Wasted Energy (AQA GCSE Physics)

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Wasted Energy

  • In practice, most systems tend to be open systems
  • When energy transfers occur that are not useful, these are described as energy being dissipated to the surroundings 
    • Dissipated just means spread out
    • This is considered to be wasted energy

  • Often these less useful energy transfers often involve heating, light and sound
  • When energy is transferred to the thermal stores of the objects, the temperature of the objects increases
    • The particles that make up the objects vibrate more, hence the transfer pathway is by heating (of the particles)
  • Visible light is electromagnetic radiation
    • Therefore, when light is produced, energy is transferred by radiation
  • When sound is produced, the sound waves make the air particles vibrate as the wave carries energy away 
    • This increases the energy in the thermal store of the air, hence the transfer pathway is by heating (of the particles)

  

  • Useful energy can be defined as:

The energy that is transferred from store to store and used for an intended purpose

  • Wasted energy can be defined as:

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

Worked example

A student uses an electric motor to lift a load.Useful Wasted Energy Worked Example, downloadable AS & A Level Physics revision notesThe motor turns a mechanism that lifts the load. Some of the energy transfers are useful and the rest of the energy is wasted.

a) State the useful energy transfer occurring in this system.

b) State the wasted energy transfer occurring in this system.

 

Part (a)

    • The motor turns the mechanism that lifts the load
    • Therefore, the useful transfer is:
      • Energy in the kinetic store of the motor is transferred to the gravitational potential store of the load

Part (b)

    • As the motor operates, friction causes a rise in the temperature of the components and the surroundings
    • In this case, the energy transfer from the kinetic store of the motor to the thermal store of the motor and the surroundings is not useful, hence it is a wasted energy transfer
      • Energy is dissipated, by heating, to the surroundings

Exam Tip

Make sure you are able to identify "useful" and "wasted" energy as this is commonly tested in exams! When describing wasted energy, make sure to say the energy is dissipated to the surroundings, if you say the energy is simply "lost", this will not gain you the mark as it implies energy is not conserved.

Reducing Energy Loss

  • 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

Lubrication

  • Friction is a major cause of wasted energy in machines
  • For example, the gears on a bike can become hot if the rider has been cycling for a long time
    • Energy is wasted as it is transferred from the kinetic energy store of the bike to the thermal energy store of the gears and the chain
    • This friction makes them become hot and transfers energy by heating to the thermal energy store of the surrounding air

  • This wasted energy 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

  • In many situations, the energy transferred by heating is wanted. For example:
    • When heating a home
    • When boiling a kettle

  • If this energy can be prevented from dissipating, then less energy will be needed to replace the wasted energy
    • This can be achieved by surrounding the appliance with insulation

  • The effectiveness of insulation depends upon:
    • How well the insulation conducts heat
    • How thick the insulation is

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Leander

Author: Leander

Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.