P2: Thermal physics

  1. kinetic particle model of matter

  • Observations based on kinetic particle model of matter:

    • liquids take shape of their container → particles are free to move about

    • gases fill their container → particles move with complete freedom

    • solids keep their shape → particles are packed tightly together

    • gases diffuse from place to place

    • dissolved substances diffuse throughout a liquid

  • Brownian motion: particles constantly being knocked about by fast moving air and colliding with each other randomly.

  1. Pressure changes

  • Gas in a container: gas particles move around inside, bumping into the sides, constantly colliding with the walls → increases pressure

    • Increasing temperature: particles kinetic energy increases and hit the walls harder and more often → increases pressure

    • Compressing: volume decreases, particles collide more often with each other and the walls → increases pressure

  1. Matter and thermal properties

  • Thermal expansion: substances expand when temperature rises due to particles gaining energy and moving faster → push each other further apart

    • object increases volume when temperature increases → density decreases because mass stays the same

  • Uses of expansion: Rivets, thermometer, gap between railways

  • Evaporation vs boiling

  • Similar: liquid → gas

  • Evaporation:

    • Occurs at any temperature

    • Only on the surface

    • no energy required → most energetic particles escape (lowers overall temperature)

  • Boiling:

    • Only occurs at b.p

    • happens throughout the liquid

    • energy is required to overcome attractive forces

  • How to increase evaporation rate:

    • Increasing temperature

    • increasing surface area

    • Increasing wind speed

    • Decreasing humidity

  1. Thermal process

  • Conduction:

    • Transfers heat by collision between solid particles

    • Mainly occurs in solids

    • In metal, there are collisions between free electrons and solid particles → carry electric current and thermal energy

  • Convection:

    • Energy is transferred from a warmer place to a cooler place

    • Convection current: Hot liquids/gases expands and becomes lighter → rises away from heat source → gets colder and sinks back to heat source

  • Radiation:

    • Form of thermal energy transfer which does not need particles (can travel in a vacuum)

    • Hotter object → more radiation it emits

    • Black:

      • Better absorber

      • better emitter

    • White:

      • better reflecter

      • worse emitter