1.1.13 Conduction of Heat

• Conduction


  The process by which heat energy is transferred through a material without the material itself moving
• How Conduction Works

  1. Particle Interaction

  2. Energy Transfer

  3. Steady State
• Particle Interaction

  In a solid, particles are closely packed together and vibrate around fixed positions
• When one part of the solid is heated, the particles in that region gain kinetic energy and vibrate more vigorously
• Energy Transfer

  The vibrating particles collide with neighboring particles, transferring some of their kinetic energy
• Steady State

  Heat spreads through the material until the temperature is uniform, or thermal equilibrium is reached
• Factors Affecting Conduction

  • Material Type
  • Temperature Difference
  • Material Thickness
  • Surface Area
• Conductors

  • Materials like metals (e.g., copper, aluminum) have free electrons that can move easily, making them good conductors of heat
• Insulators

  • Materials like wood, plastic, and glass have tightly bound electrons and are poor conductors of heat
• A larger temperature difference between two regions

  Increases the rate of conduction
• Material Thickness

  Thicker materials slow down the rate of heat conduction
• Surface Area

  The greater the surface area through which heat is conducted, the more heat can be transferred
• Cooking Utensils

  A metal spoon heats up quickly when placed in a hot liquid because metal is a good conductor of heat
• Thermal Insulation

  Houses are insulated with materials like fiberglass or foam, which are poor conductors of heat
• Heat Sinks

  Computer processors use heat sinks made of metal to dissipate heat
• Thermal Conductivity

  A property of a material that indicates how well it conducts heat
• Thermal Conductivity

  • High Thermal Conductivity
  • Low Thermal Conductivity
• High Thermal Conductivity

  • Good conductors (e.g., metals like copper and aluminum)
• Low Thermal Conductivity

  • Poor conductors (e.g., insulating materials like rubber and fiberglass)
• Calculating Heat Transfer

  1. Use the formula: Q=k⋅A⋅ΔT⋅t

  2. Omit time for rate of heat transfer: P=k⋅A⋅ΔT
• Q is the heat transferred (in joules, J)
• k is the thermal conductivity of the material (in W/m°C)
• A is the area through which heat is transferred (in square meters, m²)
• ΔT is the temperature difference across the material (in degrees Celsius, °C)
• t is the time over which heat is transferred (in seconds, s)
• d is the thickness of the material (in meters, m)
• Worked Example

  A wall has an area of 10 m², is 0.2 meters thick, and is made of a material with a thermal conductivity of 0.5 W/m°C
• If the temperature difference across the wall is 30°C, the rate of heat transfer through the wall is 750 watts