P2 - Energy transfer by heating

• Conduction


  Energy transfer through a material without the involvement of any bulk motion of the material
• Conductors


  • Materials that conduct energy well
• Insulators


  • Materials that conduct energy poorly
• Testing rods of different materials as conductors


  1. Rods coated with wax at one end

  2. Uncoated ends heated together

  3. Wax melts fastest on rod that best conducts energy
• Metals conduct energy better than non-metals
• Copper is a better conductor than steel
• Glass conducts better than wood
• Testing sheets of materials as insulators


  1. Use different materials to insulate identical cans/beakers of hot water

  2. Measure water temperature after a fixed time

  3. Use results to determine best insulator
• Thermal conductivity


  The ability of a material to conduct heat
• The greater the thermal conductivity of a material, the more energy per second it transfers by conduction
• Materials that have low thermal conductivity are good insulators
• Factors affecting energy transfer through insulating material

  • Temperature difference across the material
  • Thickness of the material
  • Thermal conductivity of the material
• To reduce energy transfer, insulating material should have low thermal conductivity and be as thick as possible

  
• Insulating a loft


  • Layers of low thermal conductivity material fitted on the loft floor reduce energy transfer through the roof

  
• Infrared radiation


  Part of the electromagnetic spectrum, longer wavelength than visible light

  
• All objects emit and absorb infrared radiation

  
• The higher the temperature of an object, the more infrared radiation it emits in a given time

  
• Black body


  An object that absorbs all the radiation that hits it, doesn't reflect or transmit any radiation
• A perfect black body is the best possible emitter of radiation
• The intensity of black body radiation varies with wavelength, with a peak intensity at a wavelength that depends on the temperature

  
• As temperature increases, the peak intensity shifts to shorter wavelengths
• An object absorbs more radiation than it emits

  Its temperature increases

  
• The Earth's atmosphere contains greenhouse gases


  They absorb infrared radiation from the Earth's surface and emit it back, warming the Earth

  
• Without an atmosphere, the Earth's surface temperature would plunge at night
• Absorption and emission of infrared radiation

  1. Infrared radiation from the Sun warms the Earth's surface

  2. CO2 molecules absorb and re-emit infrared radiation back to the ground

  
• The Earth absorbs radiation from the Sun

  The Earth emits radiation

  
• On a very clear night, the radiation the surface of the Earth emits is absorbed by the atmosphere

  
• A parked car in sunlight with its windows closed


  The temperature inside the car becomes much higher than the outside temperature
• A parked car in a shaded area


  The inside of the car would not become as hot
• The temperature of an object increases if it absorbs more radiation than it emits

  
• The Earth's temperature depends on a lot of factors, including the absorption of infrared radiation from the Sun, and the emission of radiation from the Earth's surface and atmosphere

  
• Specific heat capacity


  The energy needed to raise the temperature of 1 kg of a substance by 1°C
• Aluminium heats up more quickly than water
• Storage heater


  Uses electricity at night to heat special bricks or concrete blocks, which then release the energy to keep the room warm

  
• Bricks in a storage heater have a high specific heat capacity, so they store lots of energy

  
• Specific heat capacity of substances (J/kg °C)

  • Water: 4200
  • Aluminium: 900
  • Lead: 130
• A small bucket of water and a large bucket of water are left in strong sunlight


  The small bucket warms up faster