If the air temperature is the same as the temperature of your skin (about 30 C),

Question

If the air temperature is the same as the temperature of your skin (about 30 C), your body cannot get rid of heat by transferring it to the air. In that case, it gets rid of the heat by evaporating water (sweat). During bicycling, a typical 70 kg person's body produces energy at a rate of about 505 W due to metabolism, 81 % of which is converted to heat.

A.

How many kilograms of water must the person's body evaporate in an hour to get rid of this heat? The heat of vaporization of water at body temperature is2.42×106J/kg.

B.

The evaporated water must, of course, be replenished, or the person will dehydrate. How many 750 mL bottles of water must the bicyclist drink per hour to replenish the lost water? (Recall that the mass of a liter of water is 1.0 kg .)

Explanation / Answer

The waste heat is given by 505 W x0.81 = 409.5 W

That is 409.5 J/s that must be taken away. Now 1 hour has 3600 seconds so teh total amount of energy that needs to be removed is

E = 409.1 J/s x 3600 s = 1.47x10^6 J

Heat of vaporization is givne as H = 2.42x10^6 J/kg so the amount of water needed is

m = E/H = 0.607 kg

Since 1 L of water has a mass of 1 kg, the bicyclist needs drink only one 750 mL bottle/hour

Related Questions

Navigate

• Browse (All)
• Browse I
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.