Unit 5

• Density tells you how much mass is packed into a given volume of space
• Finding the density of an object

  1. Measure mass using a balance

  2. If box shape, measure length, width, height and calculate volume

  3. For irregular solid, submerge in eureka can and measure displaced water volume

  4. Plug mass and volume into density formula
• Density


  Mass per unit volume
• Density is a measure of the 'compactness' of a substance
• Density relates the mass of a substance to how much space it takes up
• Density formula


  Density (ρ) = mass (m) / volume (V)
• The units of density are g/cm³ or kg/m³
• Density of an object doesn't vary with size or shape, it depends on what it's made of
• A solid object will float on a fluid if it has a lower density than the fluid
• Pressure is force per unit area
• Pressure formula


  Pressure (P) = Force (F) / Area (A)
• Pressure is usually measured in pascals (Pa) or kilopascals (kPa)
• The same force applied over a larger area creates a lower pressure
• In gases and liquids at rest, pressure acts equally in all directions
• Pressure increases with depth in gases and liquids
• Calculating pressure difference in liquids and gases

  Pressure difference = height x density x gravitational field strength
• Solids


  • Strong forces of attraction hold particles close together in a fixed regular arrangement
  • Particles can only vibrate about their fixed positions
• Liquids


  • Weaker forces of attraction between particles
  • Particles can move past each other and form irregular arrangements
  • Particles have more energy than in solids
• Gases


  • Almost no forces of attraction between particles
  • Particles have more energy than in liquids and solids
  • Particles are free to move in random directions at high speeds
• The energy in a substance's thermal energy store is held by its particles in their kinetic energy stores
• Heating a liquid


  1. Extra energy transferred to particles' kinetic energy stores, making them move faster

  2. When enough particles have enough energy, bubbles of gas form - this is boiling
• Heating a solid


  1. Extra energy makes particles vibrate faster

  2. Eventually forces between particles are partly overcome and particles start to move around - this is melting
• During melting or boiling, energy is used to break bonds between particles rather than raising temperature, so temperature stays constant
• During condensing or freezing, bonds are forming between particles, releasing energy, so temperature doesn't go down until all substance has changed state
• Boiling point


  Temperature at which a liquid becomes a gas
• Melting point


  Temperature at which a solid turns into a liquid
• Evaporation


  1. Particles escape from a liquid and become gas particles

  2. Particles near surface can escape if they have enough energy in their kinetic energy stores to overcome attractive forces
• The fastest particles (with most energy) are most likely to evaporate, decreasing the average energy in the remaining liquid, causing it to cool
• Evaporation can have a cooling effect, e.g. sweating cools you down as water evaporates
• Absolute zero is the coldest temperature possible, at -273°C or 0 Kelvin
• Kelvin scale


  Temperature scale where 0 K is absolute zero, and 1°C = 1 K
• Increasing temperature increases the energy in particles' kinetic stores
• Gases consist of small particles constantly moving in random directions, taking up little space
• Increasing the temperature of a gas doubles the average energy in the particles' kinetic energy stores
• to degrees Celsius


  Subtract 273
• Freezing point of water

  • 0°С
• Boiling point of water

  • 100 °C
• There's no degree symbol when you write a temperature in kelvins
• Absolute zero


  • -273 °C
• Celsius scale


  Subtract 273 to get degrees Celsius