A thermometer reading 4˚C is brought into a room where the temperature reading is 30˚C. If the thermometer reads 10˚C after 2 minutes, determine the temperature reading 5 minutes after the thermometer is first brought into the room.

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- September 22nd 2009, 11:28 AMmikegar813Newton's law of cooling/warming
A thermometer reading 4˚C is brought into a room where the temperature reading is 30˚C. If the thermometer reads 10˚C after 2 minutes, determine the temperature reading 5 minutes after the thermometer is first brought into the room.

- September 22nd 2009, 11:36 AMmikegar813
22.46°c?

- September 22nd 2009, 04:03 PMmr fantastic
- September 23rd 2009, 04:24 AMmikegar813
30+(4-30)e^(-k2)

k=-.5ln (10/13)

30+(-26)e^((-3)-.5ln (10/13))=12.46

2 minutes = 10°C

3 minutes = 12.46°C

10+12.46= 22.46°C

is this correct? - September 23rd 2009, 05:00 AMmr fantastic
I have no idea what this all means. Where have you used

**Newton's law of cooling/warming**to set up the differential equation? Where is your working for solving that differential equation?

Please show ALL the work leading to your answer if you want your answer reviewed. - September 23rd 2009, 05:21 AMmikegar813
H= temperature of the object at time t

H sub s= the constant surrounding temperature

t= time

H sub 0= the temperature at t=0

H-H sub s=(H sub 0-H sub s)e^(-kt)

I found this formula in my book. - September 23rd 2009, 06:25 AMHallsofIvy
So you

**didn't**actually use "Newton' law of cooling", you used a formula derived from that.

Okay, here , and your formula becomes . You want to find H when t= 5 but you still don't know what "k" is. You can find that by using the fact that H= 10 when t= 2: . Then you can find H when t= 5 by using that same k and t= 5.

Warning: This is NOT linear! H(5) is NOT equal to H(2)+ H(3)! Do NOT find H(3), just use the formula with t= 5, not 3. - September 23rd 2009, 07:30 AMmikegar813
so 'k' would not change even though the 't' time does?

- September 23rd 2009, 04:10 PMmr fantastic
k is a constant. It does not change.

Were you expected to use Newton's Law of Cooling to solve this question? If yes, then the fact that k is a constant should be clear and you shouldn't be taking shortcuts by applying formulas you don't understand. If not, then the title of your post is misleading and a lot of time might have been wasted answering this question in a way that would ultimately have made no sense to you.