Energy Assignment: need a step in the right direction

Click For Summary

Discussion Overview

The discussion revolves around a homework problem involving heat transfer between a piece of iron and water, specifically calculating the final temperature after they reach thermal equilibrium. The scope includes mathematical reasoning and conceptual understanding of energy conservation in thermal processes.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant expresses confusion about how to start the problem, noting a lack of sufficient variables to apply the heat transfer equation.
  • Another participant suggests that the iron will cool down while the water heats up, indicating that the thermal energy lost by the iron equals the thermal energy gained by the water.
  • There is a proposal to set the heat lost by the iron equal to the heat gained by the water, leading to the equation Qiron + Qwater = 0.
  • A participant shares their attempt at solving the problem, showing their calculations and expressing doubt about the reasonableness of their final temperature result.
  • Another participant mentions confusion after consulting a friend who suggested a different approach, leading to a lower temperature result.
  • One participant reassures that the final temperature of 15.2°C is reasonable given the mass difference between the water and the iron.

Areas of Agreement / Disagreement

Participants generally agree on the principle of energy conservation in this context, but there is uncertainty regarding the calculations and the interpretation of the results. Multiple approaches and interpretations of the equations are presented without a consensus on the final answer.

Contextual Notes

Participants express uncertainty about the algebraic steps and the assumptions made in their calculations. There is a reliance on specific heat capacities and the mass of the substances involved, which may affect the outcomes.

Who May Find This Useful

Students working on heat transfer problems in thermodynamics, particularly those involving phase changes and energy conservation principles.

Phan
Messages
32
Reaction score
0

Homework Statement


A 5.0g piece of iron at 75.0C is added to 150.0g of water at 15.0C. What is the final temperature of the iron and the water? (use c=0.444J/g*C for iron).


Homework Equations


Q = mc(T2-T1)


The Attempt at a Solution


In all honesty, I have no idea how to start this question at all. Most of the other questions on this assignment deal with enthalpy changes, but this one seems to be missing some information that is needed to solve it. Previous questions generally gave another variable, like the Q (heat change) value for either one of the iron or the water. Yet, I am missing more than 2/3 of the variables in the Q=mcdeltaT formula, so I have no idea what I can calculate.

If anyone can point me in the right direction (for a hopefully straightfoward problem), it would be much appreciated. Thanks.
 
Physics news on Phys.org
It appears that the iron's going to cool down and the water's going to heat up. If energy is conserved, what can you say about the thermal energy lost by the iron vs. the thermal energy gained by the water?
 
Mapes said:
It appears that the iron's going to cool down and the water's going to heat up. If energy is conserved, what can you say about the thermal energy lost by the iron vs. the thermal energy gained by the water?

So...

Qiron = -Qwater or vice versa?
Do I set them equal to each other and then solve?
 
My brother asked me the same question about a month ago, LOL.

Yes, Qiron + Qwater = 0

Solve for T2 (the final temperature).
 
scarecrow said:
My brother asked me the same question about a month ago, LOL.

Yes, Qiron + Qwater = 0

Solve for T2 (the final temperature).
I am assuming that I am doing this because the total heat change between the two is 0?
Well, here is what I did (I suck as using symbols right now, so bear with me for the delta Ts):


Qiron + Qwater = 0
mc(T-T1) + mc(T-T1) = 0

[5.0(0.444)(T-75)] + [150(4.184)(T-15)] = 0
[2.22T-166.5] + [627.6T-9414] = 0
[629.82T - 9580.5] = 0
T = 9580.5/629.82
T = 15.21C

Yet, this answer seems very small... so did I mess up in my algebra somewhere?
 
I just consulted a friend, and he said to use:

Qiron=Qwater...

I'm confused now, as this nets me less that 15C?
 
Last edited:
Yes, T = 15.2 C. Think about it carefully: you have 150 g of water and put only 5 g of iron at 75.0 C, so the temperature isn't going to increase by much.

Energy is conserved: the heat lost by the iron is transferred to the water, therefore Qwater = -Qiron, i.e., Qwater + Qiron = 0.
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 11 ·
Replies
11
Views
6K
Replies
13
Views
6K
Replies
2
Views
2K
  • · Replies 1 ·
Replies
1
Views
5K
  • · Replies 1 ·
Replies
1
Views
5K
  • · Replies 7 ·
Replies
7
Views
18K
  • · Replies 1 ·
Replies
1
Views
32K