# Pressure

Pressure is the force per unit area. Pressure = Force Unit: Pascal Pascal Symbol: Pa

Area 1 pascal is a force of 1 newton per square metre.

In some situations a high concentrated force is useful so a small area of contact is desirable e.g. scissors, knife

In others low pressure is better and so a large area of contact is in the design e.g. wide tyres, school bag straps,

2 Factors Affecting Pressure 1. the size of the force – the greater the force the greater the pressure.

2. the area of contact – the smaller the area the greater the pressure.

Simple Pressure Calculations

Calculate the pressure of the block on a table top. A 300 N block with one side of 6 m2 and another side of 15 m2 Dimensions of the rectangular block: 2 m, 3 m and 5 m.

Pressure = Force ÷ Area Pressure = Force ÷ Area

= F ÷ a = F ÷ a

= 300 N ÷ 6 m2 = 300 N ÷ 15 m2

= 50 N/m2 = 20 N/m2

= 50 Pa = 20Pa

Pressure in Fluids Three factors influence the pressure in a fluid.

a) Depth: the greater the depth the greater the pressure.

b) Density: the greater the density of a liquid the greater the pressure at a particular depth.

c) Gravity: the greater the force of gravity the greater the pressure at a particular depth.

Investigate the Relationship Between Pressure and Depth for a Liquid

Take a 2l drinks bottle, put 3 hole along one side, place blu-tak over the holes, fill the bottle with water.

Over a sink or waste paper, remove the 3 bits of blu-tac at the same time, watch to see how far each jet of water goes!

Pressure in a fluid acts equally in all directions perpendicular to the surface of the object.

Air bubbles rising in a liquid are spherical in shape indicating pressure is equal in all directions on it.

The bubbles expand as they rise indicating that the pressure decreases with decrease in depth.

To Show That Air Has Mass & The Air Occupies Space

Take a meter stick, balance it at the 50cm mark, to one end attach an empty balloon, to the other end attach a blown up balloon, watch which way the meter stick tips! Air has mass & it is attracted to the Earth by gravity.

The pressure of the atmosphere = Weight of the Atmosphere = 101,325 Pa

Surface Area of the Earth

The Balloon take up space, push it in to a basin of water, what happens!

Atmospheric Pressure Experiments

A: Fill a glass with water, Slip a card across the top to cover it perfectly. Put one hand over the card and carefully turn the glass upside down. Remove your hand from the card. The card and water stay in place.

B: Put about 2 cm of water into an empty metal can. Heat the water to boiling. Let it steam for a few minutes to drive out all the air. Grip the can by a beaker holder and quickly turn it upside down into a trough of cold water - the can collapses inward very rapidly. The steam filling the can condensed on cooling reducing the pressure inside the can.

Atmospheric Pressure Varies with Height

The atmosphere is a fluid. Therefore the pressure in the atmosphere is affected by depth. Depth and height are opposite – the greater the depth the lower the height. Pressure increases with depth so therefore the lower the height the greater the pressure. Atmospheric pressure on top of Mt Everest is much lower than at sea level – about 50% lower. The atmospheric pressure at the bottom of the deepest mine is greater than at sea level.

Atmospheric Pressure and Weather Changes in atmospheric pressure cause a change in the weather.

High Atmospheric Pressure (Anticyclonic Weather)

Calm & clear sunny dry days and calm clear nights.

Hot days and cool nights in summer.

Cold frost nights in winter.

Low Atmospheric Pressure (Cyclonic Weather)

Windy, cloudy, very humid and wet days and nights.

Cool days and nights in summer.

Mild days and nights in winter.

WeatherCharts or weather maps show lines called isobars drawn on a map of a particular area. An isobar is a line that joins places that have the same atmospheric pressure. The isobars often form patterns like circles. The numbers on the isobars represent the atmospheric pressure in millibars.

A high pressure weather system is indicated by the isobar number decreasing from the centre of the ‘circles of isobars’ – the highest pressure is in the centre ‘circle’. Usually marked with a H

If the isobar numbers are decreasing towards the centre then it is a low pressure weather system, marked by L

Measuring Pressure A: measure the height of mercury in a mercury barometer for atmospheric pressure.

B: use an aneroid barometer (no-liquid barometer) for atmospheric pressure.

C: use a Bourdon gauge to measure pressure in gas cylinders and boilers.