How Does Absorptivity Affect Heat Radiation in Small Temperature Differences?

[neelakash]

Homework Statement

A small body at temperature θ is suspended in an evacuated enclosure with walls at uniform temperature θ0.If (θ-θ0) is small show that the the rate at which the body loses heat bt radiation is given by

dQ/dt=4(θ0)^3*Aaσ(θ-θ0)
where A is the area and a is the sbsortivity of the body.

Homework Equations

The Attempt at a Solution

We should use the Stefan's law:Q=σ[θ^4-(θ0)^4]
the differentiation may be carried out...But I do not understand how to replace σ

 

[hage567]

$$\sigma$$ is a constant. You don't replace it with anything.