Q11DQ When you blow on the back of you... [FREE SOLUTION]

91影视

University Physics with Modern Physics

Hugh D. Young, Roger A. Freedman

Physics

14th edition Edition 路 1596 pages

ISBN: 9780321973610

Chapter 3: Q11DQ (page 639)

When you blow on the back of your hand with your mouth wide open, your breath feels warm. But if you partially close your mouth to form an 鈥渙鈥� and then blow on your hand, your breath feels cool. Why?

Short Answer

Expert verified

With your mouth wide open, the exited gases emerge roughly at the same temperature as your body, you feel warm. But, when partially closed mouth, the adiabatic expansion occurs, the temperature of the exhaled gases/ air decreases near the lips and you feel cool.

Step by step solution

Understanding Joule-Thomson effect

In this effect, the variation in temperature occurs when fluid flows/moves from the region having high pressure in the respective low-pressure region. This Joule-Thomson effect is also termed as Joule-Kelvin effect.

Analysis of reason behind breathe feels warmer and colder

When the mouth is wide open and then the air coming from the mouth and it travels through a non-compressed area, so no temperature decrement takes place and it feels warmer.

While you close your mouth to make an 鈥渙鈥� shape and then blow air/gases on your hand, the exhaled gases will feel much cooler.

In this situation, the exhaled gases undergo a rapid expansion as they pass out into the atmosphere and the process takes so quickly that there is not enough time for appreciable heat to flow occur. Essentially in adiabatic expansion, as they emerge/ come out from between the lips, then the temperature of the exhaled gases/ air decreases near the lips and you feel cool.

Hence, with your mouth wide open, your breath will feel warm on your hand, because the exhaled gases emerge roughly at the same temperature as your body.