Physics 1 heat

• Conduction


  • In a non-metal (plastic/rubber), there are many atoms and many electrons but an insignificant number of free electrons
  • In a metal, there are a significant number of free electrons that are able to move and transfer heat across the metal
• Conduction in a metal

  1. Free electrons near the heat source speed up and carry energy to other parts of the metal

  2. They collide with particles (atoms or molecules) in other parts of the metal and transfer energy
• Metals are better heat conductors than non-metals (such as plastic or rubber) because non-metals have an insignificant number of free electrons
• Convection


  • A convection current occurs when a fluid is heated and becomes less dense and rises, and is then replaced by cooler surrounding particles
  • Occurs in fluids (liquids and gases)
• Density


  Mass / Volume
• When substances are heated, they become less dense
• Liquids and gases are fluids (particles can move), not solids
• Convection


  1. A part of the fluid (liquid or gas) is heated and becomes less dense, it rises, and is replaced by cooler surrounding fluid

  2. As less dense fluid rises, heat energy is carried away from the heat source by the molecules
• Convection can occur in liquids and gases, but not in solids because the molecules in a solid do not move away from their fixed position
• Convection currents in coastal breezes

  • At the start of a sunny day, heat energy from the sun is supplied to the land and to the sea
  • The land increases in temperature faster than the sea
  • The air above the land becomes less dense and expands, it then rises
  • This makes cooler, more dense air move towards the land from above the sea
  • A convection current is formed
• Radiation


  • The movement of heat from one place to another without the use of particles
  • The energy is transferred in small packages of energy from one place to another
• Bimetallic strip


  • Contains two metals on each side (e.g. copper, iron)
  • When heated, copper expands at a greater rate than iron, causing the strip to bend inwards with the copper side on the outside
  • When cooled below room temperature, the copper would contract at a greater rate than the iron and the strip would curve the opposite way with the copper on the inside of the curve
• Heat insulation


  • Conducted - pick up heat
  • Emitter - gives off heat
  • Plastic is a bad conductor of heat so the plastic lid of a flask reduces heat loss by conduction
  • Shiny silver reflects the heat back inside the flask
  • Shiny silver surfaces are the worst emitters of heat, this reduces heat losses
  • To reduce heat energy flow as much as possible from a hot object, it should be surrounded by a vacuum (space with no particles) and its surface should be shiny silver
  • A simple way of reducing heat energy flow from a hot object is to insulate it with a thermal insulator material such as wool or polystyrene that consists of a lot of trapped air within a poor conducting material, as the air is a very poor heat conductor and convection currents are minimised