Thermodynamics - Use conservation of energy to answer these questions....

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Discussion Overview

The discussion revolves around the application of conservation of energy principles in thermodynamics, specifically in relation to a worksheet involving heat transfer events. Participants explore how to calculate heat quantities (Q1 and Q2) for various events and assess the feasibility of these events based on thermodynamic laws.

Discussion Character

  • Homework-related
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants express confusion about how to determine Q1 and Q2 for the third and fourth events, questioning if these values could be zero due to constant temperatures.
  • One participant suggests that energy generation by fusion in the sun means its temperature does not decrease, prompting questions about entropy changes.
  • Several participants argue that Event 2 is impossible, citing that heat cannot flow from a colder object to a hotter one, and they express uncertainty regarding Events 3 and 4.
  • One participant believes Event 3 is possible since heat moves from hot to cold, while asserting that Event 4 is impossible due to the temperature relationship between Earth and the sun.
  • Another participant questions the clarity of the worksheet's temperature data, particularly why the final temperatures are the same for different events.
  • There is a discussion about the heat capacity of Earth's atmosphere and how much a specific amount of energy (57MJ) would change its temperature, along with considerations of heat re-radiated to space.

Areas of Agreement / Disagreement

Participants generally disagree on the feasibility of Events 2, 3, and 4, with multiple competing views on the implications of heat transfer and temperature changes. The discussion remains unresolved regarding the specific calculations and interpretations of the events presented in the worksheet.

Contextual Notes

Participants express uncertainty about the assumptions underlying the worksheet's questions and the definitions of terms used, particularly in relation to heat transfer and temperature changes. There are also unresolved mathematical steps regarding the calculations of energy changes.

lc99
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Homework Statement



upload_2018-4-25_20-15-23.png

Homework Equations

The Attempt at a Solution


I understand how they got Q1 and Q2 for the first two columns, but how am i suppose to find Q1 and Q1 for the 3rd and 4th event?

Isn't it just 0? Cause it seems like the temperatures just stayed constant. I'm not entirely sure what the worksheet is asking me to do.

I think this may relate to carnot engine and heat engines that are reversible
 

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lc99 said:

Homework Statement



View attachment 224684

Homework Equations

The Attempt at a Solution


I understand how they got Q1 and Q2 for the first two columns, but how am i suppose to find Q1 and Q1 for the 3rd and 4th event?

Isn't it just 0? Cause it seems like the temperatures just stayed constant. I'm not entirely sure what the worksheet is asking me to do.

I think this may relate to carnot engine and heat engines that are reversible
Energy is being generated by fusion within the sun, so its temperature is not decreasing. What are your answers now?

What are your assessments of the changes in entropy?
 
Event 2 is impossible for sure because even if it doesn't break conservation's law heat can't go from a colder object to an hotter one. I don't understand event 3 and 4.
 
dRic2 said:
Event 2 is impossible for sure because even if it doesn't break conservation's law heat can't go from a colder object to an hotter one. I don't understand event 3 and 4.
By the same rationale, what is your assessment of whether Event 3 or Event 4 is possible?
 
Chestermiller said:
By the same rationale, what is your assessment of whether Event 3 or Event 4 is possible?

I'm not the OP so I don't know exactly what the question is... it seems nonsense to me.
 
dRic2 said:
I'm not the OP so I don't know exactly what the question is... it seems nonsense to me.
Oops. Sorry, I mistook you for the OP.
 
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Never mind, It happens to me as well :biggrin::biggrin:
 
Chestermiller said:
By the same rationale, what is your assessment of whether Event 3 or Event 4 is possible?
Kinda got some insight from the above poster. I think event 3 is is possible since heat is moving from hot object to cold object.
Event 4 is impossible because Earth can transfer heat to the sun if it is colder than the sun.
 
dRic2 said:
Event 2 is impossible for sure because even if it doesn't break conservation's law heat can't go from a colder object to an hotter one. I don't understand event 3 and 4.

Oh i get it. Cause cooling doesn't give heat?
 
  • #10
lc99 said:
I understand how they got Q1 and Q2 for the first two columns, but how am i suppose to find Q1 and Q1 for the 3rd and 4th event?
"Sun(1) transfers 58MJ of thermal energy to earth(2)." What about that is unclear?
 
  • #11
tnich said:
"Sun(1) transfers 58MJ of thermal energy to earth(2)." What about that is unclear?
I'm more confused with the temperatures that they listed in the chart. Why would they use the same numbers for final temp?
 
  • #12
lc99 said:
I'm more confused with the temperatures that they listed in the chart. Why would they use the same numbers for final temp?
The heat capacity of Earth's atmosphere is about 1X107J/m2/°C, that is, for each square meter of Earth's surface, it takes 1X107J to change the temperature of the atmosphere above it by 1°C. The radius of the Earth is about 6.4X106m. So how much does 57MJ change the temperature of the atmosphere? (Notice that we are just talking about the atmosphere here, without even considering anything below the surface of the planet.)
 
  • #13
tnich said:
The heat capacity of Earth's atmosphere is about 1X107J/m2/°C, that is, for each square meter of Earth's surface, it takes 1X107J to change the temperature of the atmosphere above it by 1°C. The radius of the Earth is about 6.4X106m. So how much does 57MJ change the temperature of the atmosphere? (Notice that we are just talking about the atmosphere here, without even considering anything below the surface of the planet.)
What about the heat re- radiated to space. This is pretty much in balance with the heat received from the sun.
 
  • #14
Chestermiller said:
What about the heat re- radiated to space. This is pretty much in balance with the heat received from the sun.
That is true on average, but daytime temperatures are several degrees higher than nighttime temperatures, so there is a transient warming effect of the sun. My point was to make clear the order of magnitude of the temperature change that 57MJ of energy would cause.
 

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