Solving Heat Problems: 2 Questions & Exam Tomorrow

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The discussion focuses on two heat-related problems for an upcoming exam. The first problem involves calculating heat loss through a room's roof, requiring knowledge of thermal resistance or conductivity to determine heat flow per square meter. The second problem pertains to converting ice at -30°C to steam at 140°C, which involves multiple steps including heating the ice, melting it, heating the water, evaporating the water, and heating the steam. Participants emphasize the importance of understanding specific heat and latent heat for each phase change, suggesting a method to convert each section into a heating water equivalent for easier calculations. Overall, the conversation highlights the need for a solid grasp of thermal concepts to solve these problems effectively.
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Hi, I have 2 questions and an exam tomorrow, and I have no idea how my teacher answered these 2 problems, if someone can guide me through how to solve these problems i would really appreciate it. I am not looking for an answer just a step by step how to solve it and I am not really sure how to start.
How much heat is lost in one day through the roof of a 5.00m by 5.00m room?The room is maintained at a temperature of 20°C but the outdoor temperature is only -4°C.

How much heat, Q, is required to convert 0.500kg of ice at -30.0°C to steam at 140.0°C
For this one, i think my teacher would convert the ice to a liquid and then to a solid, and then taking taking some equation at each part, lol I am not really sure

thanks in advance :)
 
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shemer77 said:
Hi, I have 2 questions and an exam tomorrow, and I have no idea how my teacher answered these 2 problems, if someone can guide me through how to solve these problems i would really appreciate it. I am not looking for an answer just a step by step how to solve it and I am not really sure how to start.

For this one, i think my teacher would convert the ice to a liquid and then to a solid, and then taking taking some equation at each part, lol I am not really sure thanks in advance :)

For the roof, I think you need to know the thermal resistance or thermal conductivity of the material of the roof. With that you can calculate the heat flow per square meter [Joules per sec per square meter] then scale it up for the room size and the time period.

For the ice to steam.

Heat the ice - what is the specific heat for ice? [it is 1 for water]
melt the ice - what is the latent heat for melting ice?
Heat the water
evaportate the water - what is the latent heat of evaportation?
heat the steam - what is the specific heat for steam?

NOTE: Suppose the latent heat for melting ice was 50 times the specific heat for water -it's NOT - the getting from ice at -10 to water at +10 would be the same as heating water from +5 to +75, ie 70 degrees.

You could convert each section to a "heating water" equivalent and do the final calculation in one go.

***Saying latent heat is 50 times specific heat I mean suppose you need X Joules to heat a mass of water by 1 degree but need 50X Joules to melt the same mass of ice.
 
PeterO said:
For the roof, I think you need to know the thermal resistance or thermal conductivity of the material of the roof. With that you can calculate the heat flow per square meter [Joules per sec per square meter] then scale it up for the room size and the time period.

For the ice to steam.

Heat the ice - what is the specific heat for ice? [it is 1 for water]
melt the ice - what is the latent heat for melting ice?
Heat the water
evaportate the water - what is the latent heat of evaportation?
heat the steam - what is the specific heat for steam?

NOTE: Suppose the latent heat for melting ice was 50 times the specific heat for water -it's NOT - the getting from ice at -10 to water at +10 would be the same as heating water from +5 to +75, ie 70 degrees.

You could convert each section to a "heating water" equivalent and do the final calculation in one go.

***Saying latent heat is 50 times specific heat I mean suppose you need X Joules to heat a mass of water by 1 degree but need 50X Joules to melt the same mass of ice.


sorry, i didnt really learn this stuff. How would I go about calculating the heat flow?

and for the ice to steam, what do you mean by convert each section to a heating water equivalent?
 
shemer77 said:
sorry, i didnt really learn this stuff. How would I go about calculating the heat flow?

and for the ice to steam, what do you mean by convert each section to a heating water equivalent?

If you don't immediately understand the conversions, ignore them and work it out the "standard" way.

There is a conductance [or its inverse resistance] which tells you the number of joules per second per square metre per degree difference each side. You need to cram that stuff now.

[You may have seen ceiling insulation bats. They can be R2, R3, R4 etc bats - that is referring to their thermal resistance.
 

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