In an experiment with an electric bell placed inside an enclosed evacuated glass bell jar, why is the ringing of the bell not heard outside?

Introduction / Context:
The classic school demonstration with an electric bell placed inside a glass bell jar helps show that sound needs a material medium for its transmission. When air is gradually removed from the jar by a vacuum pump, the sound of the bell becomes fainter and finally cannot be heard. This question checks if you understand the basic requirement of a medium for sound waves and why a ringing bell in a vacuum is not heard outside.

Given Data / Assumptions:

• An electric bell is ringing inside a glass bell jar.
• The bell jar has been evacuated so that there is almost no air inside.
• Observers stand outside the jar and try to hear the sound.
• The electric supply to the bell continues, so the hammer still strikes the bell.

Concept / Approach:
Sound is a mechanical wave. It consists of compressions and rarefactions of particles of a medium such as air, water or a solid. If there is no medium, compressions and rarefactions cannot travel. Vacuum is a region with no material particles, so sound cannot propagate through it. In the experiment, the bell continues to vibrate mechanically, but because the air has been removed from the jar, there are no air particles inside to carry the sound to the glass walls and then to the air outside.

Step-by-Step Solution:
Step 1: Recognise that the bell mechanism continues to work, because it is connected to an electric supply. Step 2: Recall that sound requires a material medium to travel, usually air in such experiments. Step 3: Understand that when the bell jar is evacuated, air is removed and the inside space becomes almost a vacuum. Step 4: In vacuum, there are no particles to vibrate and pass on sound energy from the bell to the walls of the jar. Step 5: Therefore, even though the bell vibrates, sound waves do not reach the outside, so the ringing is not heard.

Verification / Alternative check:
When the jar is first filled with air, the bell is clearly audible, which shows that the jar glass itself does not completely block sound. As air is pumped out, the sound gradually decreases, matching the idea that fewer air particles are available to transmit the wave. If air is let back into the jar, the sound becomes loud again. This change happens even though the bell volume and glass remain the same, proving that the absence or presence of air is the key factor.

Why Other Options Are Wrong:
The ringing is not loud enough is incorrect because the bell is loud when air is present.
The glass of the bell jar absorbs the sound waves is wrong, since sound is heard through the same glass before evacuation.
The bell jar is too small is irrelevant, because size does not suddenly change as air is removed.
The electric circuit stops working in vacuum is incorrect; the bell continues to vibrate mechanically.

Common Pitfalls:
Students sometimes think that glass blocks sound completely because it looks solid and transparent. In reality, some sound can pass through glass. Another common confusion is between sound waves and electromagnetic waves such as light. Light can travel through vacuum, but sound cannot. Keeping this difference clear helps in many physics questions about waves and media.

Final Answer:
The ringing is not heard because there is no air or any other gas in the bell jar to transmit sound.