Hydrostatics Physics - Physics Notes | Study General Science(Prelims) by IRS Divey Sethi - UPSC

                                                                         Hydrostatics Physics

Fluid pressure

• The force acting normally on unit area of a surface due to a fluid is called the fluid pressure. Thus,
• Pressure = force/area

Pascal’s law

• The pressure exerted anywhere in a mass of confined fluid is transmitted equally and undiminished in all directions throughout the liquid.
• Braham’s hydraulic press works on this principle.

Archimedes's Principle

• According to the principle when a body ( totally or partially ) immersed in a fluid, it appears to lose a part of its weight and the apparent loss of weight is equal to the weight of fluid displaced.

• Apparent weight of body,= Actual Weight of Body-Up-thrust.
• Buoyant force depends on the density of the fluid and not on the density of body acting on centre of gravity of fluid.
• When a body is immersed in a fluid then if :
  • The weight W of body is more than the up-thrust , the body will sink.
  • The weight W of the body is equal to up-thrust , the body floats with whole of its volume inside the liquid.
• When an ice block floats in water the water level will remain the same when all the ice melts into water.

Principle of floatation

• According to this principle, a body floats in a liquid if weight of the fluid displaced by the immersed portion of the body is equal to the weight of the body.
• The apparent weight of the floating body is zero.

Bernoulli's principle

• It relates pressure, velocity, and height for a non-viscous fluid with steady flow.
• A consequence is that, for horizontal flow, as the speed of a fluid increases, the pressure it exerts decreases.
• the principle explains the lift of an airplane in motion. As the speed of the plane increases, air flows faster over the curved top of the wing than underneath.
• The upward pressure exerted by the air under the wing is thus greater than the pressure exerted downward above the wing, resulting in a net upward force, or lift.

Surface tension

• Every liquid has a property that its free surface behaves like a stretched elastic membrane and has a natural tendency to constant, i.e., the free surface of a liquid tends to occupy a minimum surface area.
•  This property of liquids is called the surface tension.

Surface tension

• The surface tension of a liquid results from an imbalance of intermolecular attractive forces, the cohesive forces between molecules:
  • A molecule in the bulk liquid experiences cohesive forces with other molecules in all directions.
  • A molecule at the surface of a liquid experiences only net inward cohesive forces.
• The molecules at the surface of this sample of liquid water are not surrounded by other water molecules. The molecules inside the sample are surrounded by other molecules.

• The unbalanced attraction of molecules at the surface of a liquid tends to pull the molecules back into the bulk liquid leaving the minimum number of molecules on the surface.
• It required energy to increase the surface area of a liquid because a larger surface area contains more molecules in the unbalanced situation.

Factors affecting surface tension

• Temperature:- the surface tension of a liquid decreases with the rise in temperature.
• Soluble impurities:- if the impurity is less soluble the surface tension of the liquid decreases. If the impurity is more soluble, the surface tension of liquid increases.
• Insoluble impurities:- the surface tension of liquid decreases due contamination.

Adhesive Forces

• Forces of attraction between a liquid and a solid surface are called adhesive forces.
• The difference in strength between cohesive forces and adhesive forces determine the behaviour of a liquid in contact with a solid surface.
  • Water does not wet waxed surfaces because the cohesive forces within the drops are stronger than the adhesive forces between the drops and the wax.
  • Water wets glass and spreads out on it because the adhesive forces between the liquid and the glass are stronger than the cohesive forces within the water.
• When liquid water is confined in a tube, its surface (meniscus) has a concave shape because water wets the surface and creeps up the side.

• Mercury does not wet glass - the cohesive forces within the drops are stronger than the adhesive forces between the drops and glass.
• When liquid mercury is confined in a tube, its surface (meniscus) has a convex shape because the cohesive forces in liquid mercury tend to draw it into a drop.

Capillary Action

• Capillary action is the rise of a liquid that wets a tube up the inside of a small diameter tube (i.e., a capillary) immersed in the liquid.
  • The liquid creeps up the inside of the tube (as a result of adhesive forces between the liquid and the inner walls of the tube) until the adhesive and cohesive forces of the liquid are balanced by the weight of the liquid.
  • The smaller the diameter of the tube, the higher the liquid rises.