Wasted Energy (Edexcel GCSE Physics)

Wasted Energy

• 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

• Useful energy can be defined as:

  An energy transfer that serves an intended purpose

• Wasted energy can be defined as:

  An energy transfer that is not useful for the intended purpose and is dissipated to the surroundings 

Example: A Bat Hitting a Ball

• The moving bat has energy in its kinetic store

• Some of that energy is transferred usefully to the kinetic store of the ball

• Some of that energy is transferred from the kinetic store of the bat to the thermal store of the ball mechanically due to the impact of the bat on the ball

• Some of that energy is dissipated by heating to the thermal store of the bat, the ball, and the surroundings

Energy transfers taking place when a bat hits a ball 

Dissipation of Energy

• Energy transferred by heating and by radiation have a tendency to spread out to the surroundings

  • This is known as dissipation of energy

• Dissipated energy is very difficult to "gather" so that it can be used again

  • As a result, the energy becomes less useful

  • Because of this, whenever a process produces unwanted heating, light or sound, the energy is dissipated and essentially wasted

• Not all dissipated energy is wasted energy as the following examples show:

Example 1: A Television

• The useful energy transfers occurring in a television:

  • Energy is transferred electrically from the mains supply and is dissipated to the surroundings by radiation as visible light, and by heating as sound waves 

• The wasted energy transfers occurring in a television:

  • Energy is transferred electrically from the mains supply to the thermal store of the television and is then dissipated to the surroundings by heating 

Useful and wasted energy transfers in a television

Example 2: Heaters

• The useful energy transfers occurring in a heater:

  • Energy is transferred electrically from the mains supply to the thermal store of the heating element and is then dissipated to the surroundings by heating 

• The wasted energy transfers occurring in a heater:

  • Energy is transferred electrically from the mains supply to the thermal store of the heating element and is then dissipated to the surroundings by radiation as visible light 

Useful and wasted energy transfers in an electric heater 

Reducing Energy Loss

• 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

Lubrication

• Friction is a major cause of wasted energy transfers in machines

• For example, the gears on a bike can become hot if the rider has been cycling for a long time

  • Energy is transferred wastefully from the kinetic energy store of the bike to the thermal energy store of the gears and the chain

  • Since the energy is originally transferred from the kinetic store of the rider to the kinetic store of the bike, this means that the person has to do more work to make the bike move

• This wasted energy transfer can be reduced if the amount of friction can be reduced

  • This can be achieved by lubricating the parts that rub together

Lubrication helps reduce friction in the parts of a cycle

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