Heat

• Thermal energy

  Result of the movement of tiny particles in solids, liquids and gases
• Temperature 


  The measure of how hot an object is.

  SI unit : Kelvin K
• 0K = absolute zero = -273°c
• 100K = -173°c
• 45°c = 273+45 = 318K
• In water, water contracts and volume decreases from 8°c to 4°c. Density increases hence. From 4°c to 0°c, water expands and volume increases, density decreases and ice floats onto surface of water for aquatic animals to survive.
• At 4°c, water has the smallest volume and highest density.
• Thermal equilibrium is reached when there is no net movement of thermal energy between two bodies
• Thermal energy is transferred only when there is a difference in temperature.
• Conduction is the transfer of thermal energy through a material medium due to vibrations of particles and collisions, without any flow of the medium.
• Main mechanism of conduction is the vibration and collision of particles and free electron diffusion in metals.
• Thermal energy transfer in a solid through conduction

  When the medium is heated at one end, the particles gain kinetic energy and vibrate faster about their fixed positions. These vibrating particles collide with their less energetic neighbours and kinetic energy is transferred to these neighbouring particles. Thermal energy is passed along the medium by the vibrating particles

  There is no net movement of particles during the process of conduction.
• There is no transfer of thermal energy through conduction in vacuum as there is no medium present
• In solids, particles are arranged closer together hence collision is more frequent and conduction of thermal energy is more efficient.
• Uses of insulators

  • animals like bears have fur that trap pockets of air to minimise thermal energy lost during winter
  • Woolen clothes
  • Double-glazed windows used to insulate homes
• Uses of conductors

  • Cooking utensils
  • Heating elements in ovens, kettles
• Convection is the transfer of thermal energy in a fluid through mass movements of particles due to density differences.
• Convection only occurs in fluids. And gases
• Sea breezes in day time

  Land mass heats up faster than water. The land heats up the air above it. Air expands, becomes less dense than surrounding air and rises. Denser air from above the sea will flow towards land and occupy the space.
• Explain why are air conditioners installed nearer the ceiling of rooms.

  The warm air at the top is cooled, becomes less dense and sinks to the bottom of the room. This pushes the warmer, less dense air at the bottom to rise and occupy the space. The air in turn gets cooled and this sets up a convection current which helps to cool the room.
• State one difference and one similarity between conduction and convention

  Difference:

  There is net movement of particles in convection but none in conduction

  Similarity:

  Both require the presence of a material medium.
• Radiation is the transfer of thermal energy in the form of electromagnetic waves known as infrared radiation without the aid of a medium.
• A good absorber of infrared radiation would gain thermal energy faster.
• A good emitter is thermal energy will lose thermal energy faster
• Radiation affected by

  • colour and texture of surface
  • temperature
  • surface area
• Something with a greater temperature will emit infrared radiation at a greater rate than something with a lower temperature
• Vacuum in a thermos reduces rate of thermal energy lost by conduction and convection as both processes require the presence of a medium
• When air is confined to a small space / small air pockets, it reduced convection currents and thermal energy transfer by convection is reduced.
• When they focus on the type of material ( metal, air) it is conduction.

  When they focus on appearance and texture ( black, rough) it is radiation.
• Why is the frozen pack placed at the top of the box rather than the bottom?

  Air at the top will lose thermal energy, contracting and becoming more dense, hence sinking down. Hotter air at the bottom, which Particles are now less dense, will rise and form a convection current. Placing the frozen packet at the bottom would result in warmer, less dense air at the top to remain warm and unable to be cooled.
• How do igloos keep occupants relatively warm?

  • have pockets of air in between snow that are poor conductors of thermal energy
  • White, smooth snow that are poor emitters of infrared radiation so they retain thermal energy
  • Lowered entrance traps hot air that has lower density and rose due to convection