Another 8.41 Question

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Grace Ramey 2K
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Joined: Thu Jul 27, 2017 3:01 am

Another 8.41 Question

Postby Grace Ramey 2K » Sun Jan 14, 2018 7:37 pm

I have a follow-up question for 8.41:

The solutions manual, when setting the heat change of the ice cube and the heat change of the water equal, puts a negative sign in front of the heat change for the water (which is on the left side of the equals sign in the equation below). The equation comes out looking like this (Tf = final temperature):

-(1.67x10^3 J/°C)(Tf) + 7.5x10^4 J = 1.67x10^4 J + (209 J/°C)(Tf)

Can someone explain that negative sign to me? The only thing I can think of is that it is made negative because the water loses heat, but I am not sure about this.

Thank you for your help!

Ethan-Van To Dis2L
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Joined: Fri Sep 29, 2017 7:05 am

Re: Another 8.41 Question

Postby Ethan-Van To Dis2L » Sun Jan 14, 2018 8:12 pm

Essentially yes. The negative sign is due to the loss of heat in the water, and then that heat enters the ice. Think of it as two systems. The heat leaves one system (negative sign) to enter another (positive sign)

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Re: Another 8.41 Question

Postby Seth_Evasco1L » Sun Jan 14, 2018 8:15 pm

You can think of the ice cube as the system and the water as the surroundings. In relation to the system, the system is gaining heat (positive) and the surroundings are losing heat (negative). The heat transfer is from the surroundings to the system.

Krupa Prajapati
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Joined: Sat Jul 22, 2017 3:00 am

Re: Another 8.41 Question

Postby Krupa Prajapati » Sun Jan 14, 2018 8:17 pm

Hi Grace,

Since q simply represents the amount of heat transferred to a system, q ice = -q water. The ice, the system, is gaining heat while the water, the surrounding, is loosing the same amount of heat. Therefore, when we substitute q for m*c*(t final - t initial) we put a negative in front of the q value for water. Hope this helps!

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