8.21 Heat Lost vs. Heat Gained

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Kathleen Vidanes 1E
Posts: 62
Joined: Fri Sep 29, 2017 7:07 am

8.21 Heat Lost vs. Heat Gained

Postby Kathleen Vidanes 1E » Mon Jan 22, 2018 3:37 pm

On the homework question 8.21, why is the heat lost by metal a positive value, while the heat gained by water is negative? Shouldn't a loss of heat be a negative value?

Hannah Guo 3D
Posts: 56
Joined: Fri Sep 29, 2017 7:06 am

Re: 8.21 Heat Lost vs. Heat Gained

Postby Hannah Guo 3D » Mon Jan 22, 2018 3:47 pm

In this case, q(copper)=-q(water) or -q(copper)=q(water) are the same. The negative sign just indicates that the heat lost by one is gained by the other. As long as q(copper) + q(water) = 0, in another word, as long as the system follows the conservation of energy, the calculation is correct.

Hope it helps!

Kathleen Vidanes 1E
Posts: 62
Joined: Fri Sep 29, 2017 7:07 am

Re: 8.21 Heat Lost vs. Heat Gained

Postby Kathleen Vidanes 1E » Mon Jan 22, 2018 4:08 pm

Oh okay, thank you so much! That definitely helped. Follow up question, does that mean that the sum of multiple q's always equals 0? Or is that only when no energy is lost to the surroundings?

Natalie LeRaybaud 1G
Posts: 54
Joined: Thu Jul 13, 2017 3:00 am

Re: 8.21 Heat Lost vs. Heat Gained

Postby Natalie LeRaybaud 1G » Mon Jan 22, 2018 4:20 pm

Not always. Sometimes there are other factors present in a system to take into account that can have an impact on the overall energy. However, when there is a direct heat transfer such as in this case the Copper transferring heat to the water, then the sum of the qs in the system equals zero.


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