energy transfer by heating

• Thermal conductivity


  A measure of how good something is at conducting
• Thermal insulator


  Thermal insulators reduce energy transfers (prevent heat loss to surroundings and hence have a low thermal conductivity)
• Thermal conductor


  Good at transferring heat energy
• Specific heat capacity


  The amount of energy needed to change the temperature of 1Kg of the substance by 1°C. Its units are J/Kg/°C

  
• Joulemeter


  Energy meter (measures energy supplied)
• The higher the Thermal conductivity of a material

  The higher the rate of energy transfer by conduction across the material
• Metals


  • Metals are the best conductors of energy, Copper is a better conductor than steel
• Non-metals


  • Non-metal material (like wool and fibreglass) are the best insulators
• Factors affecting insulation


  • Thickness of material
  • Thermal conductivity
• Putting the same amount of heat into some materials gives a bigger temperature rise than in other materials
• Specific heat capacity


  The energy needed to raise the temperature of 1kg of a material by 1°C
• Measuring specific heat capacity

  1. Heat a metal block of known mass

  2. Use a joulemeter to measure the energy supplied ΔE

  3. Use a thermometer to measure the temperature rise Δθ

  4. Insert measurements into the equation to calculate specific heat capacity

  
• Storage heaters


  • Use electricity at night (off peak hours) to heat special bricks (which have a high specific heat capacity)
  • The bricks store lots of energy and take time to heat up and cool down
  • During the day (on peak) they release heat slowly when the heater element is on and cool down slowly when it is off
• How to prevent heat loss from a house

  • Loft insulation
  • Cavity wall insulation
  • Double glazed windows
  • Aluminium foil behind radiators
  • External walls with thicker bricks
• Electromagnetic radiation


  Transverse waves that travel at 300,000,000 m/s. Includes radio, microwave, infrared, visible, Ultraviolet, X-ray and gamma waves
• Infrared radiation


  An electromagnetic wave. Emitted by warm objects. Also known as heat or thermal radiation
• Black body


  A body that absorbs all the radiation that hits it
• Black body radiation


  The radiation emitted by a perfect black body
• Greenhouse gases


  Gases that contribute to the greenhouse effect by absorbing infrared radiation
• The Sun emits all types of electromagnetic radiation
• Infrared radiation consists purely of electromagnetic waves of a certain range of frequencies
• The hotter an object is, the more infrared radiation it emits in a given time
• What happens to infrared waves when they strike different surfaces

  • Dark matt surfaces absorb infrared radiation much better than light glossy surfaces
  • Silvered surfaces reflect nearly all heat radiation falling on them
  • Dark matt surfaces also emit more infrared radiation
• The temperature of an object will increase


  If it absorbs more radiation than it emits
• The Earth's temperature depends on

  The absorption of infrared radiation
• Greenhouse effect


  Greenhouse gases in the atmosphere (CO2, CH4 & H2O) absorb infrared radiation preventing it escaping into space, making the Earth warmer than it would be if these gases were not present
• No system is perfect. Whenever there is a change in a system, energy is transferred and some of that energy is dissipated.
• Dissipation


  Ways in which energy is wasted. Any energy that is not transferred to useful energy stores is said to be wasted because it is lost to the surroundings.
• Electrical cables warming up


  • Energy is dissipated to the surrounding air
• Energy dissipation in a mechanical system

  1. Energy is dissipated when two surfaces rub together

  2. Work is done against friction which causes heating of the two surfaces

  3. Internal (thermal) energy of the surfaces increases
• Lubricant


  Reduces friction and so less heat is wasted
• Using LED light bulbs instead of filament lamps

  Causes less energy to be wasted
• Examples of dissipation


  • Heating up the surroundings
  • Sound waves
• The ways in which energy is dissipated depends on the system
• Radio or set of speakers


  • Electrical work is transferred into useful sound waves and infrared radiation is dissipated - ie wasted as heat energy
• Tumble dryer


  • Electrical work is transferred into useful internal (thermal) energy which helps to dry clothes
  • Energy is dissipated by sound waves