Explain the energy transfer (in 10.22 above) as the result of collisions…

The idea

A metal conductor contains a lattice of positive ions with free electrons moving between them. When a potential difference is applied, the electrons accelerate along the wire, but their path is interrupted: they collide with the ions of the lattice. Each collision transfers kinetic energy from the electrons to the lattice ions, increasing the vibration of the ions. This raised vibration is the thermal energy of the conductor, so the metal warms up and passes heat to its surroundings. The electrons keep drifting because the supply keeps doing work on them, so the transfer continues. The greater the resistance, the more frequent or energetic the collisions and the greater the heating effect for a given current.

What earns the marks

• States that free electrons move through the metal and collide with the ions in the lattice.
• Explains that each collision transfers kinetic energy from the electrons to the lattice ions.
• Links the increased vibration of the ions to a rise in temperature of the conductor.
• Connects the collisions to the resistance of the component and the heating effect produced.
• Recognises that the supply continues to do work on the electrons so the transfer is maintained.

Common mistakes and how to put them right

• Says electrons are destroyed in the collisions.

  Put it right: State that electrons transfer kinetic energy to the ions and continue to drift.

• Claims the ions move freely and collide with stationary electrons.

  Put it right: Explain that the ions form the fixed lattice and the electrons move between them.

• Describes the collisions as producing electrical energy.

  Put it right: State that kinetic energy of electrons becomes thermal energy of the lattice.

• Omits the lattice ions and refers only to atoms in general.

  Put it right: Name the ions in the lattice as the collision partners.

Exam tips

• Use the phrase collisions between electrons and ions in the lattice to match the required explanation.
• Track the energy chain: electrical work on electrons, kinetic energy, collisions, vibration of ions, thermal energy.
• Link the frequency or energy of collisions to resistance to strengthen the explanation.
• Keep the ions fixed in the lattice and the electrons moving, so the roles are clear.