# Homework Help: Thermal Energy Problem HELP PLEASE

1. Nov 17, 2005

### ilovemoney518

Hi guys I have these 2 thermal energy problems that I keep getting wrong answer, I really want to know the answer and the way to solve it. Thank u very much!

1. The average thermal conductivity of the walls (including windows) and roof of a house in Figure P11.32 is 4.8 10-4 kW/m°C, and their average thickness is 20.0 cm. The house is heated with natural gas, with a heat of combustion (energy given off per cubic meter of gas burned) of 9300 kcal/m3. How many cubic meters of gas must be burned each day to maintain an inside temperature of 24.5°C if the outside temperature is 0.0°C? Disregard radiation and loss by heat through the ground.

2. When you jog, most of the food energy you burn above your basal metabolic rate (BMR) ends up as internal energy that would raise your body temperature if it were not eliminated. The evaporation of perspiration is the primary mechanism for eliminating this energy. Determine the amount of water you lose to evaporation when running for 30 minutes at a rate that uses 400 kcal/h above your BMR. (That amount is often considered to by the "maximum fat-burning" energy output.) The metabolism of 1 gram of fat generates approximately 8.5 kcal of energy and produces approximately 1 gram of water. (The hydrogen atoms in the fat molecule are transferred to oxygen to form water.) What fraction of your need for water will be provided by fat metabolism? (The latent heat of vaporization of water at room temperature is 2.5 106 J/kg.)

For the first problem the answer I got is 119.96. But it was wrong. I can't afford to try more cuz I only have 3 chances and I am totally lost. The second problem I have no clue, could anyone guide me a little?

Last edited: Nov 17, 2005
2. Nov 17, 2005

### Astronuc

Staff Emeritus
What equation are you using for problem 1? Please show some work.

On two - on is asked to "Determine the amount of water you lose to evaporation when running for 30 minutes at a rate that uses 400 kcal/h above your BMR." From that one can determine total kcal.

Then use "The metabolism of 1 gram of fat generates approximately 8.5 kcal of energy and produces approximately 1 gram of water." find production of water from fat metabolism.

The total water loss (from evaporation) is found using "The latent heat of vaporization of water at room temperature is 2.5x106 J/kg."