Solids liquids gases (pressure

• the density of something is the mass per unit volume (kg/m3)
• density = mass / volume
• A less dense mass floats on a denser liquid
• a less dense liquid floats on a denser one
• find density of a liquid:

  • find mass of measuring cylinder by putting on balance before adding liquid & measuring new mass
  • subtract mass of liquid minus mass of cylinder to avoid zero error
  • volume read from cylinder
  • find density using equation
• find density of a solid:

  • measure mass of solid by placing on a balance
  • if solid is regularly shaped = measure dimensions using ruler & then use a formula to find volume
  • if irregular = immerse in water & measure volume of water displaced. This is the volume of the solid
  • find density using equation
• pressure is the force per unit area & is measured in pascals (Pa)
• pressure = force / area
• When measuring the volume of water, ensure the volume is read at eye level to avoid parallax errors
• a force over a small area creates a high pressure

  • the same force over a large area creates a low pressure
• the pressure at a point in a gas or liquid at rest acts equally in all directions & causes a force at right angles to any surface

  • pressure in a fluid is created from the movement of particles as they collide with a surface
• the pressure beneath a liquid surface increases with depth, the density of the liquid and the gravitational field strength
• Pressure difference = height x density x gravitational field strength
• • fluids with higher density have more particles per unit volume, hence greater weight
  • weight depends upon gravitational field strength
• Fluids are liquids & gases
• the pressure of water increases with depth

  • pressure is greatest at the bottom
  • this is as the greatest weight of the water is pushing on the container walls
  • at a given depth, fluid pressure is the same in all directions
• the weight of the atmosphere creates pressure

  • atmospheric pressure is greatest at lower altitudes (about 100 KPa = 100,000 Pa)
  • atmospheric pressure is lower at higher altitudes
• total pressure = atmospheric pressure + water pressure
• gas molecules move rapidly & randomly due to collisions with other gas molecules
• • gas molecules move around randomly at high speeds
  • they collide with the walls of the container & exert a force
  • the force acts over the area of the container & exerts a pressure
  • as pressure = force / area
• • when a gas is heated its molecules gain kinetic energy
  • they move faster & collide with container walls with more force
  • this increases the pressure
• the temperature at which the pressure is zero is called absolute zero = -273C
• • when a gas is cooled, its molecules have less kinetic energy
  • they move more slowly
  • absolute zero is -273C & at this temperature, the molecules have zero kinetic energy
• absolute zero is the start of the Kelvin scale
• • 0K = C + 273
• • absolute zero = -273C & 0K
  • freezing point of water = 0C & 273K
  • boiling point of water = 100C & 373K

  
• • to convert Celsius to kelvin = + 273
  • convert kelvin to Celsius = - 273
• temperature in kelvin = temperature in C + 273
• for a gas at fixed mass or volume, where the temperature is measured in kelvin:

  • p1 / t1 = p2 / t2
• For a fixed mass at constant volume, its pressure is directly proportional to its temperature in Kelvin
• the gas law:

  • p1 /t1 = p2 / t2
  • where p = pressure (Pa)
  • t = temperature (K)
• investigating gas law to find absolute zero:

  • fixed mass & volume of gas heated in water bath
  • as temperature increases in C, pressure is recorded
  • a graph of temperature in Pa is plotted against temperature in C
  • if line is drawn back to where pressure is zero, absolute temperature is found to be -273C (under perfect conditions)

  
• at constant temperature = if the volume increases, the pressure decreases as the molecules collide less frequently with the walls & over a greater area
• for a gas at fixed mass & temperature:

  • p1V1 = p2V2
  • known as Boyle’s law
• • gas pressure is caused by gas particles colliding with walls of container
  • the force they exert over area of container walls exerts pressure
  • if volume decreases, the gas particles speed stays the same however they have less distance to travel
  • therefore they collide with container walls more frequently & more pressure is exerted
• boyles’s law:

  • for a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume
  • p1V1 = p2V2
• the temperature in Kelvin of a gas is proportional to the average kinetic energy of the molecules

  • the higher the temperature, the greater the average kinetic energy & so the faster the average speed of the molecules