Dissipation of Energy (Edexcel GCSE Physics: Combined Science)

• Unintended or wasted energy transfers are inevitable
• There is no such thing as a perfect energy transfer
• Most wasted energy transfers result in heating of the objects and the surroundings
• We say this energy is dissipated (spread out) to the thermal store of the surroundings

 

• Work done against air resistance, frictional forces, and resistance in wires all result in heating
• That energy is transferred to the thermal store of the surroundings increasing the temperature of the air particles and surrounding objects 
• Once energy is in the thermal store of the surroundings, it can not be 'gathered' for any specific use
• Therefore, it is referred to as wasted energy

 

Example 1: An Electric Kettle Boiling Water

• Useful energy transfer
  • Energy is transferred by heating from the thermal store of the heating element in the kettle to the thermal store of the water
• Unuseful energy transfers
  • Energy is transferred from the thermal store of the water to the thermal store of the kettle casing
  • Energy is transferred from the thermal store of the kettle casing to the thermal store of the surroundings and the temperature of the air in the room will increase slightly
  • Energy is transferred from the thermal store of the water to the thermal store of the surroundings as water evaporates

 

Example 2: An Electric Circuit Lighting a Filament Bulb

• Useful energy transfers
  • Energy is transferred from the chemical store of the bulb to the thermal store of the filament wire (the fact that the filament wire glows hot is how the light is produced)
  • Energy is transferred from the thermal store of the filament wire to the thermal store of the surroundings by radiation as visible light (EM radiation)
• Unuseful energy transfers
  • Energy is transferred from the chemical store of the cell to the thermal store of the wires due to resistance 
  • Energy is transferred from the chemical store of the cell to the thermal store of the bulb casing (the metal and glass that make up the bulb)
  • Energy is transferred from the thermal store of the filament wire to the thermal store of the surroundings (most of the energy transferred away from the bulb is by heating rather than as light)

• Mechanical processes can become wasteful when they cause a rise in temperature
• These processes often involve friction
  • When friction acts, it has the effect of transferring energy from the kinetic store by heating to the objects and the surroundings
  • This energy cannot be used in a useful way, therefore it is called wasted energy
  • Energy that is transferred to the surrounding is said to be dissipated (spread out) to the surroundings

 

• There are many situations where energy transfers are actually unwanted:
  
  • Keeping a house warm
  • Keeping a hot drink hot or cold
  • Friction of mechanical parts

Insulated mugs are used to maintain the temperature of hot or cold drinks

• When an appliance is used for heating something (a kettle, a heater, a tumble drier, a central heating system etc.), the appliance requires a lot of energy
  • It can become expensive for a household to run such appliances
  • The production of electricity using fossil fuels produces greenhouse gases which contribute to global warming
  • The combustion of (methane) gas produces greenhouse gases which contribute to global warming

• Therefore, it is often useful to explore ways of reducing unwanted energy transfers

• Energy that is dissipated to the surroundings is often the main source of wasted energy transfers
• If these unwanted energy transfers can be prevented, or reduced, the useful energy transfers can be made more efficient

Insulation

• Insulation reduces energy transfers from conduction 

 

• The effectiveness of an insulator is dependent upon:

• • The thermal conductivity of the material
    • The lower the conductivity, the less energy is transferred 

• • The density of the material
    • The more dense the insulator, the more conduction can occur
    • In a denser material, the particles are closer together so they can transfer energy to one another more easily

• • The thickness of the material
    • The thicker the material, the better it will insulate

 

• Insulating the loft of a house lowers its rate of cooling, meaning less energy is transferred to the surroundings (outside)
• The insulation is often made from fibreglass (or glass fibre)
  • This is a reinforced plastic material composed of woven material with glass fibres laid across and held together
  • The air trapped between the fibres makes it a good insulator

• The gaps or cavities between external walls are often filled with insulation
  • This is called cavity wall insulation
  • This is often done by drilling a hole through the external wall to reach the cavity and filling it with a special type of foam which is made from blown mineral fibre filled with gas
  • This lowers the conduction of heat through the walls from the inside to the outside

Less energy is transferred by conduction if the cavity is insulated