Explain that mechanical processes become wasteful when they cause a rise…

The idea

Mechanical processes such as friction between moving surfaces, air resistance or the deformation of a material transfer energy to the thermal store of the objects and surroundings. When this raises the temperature, the energy becomes spread out into the environment and is described as dissipated. Such processes are called wasteful because the energy is no longer available to do the useful mechanical work it was originally transferred for — for example, a sliding block slows as friction heats the surfaces, and that thermal energy cannot easily be recovered to keep the block moving. Learners should explain the link between the temperature rise, the dissipation to the surroundings and the loss of useful energy, using a concrete example and identifying the stores involved.

What earns the marks

• Identifies that mechanical processes such as friction or air resistance transfer energy to the thermal store of the objects and surroundings.
• Explains that this transfer causes a rise in temperature of the surroundings.
• States that the energy becomes dissipated, meaning it is spread out into the environment.
• Explains that the process is wasteful because the dissipated energy is no longer available for the useful mechanical work.
• Applies the explanation to a named example, such as a sliding block slowing as friction heats the surfaces.

Common mistakes and how to put them right

• States that the energy is destroyed by friction.

  Put it right by explaining that the energy is transferred to the thermal store of the surroundings.

• Describes the process as wasteful without explaining why.

  Put it right: Link the temperature rise and dissipation to the loss of useful energy.

• Confuses dissipation with a loss of total energy.

  Put it right: Keep the total energy constant while noting that the useful energy decreases.

Exam tips

• Name the mechanical process and the surfaces or surroundings involved before explaining the temperature rise.
• Use the term dissipated and explain what it means in terms of energy spreading into the surroundings.
• Explain why the process is wasteful by contrasting the useful energy with the energy now in the thermal store of the surroundings.