Physics 11–12 · Year 12
Cathode rays in Crookes tubes: Maltese cross, paddle wheel and magnetic deflection
Module 8: From the Universe to the Atom (Structure of the Atom)
Teacher-led demonstration
A teacher carries out this activity as a demonstration. Its materials, steps, variables, misconceptions and sources are not published on the open web, and neither is any result, control or note that states a number or an amount: tutors and administrators read them on the learning platform, with the safety card first. The idea, the hazards and the curriculum references are below.
Open the procedure on the learning platform (tutor or administrator sign-in)
School laboratory, not for home
In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
The idea
Cathode rays travel in straight lines, carry momentum and are deflected by magnetic and electric fields as negative particles, the evidence that led to the electron.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- EHT supply
- evacuated glass tubes
- X-rays from the tubes, more at higher operating voltages
Controls
At least one of them states a number or an amount, which may be part of the method, so the controls are shown with the method to tutors and administrators on the learning platform.
Note
It states a number or an amount, which may be part of the method, so it is shown with the method to tutors and administrators on the learning platform.
What you should see
A crisp cross-shaped shadow shows straight-line travel from the cathode; the paddle wheel rolls away from the cathode, which Crookes took as evidence that the rays carry momentum, although J. J. Thomson later showed that their momentum is far too small and that the vanes are pushed mainly because the rays heat them; the beam bends at right angles to the magnetic field in the direction expected for negative charge and reverses with the pole. The whole set is consistent with a stream of fast negative particles.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH12-15PH11/12-5PH11/12-7
- Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-04PY-12WS-07
- Australian Curriculum v9No Australian Curriculum v9 code is listed.