Pressure

=** 1.8 Pressure **= • Relate (without calculation) pressure to force and area, using appropriate examples

Pressure is the amount of __ force __ acting on a certain __ area __. Pressure is measured in __p__ __ ascals __. [Pa] = Newtons per square metre. [ N/m 2 ] 1N/m 2 = 1Pa

What is the pressure on bear's feet when the mass of a bear is 150kg and size of a foot of the bear is 10cm2 ? P = Force / Area F= ma = 150kg x 10m/s2 = 1500N A: A bear has 4 feet so the area the force acting on it is 10cm2 x 4 = 40 cm2 Therefore, the pressure = F/A = 1500N / 40 cm 2 = __37.5 N/cm__ 2 1 Pascal is the same value as 1N/m2 where 1 cm2 = 1 cm x 1 cm = 0.01m x 0.01m = 0.0001m2 as 1cm = 0.01m So, __37.5 N/cm2__ is equal to 37.5N / 0.0001 m 2 The final pressure value on the bear's feet is 375000N/m 2 which is the same as 375000Pa (375kPa).
 * Pressure of a bear question**

• Describe the simple mercury barometer and its use in measuring atmospheric pressure Any liquid is held in its container by its own weight. This causes pressure on the container and any object in the liquid.

Pressure in fluids follows 4 main properties: 1) Pressure acts in all directions 2) Pressure increases with depth 3) Pressure depends on the density of the fluid 4) Pressure does not depend on the shape of the container An instrument that measures atmospheric pressure (the pressure we feel due to all the gases in the atmosphere above and around us) - atmospheric pressure at sea level is 100 kPa
 * **Barometer**

The barometer is filled with mercury which rises (high pressure) and falls (low pressure) as the atmospheric pressure changes. media type="youtube" key="EkDhlzA-lwI" width="560" height="315" [|The history of barometer] TED Ed Published on 28 Jul 2014 || Image from Thomson Higher education Mercury barometer is a long glass tube with one bottom end dipped in a dish of liquid mercury and sealed at the top. It keeps a column of mercury up the tube showing Atmospheric pressure. It is measured in millimetres of mercury [mm Hg].

The pressure at sea level is 760 mm Hg. ||

• Relate (without calculation) the pressure beneath a liquid surface to depth and to density, using appropriate examples • Recall and use the equation p = F/A Image from[| Toya.net] ||  || //Volume of liquid// V = A x depth of liquid (h).
 * Supplement **
 * • Recall and use the equation p = hρg **
 * [[image:Pressure on Dams.jpg width="513" height="176"]]

//Density of liquid//
 * ρ **= m/V, m = ** ρ ** V =** ρ ** A h

//Pressure [P]// = Force/Area = Weight of liquid / Area = m g / A = __** ρ **__ g __ A h __ / A
 * = ρ g h **

Calculate the pressure at depth from calctool || ||

• Use and describe the use of a manometer Manometers are used to measure gas and liquid pressures. It measures a difference in pressure. It involves 2 tubes being filled with a liquid (usually mercury). One of the tubes is attached to a gas supply. The difference in height of the liquids gives the excess pressure. To find the pressure of the gas, atmospheric pressure is added to the height difference. The height difference indicates the extra pressure that the gas supply has on top of atmospheric pressure. To find the actual pressure of the gas, we need to add on the value of atmospheric pressure.
 * ** (a) P gas pressure in (a) < 760 mm Hg ** (It's below atmospheric pressure)

P = 760 mm Hg - The different value in height of between two columns.


 * (b) P gas pressure in (b) > 760 mm Hg ** (It's above atmospheric pressure)

P = 760 mm Hg + The different value in height between two columns. || || 1. The open manometer in (a) is filled with mercury and connected to a container of gas. The mercury level is 90.0 mm higher in the arm of the tube connected to the gas Atmospheric pressure is 760 mmHg. What is the pressure of the hydrogen? Show your working. Answer: 670 mmHg

2. The open manometer in (b) is filled with mercury and connected to a container of gas. The mercury level is 70.0 mm lower in the arm of the tube connected to the gas. Atmospheric pressure is 760 mmHg. What is the pressure of the hydrogen? Show your working Answer: 830 mmHg

Image and questions from [|M.C.H.S Chemistry Page] || media type="youtube" key="L4A8yrmJ6kU" width="560" height="315" [|Manometer] Published on 1 Sep 2013 Gas law media type="custom" key="21409966"