At ground level, the air pressure measured with a barometer is 1000 mb. The barometer is lifted upward by a weather balloon. When the balloon reaches 2 km above the ground, the measured air pressure is 800 mb. Explain why the air pressure decreased. After the balloon goes up another 2 km (now 4 km above the ground), will the measured air pressure be exactly 600 mb, lower than 600 mb, or higher than 600 mb? Explain the reason for your answer.
Kinetic Theory of Gas
The Kinetic Theory of gases is a classical model of gases, according to which gases are composed of molecules/particles that are in random motion. While undergoing this random motion, kinetic energy in molecules can assume random velocity across all directions. It also says that the constituent particles/molecules undergo elastic collision, which means that the total kinetic energy remains constant before and after the collision. The average kinetic energy of the particles also determines the pressure of the gas.
P-V Diagram
A P-V diagram is a very important tool of the branch of physics known as thermodynamics, which is used to analyze the working and hence the efficiency of thermodynamic engines. As the name suggests, it is used to measure the changes in pressure (P) and volume (V) corresponding to the thermodynamic system under study. The P-V diagram is used as an indicator diagram to control the given thermodynamic system.
At ground level, the air pressure measured with a barometer is 1000 mb. The barometer is lifted upward by a weather balloon. When the balloon reaches 2 km above the ground, the measured air pressure is 800 mb. Explain why the air pressure decreased.
After the balloon goes up another 2 km (now 4 km above the ground), will the measured air pressure be exactly 600 mb, lower than 600 mb, or higher than 600 mb? Explain the reason for your answer.
Hint: This can be a little tricky. You need to consider how air density changes as you move upward. See the figure below. If air density does not change as you move upward, then the "block" or air from 2- 4 km weighs the same as the "block" of air from 0 - 2 km and the air pressure will decrease uniformly to be exactly 600 mb at 4 km. If air density changes as you move upward, then the "block" of air from 2 - 4 km will not weigh the same as the block of air from 0 - 2 km and so the air pressure will not be exactly 600 mb at 4 km.
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