I am given a formula; Pressure p = Force [Newtons] / area [metres^2] I want to work out the pressure acting on a liquid surface. Therefore; Area A = 2 • π • 52 + 2 • π • 5 • 5 = 12494.10 cm2 = 124.940 mm2 = 0.00012494 m2 Therefore the average pressure of liquid acting on the jar is; = 3.924 / 0.00012494 Pressure p = 4.560 PSI My first part of confusion is weather I am right in deciding to multiply 0.400 milliliters of liquid by 9.81 N to say that the force acting is 3.924 N on the liquid before I decide that the force of the liquid acting on the jar is 4.560 PSI Next the result of pressure p = 4.560 psi I understand is the pressure acting on the jar, but how do I know if this pressure is absolute or gauge? If this exercise is theory and no experiment has been carried out, then in theory how would I add gauge pressure to absolute if indeed the pressure was absolute? Or is the result in pressure 4.560 psi acting on the jar gauge pressure, how would I know?
Your question seems very confused. First you say you want the pressure acting on the liquid surface then you start discussing the pressure on the inside of the jar. The pressure on the surface of liquid is about 14.7 psi. The pressure on the inside of the jar will be 14.7 psi + the pressure from the weight of the liquid which will increase with depth, by the equation \(P= \rho g h\). The pressure from the liquid will be tiny unless it is a really big jar! A column of water about 35 ft high will yield an additional pressure of 14.7 psi.
Harry2: The pressure due to what acting on the liquid surface? The air? Atmospheric pressure at sea level is about 100,000 Pascal. What liquid are you talking about? 1 millilitre of water has a mass of 1 gram and therefore a weight of 0.0098 Newton. 1 litre of water would have a mass of 1 kg and weight 9.81 N. But why are you using the weight of the liquid? Are you trying to work out the pressure on the top surface of the liquid, or the bottom surface (at the bottom of the jar), or on the sides, or what? Gauge pressure usually means absolute pressure minus atmospheric pressure. So, when you pump up your car tyres to 30 psi, for example, that's the gauge pressure - it means the pressure in the tyre is 30 psi above atmospheric. absolute pressure = gauge pressure + atmospheric pressure.
