Issacphysics Thermal physics question

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EVEsali
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How much water at 52∘C must I add to 19kg of water at 21∘C
to make it the right temperature, 37∘C for me to bath a baby?

specific heat capacity of water for the question: 4180Jkg−1K−1

if anyone has any ideas about how to do this please let me know
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EVEsali
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(Original post by ed_e12)
treat the 2 masses of water as separate objects. For the 52 degree water, it must undergo a temperature change of 15 degrees to reach the safe temperature of 37. Using the formula energy = mass × shc × temperature change, the energy transferred from the 19kg of 52 degree water to the mass of 21 degree water must be 19×4180×(52-37)=1191300j

now we know that the energy change of the 21 degree water is also 1191300 because, assuming no energy is dissipated, energy must be conserved in the system. So we know the energy, the specific heat capacity and the temperature change (37-21). By rearranging the above formula, e=mct we get m=e/ct

putting in numbers we arrive at 1191300/(4180×(37-21))=17.8125

since this is isaac physics they're very picky about significant figures so check what degree of accuracy you are supplied in the question, i expect 2 or 3 significant figures giving you a result of 18kg or 17.8kg
thanks so much that makes so much sense !!!
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EVEsali
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I've checked on Issac and its saying that the answer is incorrect thanks for the help tho
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Ed_e12
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(Original post by EVEsali)
I've checked on Issac and its saying that the answer is incorrect thanks for the help tho
I can't even find the question on Isaac so I don't know why it's wrong
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EVEsali
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(Original post by Ed_e12)
I can't even find the question on Isaac so I don't know why it's wrong
It’s G3 heat capacity question G3.7
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RogerOxon
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(Original post by EVEsali)
How much water at 52∘C must I add to 19kg of water at 21∘C
to make it the right temperature, 37∘C for me to bath a baby?

specific heat capacity of water for the question: 4180Jkg−1K−1

if anyone has any ideas about how to do this please let me know
The specific heat capacity is irrelevant, as it will cancel out. This reduces to a weighted average:

52x+21*19=(x+19)37

I get an answer around 20Kg, which makes sense as 37 is just over half way between 21 and 52C, so you need just over 19Kg.

Edit: Added signs again. Bloody mobile app.
Last edited by RogerOxon; 1 year ago
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EVEsali
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(Original post by RogerOxon)
The specific heat capacity is irrelevant, as it will cancel out. This reduces to a weighted average:

52x  21*19=(x 19)37

I get an answer around 20Kg, which makes sense as 37 is just over half way between 21 and 52C, so you need just over 19Kg.
Issac is happy with that thanks !
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Ed_e12
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(Original post by EVEsali)
It’s G3 heat capacity question G3.7
Sorry now I've looked on isaac I know what I did wrong, basically it's exactly the same method but swap around the temperatures. I took it to be 19kg of 51°C water but it's actually 19kg of 21°C water
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