#### Topics

1. Write a  short note on Pressure.

Pressure is an effect which occurs when a force is applied on a surface. Pressure is the amount of force acting on a unit area. The symbol of pressure is p.  The SI unit for pressure is the Pascal (Pa), equal to one Newton per square meter (N/m2 or kgm-1. ). This special name for the unit was added in 1971; before that, pressure in SI was expressed simply as N/m2. The standard atmosphere (atm) is an established constant.  It is approximately equal to typical air pressure at earth mean sea level.

2. Write a short note on Atmospheric Pressure?

Atmospheric pressure is the force per unit area exerted against a surface by the weight of air above that surface in the Earth’s atmosphere.  In most circumstances, atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point.  Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location.  Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases   with increasing elevation.   A column of air one square centimeter in cross- section, measured  from sea level to the top of the atmosphere, has  a mass of about a kilogram  and a  weight of  9.8N( 2.2Ibs)  ( and a  column  one square  inch in cross section would weigh about 14.7 Ib (63N).

3. Write a short note on Aneroid Barometer.

Aneroid barometer was invented by the French engineer and inventor Lucien  Vidie.  It uses a small, flexible metal box called an aneroid cell.  This aneroid capsule (cell) is made from an alloy of Beryllium and Copper.  The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure causes the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and  displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen.  Inaddition, the mechanism is made deliberately “stiff’ so that tapping the barometer reveals whether the pressure is rising or falling as the pointer moves.

4. Write a short note on Mercury Barometer.

Barometer is an instrument for measuring atmospheric pressure.  Two common types are the aneroid barometer and the mercurial barometer.  Evangelista Torricelli invented the mercurial barometer, known as the “Torricelli’s tube”.  Mercury barometer has a glass tube of at least 84cm in height, closed at one end, with an open mercury – filled reservoir at the base.  The weight of the mercury creates a vacuum in the top of the tube.   Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir.  High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column.   Low pressure allows the mercury to drop to a lower level in the column by lowering the force and placed on the reservoir.  Since higher temperature at the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect.

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