Transfer of heat energy and its effects

• A thermometer is used to measure temperature (K) accurately.
• Heat is also known as thermal energy. Temperature is a measurement
• Heat will be transferred from higher temperature region to lower temperature region, until thermal equilibrium.
• Expansion: When objects are heated, they gain heat and generally increase in size (volume)
• Contraction: When objects are cooled, they lose heat generally and decrease in size (volume)
• Expansion (gas) > Expansion (liquid) > Expansion (solid)
• Density = mass/volume

  • As volume increases, density decreases
  • When a substance contracts, its density increases
  • When a substance expands, its density decreases
• Bimetallic strip: A strip made out of 2 different metal strips joined together
• Expansion and contraction of bimetallic strip

  • When heated, the material outside of the curve expands more
  • When cooled, the material inside of the curve contracts more
• Examples of bimetallic strips in devices

  • Bimetallic thermometer: The bimetallic strip is made into a coil. As the temperature changes, the two metal strips expand or contract unequally, causing the coil to unwind or tighten. The pointer moves over a scale to show the temperature.
  • Fire alarm: The bimetallic strip gets hot, bends and completes the electrical circuit.
  • Thermostat: Depending on design, the bending of bimetallic strip helps to regulate and maintain temperature.
• Explain why styrofoam box is used to keep food warm (conduction)

  • Styrofoam has numerous air pockets which are very poor conductors of heat (Property of material + poor/good conductor of heat)
  • This will reduce the rate of heat transferred from the internal store of the food to the surroundings by conduction (Effect on rate of heat flow + direction of heat flow +mode of heat transfer)
  • Hence, the food is kept warm
• Conduction: A process by which heat is transferred through a medium from one particle to another, without the movement of the medium itself
• Conduction (solids) > Conduction (liquid) > Conduction (gas)
• When one end of a solid substance is heated, the particles at the hotter end gain kinetic energy and vibrate faster. These particles collide with less energetic neighbouring particles, causing them to vibrate faster. Energy is transferred from one particle to another and through this manner, heat is transferred from the hotter to the cooler end of the solid.
• Why do metals counduct heat better than non-metals?

  
  

  Metals have free moving electrons, hence are better conductors of heat than non-metals.

  When heated, the free moving elections gain kinetic energy, moving faster in the spaces between particles. They collide with other particles and transfer energy to them
• Convection (only occurs in liquids and gases) : The transfer of heat from one place to another by the movement of the medium itself.
• Convection current: Movement of water set up by differences in densities
• Convection in liquids:

  When water is heated at the bottom, it expands (volume increases), becomes less dense and rises. The colder water at the top is denser so it sinks. The difference in densities of the water sets up a convection current.
• Convection (gas) > Convection (liquid)
• warming a room (convection current)

  • Heaters are placed as low as possible.
  • Air at the bottom of the room is heated up and expands. It becomes less dense and rises. (State effect of heaters being placed as low as possible)
  • Cold air at the top is denser, so it sinks and will be heated by the heater. (State the effect of the denser gas on top)
  • The cycle continues to set up a convection current to heat the room effectively. (Result)
• Radiation: A method of heat transfer without the need for a medium. (Can occur in a vaccum)
• Emitter and absorber of radiation

  • All objects and surfaces are able to emit radiation. This emission causes heat loss and the temperature of the object or surface to fall.
  • All objects and surfaces are able to absorb radiation. This absorption causes heat gain and the temperature of the object or surface to rise.
  • Good emitters = good absorber, poor emitters = poor absorber
• An object can only be either a net emitter or absorber at any one time.

  Since heat travels from hotter region to cooler region,

  • If object is cooler than surroundings, it is a net absorber
  • If object is warmer than surroundings, it is a net emitter
• What affects the rate of heat gain or loss?

  1. Nature of surface: Black, dull, rough are good emitters/absorbers. White, shiny, smooth are poor emitters/absorbers
  2. Surface area: The greater the surface area of a body, the more heat is gained or lost per second
  3. Temperature of body: The greater the temperature difference between the body and its surroundings, the more heat is gained or lost per second
• why wool blankets are used to keep us warm (conduction)

  • Wool traps air, which is a poor conductor of heat
  • Hence, this will decrease the rate of heat transferred from the internal store of the body to the surroundings
• Why stone floor feels cool to bare feet, but a carpet feels warm (conduction)

  • Stone is a better conductor of heat compared to the carpet floor.
  • Heat will be conducted away from internal store of our feet faster while standing on the stone.
  • The temperature of our feet will fall faster, hence our feet feels cooler on stone