(35-39) A student travels on a school bus in the middle of winter from home to school. The school bus temperature is 68.0° F. The student's skin temperature is 94.4° F. Determine the net energy transfer from the student's body during the 20.00 min ride to school due to electromagnetic radiation. Note: Skin emissivity is 0.90, and the surface area of the student is 1.50m2.

The net energy transfer from the student's body during the 20-min ride to school is 139.164 BTU.

Explanation:

From Heat Transfer we determine that heat transfer rate due to electromagnetic radiation (), measured in BTU per hour, is represented by this formula:

(1)

Where:

- Emissivity, dimensionless.

- Surface area of the student, measured in square feet.

- Stefan-Boltzmann constant, measured in BTU per hour-square feet-quartic Rankine.

- Temperature of the student, measured in Rankine.

- Temperature of the bus, measured in Rankine.

If we know that , , , and , then the heat transfer rate due to electromagnetic radiation is:

Under the consideration of steady heat transfer we find that the net energy transfer from the student's body during the 20 min-ride to school is:

(2)

Where is the heat transfer time, measured in hours.

If we know that and , then the net energy transfer is:

The net energy transfer from the student's body during the 20-min ride to school is 139.164 BTU.