conductivity and insulation

• Energy


  The capacity for doing work
• Methods of energy transfer

  • Conduction
  • Convection
  • Radiation
• Conduction


  Energy is transmitted through a material by the vibration and collision of particles
• Conductors


  • Allow internal (thermal) energy to be transmitted through them easily
  • All metals are good conductors
• When one end of a metal rod is put into a fire, the energy from the flame makes the ions in the rod vibrate faster</b>
• Since the ions in the solid metal are close together, this increased vibration means that they collide with neighbouring ions more frequently
• Energy is passed on through the metal by these collisions, transmitting the energy
• More frequent collisions increase the rate of energy transfer
• Insulator


  Material that does not allow energy to flow easily
• A metal seat at a railway station will feel cold as it conducts energy away from the passenger's body, whereas a cushion on the chair would not allow energy to flow so easily
• Thermal conductivity


  A measure of how well a material conducts energy when it is heated
• Energy is transmitted by conduction, convection or radiation
• Comparing conductivities


  • The conductivity of materials can be compared by examining the time taken to transmit energy through them
  • A fan of rods made of different materials can be heated at one end with the same flame, whichever rod gets hottest first at the other end is the best conductor
  • The material that heats the quickest is said to have a high thermal conductivity
• Thermal conductivity is a measure of how well a material conducts energy when it is heated
• Typical values of thermal conductivities

  • Copper: 385.0 W/m/°C
  • Glass: 0.17 W/m/°C
  • Brick: 0.15 W/m/°C
  • Air: 0.024 W/m/°C
• 385 joules (J) of energy will flow per second through a cubic block of copper (1 m × 1 m × 1 m) when the temperature difference between its sides is 1°C
• Insulating houses


  • The choice is between materials that are poor conductors such as brick, wood, plastic and glass
  • A house built of conducting materials like copper would be very cold to live in as energy would be able to leave the house easily
• Why is it better to have a window made of two layers of glass with a layer of air trapped between them

  • Both glass and air are insulators because they have low thermal conductivities
  • The layer of air has the lowest thermal conductivity and reduces the overall conductivity of the window unit
  • Air and glass are both transparent so people can still see through the window
• Investigating methods of insulation - materials

  1. Place a small beaker into a larger beaker

  2. Fill the small beaker with hot water from a kettle

  3. Put a piece of cardboard over the beakers as a lid

  4. Place a thermometer into the smaller beaker through the hole

  5. Record the temperature of the water in the small beaker and start the stopwatch

  6. Record the temperature of the water every 2 minutes for 20 minutes

  7. Repeat steps 1-6, each time packing the space between the large beaker and small beaker with the chosen insulating material
• Energy is transmitted by conduction, convection or radiation
• The conductivity of materials can be compared by examining the time taken to transmit energy through them
• It is important to make and record measurements of temperature and time accurately, measure and observe the effect of different materials as thermal insulators, and use appropriate apparatus and methods to measure the effectiveness of different materials as thermal insulators
• Materials


  • No insulation
  • Material 1
  • Material 2
• The curve which takes the longest time for the water temperature to drop (the shallowest) should be the material which is the best insulator
• The temperature falls quickly at high temperatures and slowly at low temperatures
• When the beaker is at a high temperature, there is a big difference between the temperature of the beaker and the temperature of the surrounding air, resulting in a high rate of heat transfer
• When the beaker is at a lower temperature, there is less difference between the temperature of the beaker and the temperature of the surrounding air, resulting in a lower rate of heat transfer
• Investigating methods of insulation - thickness

  1. Wrap a sheet of newspaper around a 100 ml beaker

  2. Fill the beaker with hot water from a kettle

  3. Put a piece of cardboard over the beaker as a lid

  4. Place a thermometer into the beaker through the hole

  5. Record the temperature of the water in the beaker and start the stopwatch

  6. Record the temperature of the water every 2 minutes for 20 minutes

  7. Repeat steps 1-6, each time adding another layer of newspaper around the beaker until there are 10 layers of newspaper wrapped around the beaker
• Aim of the experiment

  • To compare the effectiveness of different thicknesses of thermal insulators
• Energy is transmitted by conduction, convection or radiation
• Thermal conductivity


  The conductivity of materials can be compared by examining the time taken to transmit energy through them
• The temperature falls quickly at high temperatures and slowly at low temperatures
• As the number of layers increases


  The gradient of each curve decreases at any given time interval
• Having more layers of paper

  Increases the insulation which means the temperature drops more slowly
• The curve for no insulation

  Has the steepest gradient at any given time interval when compared to any other curve
• The curve which takes the longest time for the water temperature to drop (the shallowest gradient)


  Shows the amount of layers that provide the best insulation