Why is the direction of temperature gradient important in thermal conductivity?

[Ronaldo95163]

I'm doing Thermal conductivity and I'm kinda confused on something wrt to the temperature gradient...why is it that ΔΘ in the first case the smaller value minus the larger but in the second example it's the larger minus the smaller?

 

[Andrew Mason]

I'm doing Thermal conductivity and I'm kinda confused on something wrt to the temperature gradient...why is it that ΔΘ in the first case the smaller value minus the larger but in the second example it's the larger minus the smaller?

Whether dθ/dx is positive or negative has no physical significance. Heat flow is always from hotter to cooler. The sign depends on where you put the origin and where the hot end is in relation to the origin.

AM

 

[Ronaldo95163]

Oh cool
So For the first example it wouldn't have mattered if 120-0 was used instead then?

 

[Andrew Mason]

Oh cool
So For the first example it wouldn't have mattered if 120-0 was used instead then?

If the hot end is at the origin (x=0) and the cool end at x=20cm then:

dθ/dx = Δθ/Δx = (0-120)/(20-0) = -6°/cm

If the cool end is at the origin (x=0) and the hot end at x=20cm then:

dθ/dx = Δθ/Δx = (120-0)/(20-0) = 6°/cm

The gradient from hot to cold is always negative: i.e. temperature decreases with distance from the hot end.

AM

 

[Ronaldo95163]

Thanks a lot man :D