Question

To prevent fogging on a car’s windshield, warm air must be discharged on its inner surface, to keep it at a temperature higher than the dew point (10°C). Consider a windshield that is 0.8 m long, a car travelling at 70 mph in ambient air (-15°C). If the car defroster discharges air at 40°C, flowing from the base of the windshield up, what should be the speed of air the defroster to prevent?

a) 83 mph
b) 95 mph
c) 107 mph
d) 120 mph

Answer 1

The student's question about the required speed of air from the car defroster to prevent windshield fogging involves thermodynamics and is based on keeping the windshield temperature above the dew point. However, the provided information is insufficient to calculate the correct answer without additional knowledge on heat transfer and fluid dynamics.

Step-by-step explanation:

The question relates to the prevention of windshield fogging in a car by using a defroster to blow warm air over its inner surface. To prevent fogging, the windshield must be kept at a temperature above the dew point. The specifics provided, including a windshield of 0.8 meters, a car traveling at 70 mph in -15°C ambient air, and a defroster discharging air at 40°C, are parameters to determine the required speed of air from the defroster to maintain visibility. However, based on the provided information, the exact speed at which the defroster should blow the air cannot be calculated directly as the relationship between these variables is not provided. Instead, the question seems to require knowledge of thermodynamics and fluid dynamics to predict the rate at which heat is transferred from the warm air to the windshield and how this maintains a temperature above the dew point to prevent fogging.