Demo Title:
Bell in vacuum and slinky wave
Demo overview
The vacuum bell, together with the slinky wave effectively demonstrate to students fundamental concepts related to the propagation of waves in media, and how essential these media are to their propagation. The vacuum bell challenges preconceptions young students may have, often due to portrayals in media, about when sound may be present. The slinky demonstration, while simple to set up, provides one of the best ways to visualize different kinds of ways. In using a reasonably easy-to-acquire item such as a slinky during workshops, we allow students the opportunity to continue building their intuition for wave propagation at home.
The bell in a vacuum demonstration is presented in the context of introducing the concept of a medium of propagation for a wave. In this sense, it serves to demonstrate what happens when sound does not have a medium. The demonstration relies on a glass jar surrounding a small brass bell. We first demonstrate the sound made by the bell when air fills the jar. We then remove air from the jar and show students how the sound from the bell no longer makes a sound (or, more accurately, sound no longer propagates from the bell out of the jar). The slinky wave demonstration complements the bell in vacuum demonstration and is usually presented immediately afterward. It helps to reinforce the concept of a medium for a wave by providing a visible medium (the slinky). In this activity, the instructor and volunteer each hold an end of a large slinky as the instructor makes movements at their end to generate waves. The instructor then explains the various waves that are seen as they are being generated. This also helps strengthen students’ intuition for other waves they may observe in life (e.g., gravity waves on a beach).
Demo instructor notes:
Bell in vacuum
- In preparation for this demonstration, we require the bell jar and the vacuum pump, which should be mounted to a cart. Ideally, the vacuum pump should be placed in an area where it is not interfering with other demonstrations, but visible so that the audience can understand that you are creating a vacuum in the jar during the demonstration.
- It is helpful to introduce the demonstration by asking the audience: “can you hear sound in space?” In my experience, with elementary school groups, about half of the students will answer ‘yes.’ Naturally, the positive response rate declines with older students.
- Introduce the bell jar with a short description and ring it to show that it is making a sound. Move throughout the room so students can see the bell and better hear the sound.
- Before creating a vacuum, emphasize that space is a vacuum so that we are really simulating space in the jar. Connect the vacuum tube to the male port on the jar. Ensure both the valve on the jar and the one on the pump are open, then turn on the pump. Vacuum until the barometer reading is no longer significantly decreasing. This should take 10-15 seconds. Once a suitable pressure has been reached, close the jar valve, then turn off the pump and close its valve. Disconnect the jar from the tube.
- Shake the jar again. Remark that while the bell is still moving, you cannot hear any sound from it. As before, move throughout the room to give the audience to see the bell move up close.
- Open the jar valve while still shaking the jar and moving around. Remark that as air gets back into the jar the sound from the bell becomes more pronounced.
- Conclude with a short summary of what happened: when there was air, we had sound. In the absence of air, sound had no medium to travel through and therefore could not reach our ears. Make connection to space; ask audience again if they think sound can be heard in space.
Slinky wave
- The only equipment required for this demonstration is a large slinky
- To begin the demonstration, ask for a volunteer among the audience. Hand them one end of the slinky, and, holding the other end, elongate the slinky in a straight line. While still holding the ends, set it on or near the floor in a place where most of the audience can see it. If some members have trouble seeing it, ask them to move closer the demonstration area.
- Explain that the slinky is a medium where both transverse and longitudinal waves can propagate. Start making transverse waves by moving your end side-to-side first in bursts, then continuously. Explain analogy with gravity waves at the beach (the term ‘gravity wave’ is too confusing; I prefer ‘water wave’ instead for lay outreach). Explain that they can see a wave traveling even though the rings of the slinky do not move very far. If the volunteer would like, ask them to make waves on their end.
- Then make compressive, or longitudinal waves by pushing/pulling the slinky along its axis. Explain similarities and differences with transverse waves, and connect to sound waves.
- (More often than not, a student will remark that waves will ‘bounce off’ the end of the slinky and return to the end it came from. This is an opportunity to talk about sound echoes if time permits.)