Suppose a cup of coï¬ee is at 100 degrees Celsius at time t = 0, it is at 70 degrees at t = 10 minutes, and it is at 50 degrees at t = 20 minutes. Compute the ambient temperature.

Respuesta :

Answer:

T ambient = 10 degrees

Explanation:

Using Newton's Law of Cooling:

T(t) = Tamb + (Ti - Tamb)*e^(-kt)  ..... Eq 1

Ti = 100

We have two points to evaluate the above equation as follows:

T = 70 @ t = 10 using Eq 1  

70 = Tamb + (100 - Tamb)*e^(-10k)   ... Eq 2

T = 50 @ t = 20 using Eq 1

50 = Tamb + (100 - Tamb)*e^(-20k)   ... Eq 3

Solving the above Eq 2 and Eq 3 simultaneously:

Using Eq 2:

(70 - Tamb) / (100 - Tamb) = e^(-10k)  

Squaring both sides we get:

((70 - Tamb) / (100 - Tamb))^2 = e^(-20k)   .... Eq 4

Substitute Eq 4 into Eq 3

50 = Tamb + (100 - Tamb)*((70 - Tamb) / (100 - Tamb))^2

After simplification:

50 = (Tamb (100-Tamb) + (70-Tamb)^2) / (100 - Tamb)

5000 - 50*Tamb = 4900 - 40*Tamb

Tamb = 100 / 10 = 10 degrees

The ambient temperature will be 10°C. The rate of heat loss is proportional to the difference between the body and ambient temperature.

What does Newton's Law of Cooling state:

The rate of heat loss is proportional to the difference between the body and ambient temperature.

[tex]T_t = T_{amb} + (T_i - T_{amb})\times e^{-kt}[/tex]

There,

[tex]T_t[/tex] - temperature at time t

[tex]T_{amb}[/tex] - ambient temperature

[tex]T_i[/tex] - Initial temperature

[tex]k[/tex] - constant

For the first situation, T = 70°C at  t = 10  

[tex]70 = T_{amb}+ (100 - T_{amb})\times e^{-10k[/tex]

[tex]\dfrac {70 -T_{amb} } {(100 - T_{amb})}= e^{-10k[/tex]..........2

For second situation, T = 50°C at  t = 20.

[tex]50 = T_{amb}+ (100 - T_{amb})\times e^{-20k[/tex].....................3

From solving equation 2, we get,

[tex]T_{amb} = 10^oC[/tex]

Therefore, the ambient temperature will be 10°C.

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