Pressure in Solids

Which is the odd one out? Why?

Bed of nails http://www.youtube.com/watch?v=JXj24tYimOI

Area Bigger Force Smaller Pressure force Pressure is the amount of …………… pushing on a certain …………… The bigger the force the …………… the pressure The bigger the area the …..………… the pressure area bigger smaller Key words Area Bigger Force Smaller

In your books Aim To investigate the effect of surface area on the size of impact with a constant force Diagram of set up Results & Conclusion And an explantion/ conclusion/ how Surface in contact Size of crater Explanation At least 6 different ex.

Spreading the Load Answer the question below in a full sentence in your book Why has Eskimo Fran fallen through the ice when Eskimo Percy hasn’t? Answer: Eskimo Percys weight is spread out over a bigger surface area because of his snow shoes!

Pressure is measured in Newton’s per square metre (N/m2) This is also called a Pascal (Pa) 1 Pa = 1 N/m2 The area of Fran’s shoes is 150cm2, He weighs 600N The area of Percy’s snow shoes is 360cm2, He weighs 750N Work out how much pressure is put on to the snow by their shoes Pressure = force ÷ area Fran = 600N ÷ 150cm2 Percy = 720N ÷ 360cm2 = 4N/cm2 = 2N/cm2

Pressure in liquids

Liquids cannot be compressed because the particles are already close together 11

This means that liquids can be used to send forces from one place to another in hydraulic systems Liquids cannot be compressed because the particles are already close together 12

YouTube - ridiculous hydraulics X YouTube - ridiculous hydraulics 13

Piston X puts pressure on the liquid Force = 20N Force = 10N Piston X puts pressure on the liquid Y X Area = 10cm2 Area = 40cm2 How do we calculate the pressure that is being applied from piston X? 14

f p a force area Force = 20N Force = 10N Y Area = 10cm2 pressure = X Ask pupils to calculate the pressure from piston x. Demonstrate how to calculate the force by rearranging the equation (higher pupils will probably only understand this point) Area = 10cm2 pressure = force Area = 40cm2 area 15

What would the pressure be at piston Y? Force = 20N Force = 10N The pressure stays the same throughout the liquid Y X The same as piston X = 1N/cm2 Area = 10cm2 Area = 40cm2 What would the pressure be at piston Y? 16

Force Area b) Force = Pressure  Area a) Pressure = = 5N/cm2 x 20cm2 10N 2cm2 = 100N = 5N/cm2

Pressure in Liquids In a liquid: Pressure acts in all directions and pressure increases with depth.

Pressure (N/m2) = 10 N/Kg x depth (m) x density (Kg/m3) The relationship between pressure and depth is shown by a water bottle with holes along its length. low pressure High pressure Pressure (N/m2) = 10 N/Kg x depth (m) x density (Kg/m3) The deeper you go, the higher the pressure The denser the liquid, the heavier it is! The pull of gravity

Force Area b) Force = Pressure  Area = 3N/cm2  30 cm2 Force = 90 N a) Pressure = Pressure = 30 10 Pressure = 3N/cm2 or Pascals (Pa)

Force Area b) Force = Pressure  Area a) Pressure = = 1N/cm2 x 100cm2 10N 10cm2 = 100N = 1N/cm2

Force Area a) Pressure = b) Force = Pressure x Area = 10N/cm2 x 200cm2 20N 2cm2 = 10N/cm2 = 2000N

Air Pressure

To begin with ... Explaining why Can you pull the Madeburg hemispheres apart? Why is this happening? 24

Atmospheric pressure us about 100 000Pa (although it changes day to day) That like having one kilogram resting on every square centimetre of your skin! The fluids inside your body are of a similar pressure so it evens out How come you don't notice this pressure? 25

A vacuum is a space where there are no particles at all The pressure outside the hemispheres is greater than the inside This pushes them together 26

Manometer We can measure the air pressure using a manometer

Manometer When the pressure acting on the left equals the pressure on the right the height either side of the manometer is the same

Manometer The pressure on the left (the air pressure) is the same as before BUT the pressure on the right is now bigger (different gas) So for it to be even… The air pressure AND the pressure caused by the difference in height from the liquid can then balance. And luckily… we have an equation for that BRILLIANT + =

Pressure = g ρ h REALLY IMPORTANT The manometer measures the difference in the pressure between two gasses Therefore if you want the actual pressure of the gas you need to add that on to what you have found using the equation

Barometer The barometer works in a slightly different way. This has a vacuum at the top, so rather than comparing two different gases it gives you the pressure of the gas acting on the surface- this is difficult to use with anything other than air pressure

Mercury Barometer When we use a Mercury barometer (Very heavy liquid) we can use a different unit of measurement. mmHg Millimetres of mercury … Is just that, can just measure the difference in height in mm Atmospheric pressure is 760mmHg

Pressure