Heating and Cooling

• Kinetic Theory states that...

  all matter consists of many particles that are constantly moving
• Heat is the transfer of thermal energy from warm too cool until thermal equilibrium is reached
• Internal Energy (U) : the total Ep and Ek of an object
• Temperature (joules)

  the average kinetic energy of an object's particles
• Converting from C -> K

  Add 273
• Converting from K -> C

  Minus 273
• Thermal Equilibrium: the point when two objects reach the same temperature and energy transfer stops
• ΔU (change in internal energy) = Q +W
• In terms of ΔU=Q+W, when should Q be +/-

  + when being heated, - when cooling
• In terms of ΔU=Q+W, when should W be +/-

  + when work is being done on the object, - when work is being done by the object
• Latent Heat: the potential energy that is released/absorbed during state change
• Why does latent heat of vaporization require more energy than the latent heat of fusion.

  In Lvap all intermolecular bonds must be broken hence more energy is required
• Evaporation: when a substance turns from liquid to gas at room temperature i.e. sweat evaporating

  • more noticeable in volatile liquids as surface bonds are weaker
• How does evaporating at room temp work?

  if the particles at the surface have sufficient energy, they can break bonds and escape through the liquids surface into the air. Causing the substance to lower in temperature as less of the high Ek particles remain.
• Energy: the ability to do work
• W=Fs

  W= work done (joules)

  F= force applied (newtons)

  s= distance (meters)
• Thermal E -> Mechanical E

  • a system with Thermal E has the ability to do mechanical work
  • the system doing the work will lose internal E, whereas the system the work is being done on will gain internal E
• efficiency (η) = energy output/energy input x100
• Conduction: the transfer of thermal energy through contact

  Two ways can occur:

  1. atomic collisions
  2. free electrons
• Insulator: a poor conductor

  • keeps things warmer/cooler for longer
• Conductor: something with the ability to conduct heat readily

  • does not hold heat but transfers
• Conduction by Collision:

  • kinetic theory states that particles within a solid are constantly moving and interacting with one another
  • when one area is heated, particles gain kinetic energy, vibrating more rapidly
  • as vibrations increase, energy is passed to neighboring particles
• Conduction by Free Electrons:

  • materials (like metals) have delocalized electrons that are free to move
  • when heated, both positive ions and electrons gain kinetic energy
  • since electrons have an insignificant mass, the small energy increase causes a large change in velocity, allowing quick energy transfer
• Thermal conductivity: the ability of a material to conduct heat

  (measured in Watts per Meter per Kelvin)
• Factors affecting conductivity rate:

  • temperature difference between the two objects - a greater temp difference results faster energy transfer
  • material thickness - thicker materials require a greater number of collisions or electron movement, therefor making energy transfer slower
  • surface area - the larger the surface area, the more contact available for heat transfer, therefore having a quicker rate
• Larger specific heat capacities take longer to change temperature than than lower specific heat capacities
• Convection: the transfer of thermal energy in a fluid (liquid or gas) by the movement of warm regions
• Convection

  • as a fluid is heated, particles gain kinetic energy and expand, becoming less dense and wants to rise
  • the colder region at the top has slower moving particles and is more dense, therefore sinks back to the bottom
• Convention causes:

  1. Forced - heat is added
  2. Natural - when the fluid at the top heats (i.e. lake)
• Radiation: the transfer of heat without matter or particles, but instead through electromagnetic radiation. hence not requiring a medium
• Electromagnetic spectrum: the range of all forms of both light and electromagnetic waves
• Electromagnetic waves travel at the speed of light and when come into contact with are partially absorbed, reflected and transmitted
• Electromagnetic radiation is emitted by all objects above absolute zero
• The wavelength and frequency of an electromagnetic wave is dependent on an objects internal energy

  • the higher the temperature, the shorter the wavelength and the higher the frequency
• If an object absorbs more energy than it emits, temperature rises

  If an object emits more energy than it absorbs, temperature decreases

  If no change occurs, object is at thermal equilibrium
• Factors affecting the rate of absorption/emission:

  • Surface area - the larger the exposed area the more area for radiant transfer
  • Temperature - the greater the temperature difference between the emitting/absorbing object, the greater the rate of energy transfer
  • Colour and Texture - characteristics determine how readily it will emit/absorb radiant energy e.g. matte black emits/absorbs faster than shiny white, matte black cools faster aswell as they radiate equally as efficient as they absorb
• Specific Heat Capacity: the energy required to change 1 kg by 1 degree kelvin

  (J Kg^-1 K^-1)