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Chapter 18: Problem 10

Blow on the back of your hand with your mouth wide open. Your breath will feel hot. Now tighten your lips into a small opening and blow again. Now your breath feels cool. Why?

Short Answer

Expert verified
The phenomenon we observe is due to the principles of fluid dynamics and heat transfer. When the mouth is wide open, warm, slow-moving air has more time for heat transfer, so it feels hot. When making a small opening, air moves faster due to Bernoulli's principle. As airflow increases, it lowers both the air pressure and its temperature. Therefore the breath feels cooler because of less heat transfer to your skin.

Step by step solution

01

Defining the Scenario

When the mouth is wide open and one blow on the back of the hand, the breath feels hot. This is because the air is relatively slow-moving, so the hand can feel much of its warmth. When one blow through pursed lips, the breath feels markedly cooler. This is due to a phenomenon known as the Bernoulli's principle.
02

Explaining Bernoulli's Principle

According to Bernoulli鈥檚 principle, the pressure in a fluid decreases as the fluid moves faster. So when air is blown out with a small opening, it moves faster and therefore the pressure of the air decreases. This decrease in pressure also leads to a decrease in temperature.
03

Discussing Heat Transfer Principle

When you blow air from your mouth, you're forcing the air out and it moves faster. As it moves faster, it has less time to transfer heat to your skin, so it feels cooler. This is similar to why a fan cools you off. It鈥檚 not actually lowering the temperature in the room. Instead, it鈥檚 increasing the speed of the air moving over your skin, which makes it feel cooler.

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