat loss from both of these modes?

(b) The skier's boyfriend, who has a surface area of 1.4 m2, is wearing a down jacket and trousers of thickness 2.5 cm and thermal conductivity of £e = 0.0207 W m−1 K−1. If his skin temperature is at a comfortable 34.2 ¢XC and the outside of his costume is at the same temperature as the ski-field air:
(i) What is the magnitude of the rate of heat loss by conduction at this ski-field temperature?
(ii) What is the lowest ski-field temperature for which the costume is adequate (i.e. comfortable) if it is possible to safely lose 58.6 W by conduction alone?


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