When to convert units of temperature?

[BlackJ]

Homework Statement

[/B]
(1)A sphere of radius 0.500 m, temperature 27.0 C, and emissivity
0.850 is located in an environment of temperature 77.0 C. At
what rate does the sphere (a) emit and (b) absorb thermal radiation?
(c) What is the sphere’s net rate of energy exchange?

(2)A cylindrical copper rod of length 1.2 m and cross-sectional
area 4.8 cm2 is insulated along its side.The ends are held at a temperature
difference of 100 degree C by having one end in a water–ice mixture
and the other in a mixture of boiling water and steam. At what rate
(a) is energy conducted by the rod and (b) does the ice melt?

Homework Equations

The Attempt at a Solution

So I'm studying the chapter about temperature, heat and the first law of thermodynamics. I start to feel confused about when the temperature should be converted. Sometimes it's Celsius sometimes it's in Kelvin. For example, could anyone explain why the first problem have to convert to Kelvin but the second one can be in Celsius?

 

[Merlin3189]

I hope someone like Chester comes along, who is an expert in thermodynamics, as I'm not.
But I think the answer here is straightforward.

In 1) radiation depends on the absolute temperature, so you convert to K.
( Only at absolute 0 is there no radiation. Just because something is at 0 ^oC, it is not incapable of radiating heat.)

In 2) conduction depends on temperature difference - which will be the same in K as it is in ^oC.
( You could even work in ^o F if you wanted, but you'd need to use a value for the conductivity expressed in those units.)

 

[Phantoful]

Same with what Merlin said. My rule is that you can pretty much exclusively use Kelvin in Physics because you can't go wrong with it; differences will be the same and relating things will be much easier, so it's nice to convert straight from the beginning. (Celsius + 273)