[PHY] Chapter 6 - Pressure

DARRELL ER (COPYRIGHTED) © TOPIC 6: PRESSURE DARRELL ER (COPYRIGHTED) ©

CHAPTER ANALYSIS THE ABOUT MASTERY EXAM WEIGHTAGE DARRELL ER (COPYRIGHTED) ©

PRESSURE PRESSURE IN FLUIDS ATMOSPHERIC PRESSURE KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

PRESSURE PRESSURE Pressure is defined as the force acting per unit area. Unit: Pascal (Pa) or Nm-2 DARRELL ER (COPYRIGHTED) ©

PRESSURE PRESSURE When calculating pressure, take note of the units whether it is in cm or m. Also, mass is in kg while weight is a force, with units Newton (N). DARRELL ER (COPYRIGHTED) © For a block with unique dimensions, the smallest base area gives the greatest pressure. Meanwhile, the largest base area gives the least pressure.

PRESSURE IN FLUIDS PRESSURE IN FLUIDS Pressure exerted by a fluid is proportional to the depth at which the body is submerged. Formula: P = ρgh The formula P = ρgh is a derivative of P = F / A. P = F / A P = Weight / A P = mg / A Since mass = density x volume, P = (density x volume x g) / A Since Volume / A = height, P = ρgh Hence for any liquid, the height of the liquid in any container or orientation will give us the pressure. This is regardless of the shape or dimension/base area of the container. DARRELL ER (COPYRIGHTED) ©

PRESSURE IN FLUIDS ATMOSPHERIC PRESSURE Atmospheric pressure is defined as the weight of air in the atmosphere per unit area of any surface. Air that is around us exerts pressure on all bodies on Earth. Hence, a more accurate formula for fluid pressure that is exposed to air is, PTotal = PFluid + Patm DARRELL ER (COPYRIGHTED) © Atmospheric pressure

MERCURY BAROMETER HYDRAULIC SYSTEM U-TUBE MANOMETER KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

MERCURY BAROMETER MERCURY BAROMETER A simple mercury barometer measures atmospheric pressure. The mercury experiences atmospheric pressure outside the column (at Y). The height, X, represents the atmospheric pressure. (76cm Hg) The volume of mercury in the column will increase or decrease with changes in atmospheric pressure such that Pressure at y = Height of X Atmosphere (atm) and centimetres of mercury (cm Hg) are common units for atm pressure At sea level, it is 1 atm or 76cm Hg. DARRELL ER (COPYRIGHTED) © Scenario Explanation Water is used instead As water’s density is much lower while atmospheric pressure remains constant, the height of the water column will be much higher. (P = ρgh) The glass tube is tilted Perpendicular height of mercury column to the reservoir is unchanged as pressure is dependent on the vertical height and not the length of the column. The barometer is brought to a higher altitude As the air is thinner at higher altitude, atmospheric pressure is lowered. Hence, height of mercury column decreases. There is a crack in the glass tube along the mercury column above the reservoir Height of mercury decreases to the same level as the reservoir as air will move from outside the tube to inside until the pressure difference is zero.

HYDRAULIC SYSTEM A hydraulic system multiplies the effort so a small effort can be used to lift a much greater load. Formula: Pressure throughout the liquid is equal. Volume of water is equal. Application: Car Brakes When a driver steps on the brake pedal, the force on the small piston exerts pressure on the brake fluid. The brake fluid transmits the pressure to the larger pistons. The pressure exerts a greater force on the larger pistons, which clamps the disc and shows down the car. HYDRAULIC SYSTEM DARRELL ER (COPYRIGHTED) © P = F1 A1 F2 A2 = V = A1d1 = A2d2 d2 d1