A homeowner finds that, if she turns down her thermostat by 3[degrees C] she can reduce the rate at which heat is lost to outside, by 10%. If the outside temperature is -5[degrees C], what are the Thermostat readings before and after the adjustment.(adsbygoogle = window.adsbygoogle || []).push({});

I'd just like verification that i haven't screwed up this problem totally...

[because i'm just sorta going along blindly, moving on when I touch something...]

My teacher gave me this equation:

d Q/ d t{ime} = (T{emp hot} - T{emp cold}) * A{surface area} * K(conductivity constant) / L{ength of conducted material}

dQ/dt = (T(h) - T(c)) * A * K / L

While i know this equation:

Q = mc/\T

However, i chose to use sole my teacher-given equation...

dQ/dt = (T(h) - T(c)) * A * K / L

and instead equated it to

/\Q//\t = (T(h) - T(c)) * A * K / L

(i used change in heat/time, i'm not so good with calculus :tongue: )

/\Q//\t = (T(h) - T(c)) * A * K / L

I decided that the orange terms would be kept constant.

Constant = (T(h) - T(c)) / (/\Q)

then i chose two thermostat readings to be equal to that constant

(Before thermostat change) Constant = (To - (-5) / (Q)

(After thermostat change) Constant = ((To - 3) - (-5) / (.9Q)

I then set the two equations equal to each other, via the constants, cancelled out the Q on both sides, Solved for To,

and got

75[degrees C] for the beginning Temperature...

Any suggestions?

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# Homework Help: Thermodynamics/Cooling Problem ^_^

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