[Science] Pressure & Its Use in Physics

Table of Contents

PRESSURE

  • Pressure : Pressure is defined as force acting normally on unit area of the surface. P = F/A = Force/Area

  • SI unit of pressure is N/M2 also called pascal (Pa). Pressure is a Scaler quantity.

  • If the area is less then the pressure will be more.

  • Atmospheric Pressure : It is that pressure which is exerted by a mercury column of 76 cm length at 0 degree c at 450 latitude at the sea level.Generally it is measured in bar.1 bar = 105 N/M2.

  • Atmospheric pressure decreases with altitude.This is why (1) It is difficult to cook on the mountain. (2) The fountain pen of a passenger leaks in Aeroplane.

  • Atmospheric pressure is measured by barometer. With the help of barometer, weather forecast can be made.

  • Sudden fall in barometric reading is the indication of storm.

  • Slow fall in barometric reading is the indication of rain.

  • Slow rise in the barometric reading is the indication of clear weather.

  • Pressure in liquid : The pressure exerted by liquid at depth h below the surface of the liquid is given as P = hdg where d is the density of the liquid.

  • In a static liquid, pressure is same at all points.

  • Pascal law : If gravitational attraction is negligible , in equilibrium condition, pressure is same at all points in a liquid.The size of vessel does not affect the pressure of the liquid.

  • Hydrolic lift, hydrolic press, hydrolic brake work on pascal law.

  • Effect of pressure on melting point :

  • The melting point of substances which expands on fusion increases with the increase in pressure. For example: wax, ghee.

  • The melting point of substances which contracts on fusion decreases with the increase in temperature. For example: ice.

  • Effect of pressure on boiling point : Boiling point of all the substances increases with the increase in pressure.

  • Pressure cooker : If the pressure is increased then the boiling point of water also increases that is why the more energy is stored.There is no effect of the height on pressure cooker.

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