87. The cube has six faces, each of which has an area of

6.0

× 10

−6

m

2

. Using Kelvin temperatures and

Eq. 19-40, we obtain

P

net

=

σεA

T

4

env

− T

4

=

5.67

× 10

−8

W

m

2

·K

4

(0.75)

2.16

× 10

−10

m

2

(123.15 K)

4

− (173.15 K)

4

=

−6.1 × 10

−9

W .