Question

In the given figure the atmospheric pressure is equal to ρgh. Which of the following is not always correct when the simple barometer is accurately measuring the atmospheric pressure?

• A. P_b = P_c
• B. P_b = P_atm
• C. P_b = 760 mm-Hg
• D. P_a is a vacuum

Answer 1

The correct option is C

P_b = 760 mm-Hg

In the given figure, pressure is equal to the same depth anywhere in the liquid. Thus, $P_b$ is equal to $P_c$. As $P_b$ is in contact with the atmosphere thus, $P_b$ is equal to P_atmosphere. Some space is left empty above the mercury column in the tube. The space $P_a$ in the tube is called Torricellian vacuum. The barometric height at normal temperature and pressure at sea level is 760 mm of Hg. This is standard atmospheric pressure, but sometimes as the atmospheric pressure changes, the measured value can be greater or less than this.