Cathode rays (A-level only): Production of cathode rays in a discharge tube.

The idea

You need to explain how cathode rays are produced when a high potential difference is applied across a partially evacuated discharge tube. The strong electric field accelerates stray charged particles, and collisions between these particles and gas atoms cause ionisation, creating positive gas ions and free electrons. The positive ions are accelerated towards the cathode. When they strike the cathode, they cause the emission of electrons from its surface. These emitted electrons are accelerated towards the anode, and some pass through a hole to form a beam called cathode rays. The process depends on the tube being evacuated enough to give electrons a long mean free path, but not so empty that there are no gas atoms to ionise.

What earns the marks

• Describe a discharge tube containing gas at low pressure with a high potential difference applied between cathode and anode.
• Explain that the strong electric field accelerates charged particles, and collisions with gas atoms cause ionisation, producing positive ions and free electrons.
• State that positive ions are accelerated towards the cathode and their bombardment causes the emission of electrons from the cathode surface.
• State that these emitted electrons are accelerated towards the anode and those passing through a gap form the cathode ray beam.
• Relate the gas pressure to the process: too high a pressure causes frequent collisions and prevents a beam, while too low a pressure gives too few gas atoms for ionisation.

Common mistakes and how to put them right

• Saying cathode rays are produced solely by electrons from gas ionisation.

  Put it right by stating that positive ions strike the cathode, causing it to emit the electrons that form the rays.

• Claiming cathode rays are electromagnetic waves like light.

  Put it right by identifying them as beams of electrons with mass and charge.

• Ignoring the role of gas atoms and treating the tube as a perfect vacuum.

  Put it right by explaining that low-pressure gas is ionised to create the positive ions needed to bombard the cathode.

Exam tips

• Use the correct terminology: cathode, anode, ionisation, positive ion bombardment, and acceleration.
• When asked to explain, link each cause to its effect, for example high potential difference causes a strong field, which accelerates positive ions to strike the cathode, releasing electrons.