10: Transfer of heat

• What is temperature a measure of?

  How hot or cold an object is
• What is the SI unit of temperature?

  Kelvin (K)
• What is thermal energy?

  Energy stored in the body due to its temperature
• How is temperature different from thermal energy?

  Temperature measures how hot or cold an object is, while thermal energy is the energy stored due to temperature
• From where does net thermal energy always flow?

  From a region of higher temperature to a region of lower temperature
• What happens when two bodies in thermal contact are in thermal equilibrium?


  There is no net transfer of thermal energy between them
• What is the net thermal energy transfer when two bodies are at the same temperature?

  0
• What occurs when a substance is heated?

  It increases in size, a process called expansion
• What occurs when a substance is cooled?

  It decreases in size, a process called contraction
• How do solids generally behave when heated and cooled?

  They expand when heated and contract when cooled
• What happens when a body expands or contracts?

  Large forces are produced, which may help us or cause problems
• What happens to water when it is heated in a pot or kettle?

  It expands and may spill out when boiling
• What occurs to air bubbles in dough when heated?

  They gain heat and expand, causing the cake to rise
• What is a rivet used for?

  To join two steel plates together
• What happens to a rivet when it is heated and then cooled?

  The contraction on cooling helps to pull the steel plates together
• What is a bimetallic strip?

  A strip made of two different metals that expand at different rates
• How does a bimetallic strip behave when heated?


  It bends with the metal that expands more on the outside of the curve
• What is the function of a thermostat?


  To keep temperature constant
• How does a bimetallic thermometer work?

  It tightens or unwinds as temperature changes, moving a pointer over a scale
• What is conduction?

  The transfer of thermal energy through a medium without any flow of the medium
• How does conduction work?

  Particles at the heated end vibrate and collide with neighboring particles, transferring energy
• Why do metals conduct thermal energy more quickly than non-metals?

  Due to the presence of free electrons in metals
• How is thermal energy transferred in non-metals?

  Through the vibration and collision of particles
• What is convection?

  The transfer of thermal energy by means of convection currents in a fluid due to a difference in density
• What is the difference between conduction and convection?

  In conduction, thermal energy is transferred from one particle to another, while in convection, it is transferred through the movement of heated particles from the warmer and cooler parts of the fluid
• What happens when water at the bottom of a flask is heated?

  It expands, becomes less dense, and rises
• How do convection currents form in liquids?

  Due to changes in density caused by heating and cooling
• What happens to air above a candle when it is heated?

  It expands, becomes less dense, and rises
• How are land and sea breezes formed?

  Land heats up faster than the sea during the day, causing air above the land to gain heat and expanding to rise and cooler air from the sea is denser and moves in to replace the warmer air. This sets up a sea breeze. At night, the land cools faster than the sea. The air above the land is now cooler than the air above the sea. A convection current is set up in the opposite direction. This is called a land breeze.
• What is radiation?

  The transfer of thermal energy in the form of electromagnetic waves without the aid of a medium
• What factors affect the rate of emission of infrared radiation?

  Colour, texture, surface area, and surface temperature
• What can be deduced about the black tin and shiny tin?

  The black tin is a good emitter of thermal energy
• What is the effect of surface area on the emission or absorption rate of infrared radiation?

  The object with the larger surface area will emit or absorb infrared radiation at a higher rate
• How does surface temperature affect the rate of emission of infrared radiation?

  The higher the temperature of an object relative to its surroundings, the higher the rate of emission
• What are the three methods of thermal energy transfer?

  • Conduction: Transfer through a medium without flow
  • Convection: Transfer through convection currents in fluids
  • Radiation: Transfer in the form of infrared radiation without a medium
• What are the uses of expansion in materials?

  • Rivets: Join steel plates
  • Bimetallic strips: Used in thermostats and thermometers
• What are the effects of heat on solids, liquids, and gases?

  • Solids: Expand when heated, contract when cooled
  • Liquids: Expand when heated, contract when cooled
  • Gases: Expand when heated, contract when cooled
• How does convection work in liquid

  When the water at the bottom of the flask is heated, it expands. Volume increases. The expanded water is less dense than the surrounding water and rises. Since the upper region is cooler, it is denser and therefore sinks. The difference in the densities of water in the different regions sets up a convection current.
• The rate of emission of radiation is affected by a few

  factors.

  • Colour

  • Surface area. ( For two objects of identical mass and material,

  the object with the larger surface area will emit

  or absorb infrared radiation at a higher rate)

  • Texture

  • Surface temperature