Question

Ceiling fans are sometimes reversible, so that they drive the air down in one season and pull it up in another season. Explain which way you should set the fan (a) for summer, (b) for winter.

Short answer

(a)

When the fan drives the air down during summer, it increases the evaporation from the human skin, making it feel cool.

(b)

The relatively hot air is at the top of the room during winter. When the fan pulls the air up, it forces the relatively warmer air to go down,which gives a warm feeling in winter.

Step-by-step solution

Concepts

Maximum fans direct the air downward.

During summer, the rate of evaporation from a surface is proportional to the rate of airflow.

Explanation

Part (a)

When the fan drives the air down during summer, it increases the evaporation from the human skin, making it feel cool because evaporation is a cooling process.

Part (b)

If the fan is set such that it pulls the air up, then relatively hot air will be present at the top of the room during winter. This hot air goes down and to the walls of the room. It gives a warm feeling to the occupants inside the room.