# OCR - P6: Matter – models and explanations

 P6.4 How does the particle model relate to pressures in fluids? (separate science only) Background to the topic What you should be able to do: An object immersed in a fluid (a liquid or a gas) experiences forces acting at right angles to all its surfaces due to the pressure of the fluid. The pressure of the fluid is due to collisions of the particles of the fluid with the surface of the object. The particles of gas in a container collide with the surfaces of the container, exerting a pressure. If the volume of the container is increased, the particles have further to travel between collisions and the pressure of the gas falls. When a gas is compressed the particles are much closer together and will collide with the walls of the container more frequently, exerting a greater outward pressure. The atmosphere of the Earth exerts a pressure perpendicular to the surface of any object in it, and this pressure is the same in all directions at a particular height. Atmospheric pressure decreases with height above the surface of the Earth. The pressure at a point in a fluid increases with depth, because it is caused by the gravitational force on the fluid above that point. A fluid with greater density will experience a greater gravitational force and so exert a greater pressure. 1. Recall that the pressure in fluids causes a force normal to any surface In your practical work you will investigate the relationship between density of an immersed object and density of the fluid and the net force on the object. You will devise an experiment to show that pressure in a fluid varies with depth and investigate the relationships between the pressure of a gas and its volume and its temperature. 2. Recall and apply the relationship between the force, the pressure, and the area in contact: pressure (Pa) = force normal to a surface (N) ÷ area of that surface (m2) 3. Recall that gases can be compressed or expanded by pressure changes and that the pressure produces a net force at right angles to any surface 4. Use the particle model of matter to explain how increasing the volume in which a gas is contained, at constant temperature, can lead to a decrease in pressure. 5. Select and apply the equation: (for a given mass of gas at constant temperature) 6. Describe a simple model of the Earth's atmosphere and of atmospheric pressure, and explain why atmospheric pressure varies with height above the surface. 7. Explain why pressure in a liquid varies with depth and density.