Science 7–10 · Year 9
Bell in a vacuum jar: sound needs a medium (teacher demonstration)
Physical sciences — Waves and motion: waves, sound and light (NSW Stage 5 focus area)
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
A ringing bell becomes almost inaudible as the air is pumped out of the jar around it, because sound needs particles to pass the vibration along.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- implosion of a chipped or scratched glass jar under vacuum
- flying glass
- pump oil mist and noise
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
The sound fades as air is removed and becomes faint even though the clapper is seen striking, and the phone's amplitude reading falls as the pumping continues. Readmitting air brings the sound back. The sound never quite disappears, because some vibration reaches the jar through the base and the bell's mounting; the foam pad reduces this, and without it the sound does not fade as far.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.