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| Pascal's law |
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| Blaise Pascal, a French physicist, discovered that the pressure in a fluid in equilibrium is the same everywhere, if the effect of gravity is neglected. |
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Consider a spherical vessel having four cylindrical tubes A, B, C and D, each fitted with airtight frictionless piston, of area of cross-section 4a, 3a, 2a and a, respectively. When the vessel is filled with an incompressible liquid so that no air gap is left inside the vessel, a force 4 'F' exerted on A is transmitted in all directions. The other piston moves outward. To keep the pistons in their place, forces 3F, F and 2F have be exerted on B, C and D, respectively. Therefore, pressure
on each is  in each case. |
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| Consider a tank containing a liquid (or fluid). Let A and B be two points inside the liquid separated by a distance 'h'. Imagine a cylinder of liquid with axis AB, cross-sectional area A and length h. Let the mass of the liquid in this imaginary cylinder be 'm'. |
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| Let P1 and P2 be the pressure at A and B. The forces acting on the cylinder are |
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 F1 = P1 A acting vertically downward |
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 F2 = P2 A acting upwards on the lower face of the cylinder. |
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 Weight (mg) = W of the liquid acting downward. |
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| As the liquid is at rest, the net force acting on it should be zero. |
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| Note |
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 If A and B were at the same height |
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| i.e. h=0 then P2 - P1=0 or P1 = P2 |
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| i.e., Pressure is same at all points inside a liquid lying on the same horizontal plane. |
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 If g = 0 at place, then P2 = P1 at any two points inside the liquid (Pascal's law neglecting effect of gravity). |
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