Question

Describe a bell jar experiment to prove sound requires a medium to travel. [5 MARKS]

Answer 1

Aim: 1 Mark
Procedure: 2 Marks
Working: 2 Marks

Aim: To show that sound needs a material medium for its propagation.

Procedure: The fact that sound needs a material medium for its propagation can be demonstrated by the bell and jar experiment. The apparatus is set up as shown in Fig. It consists of a bell jar fixed with a stopper at the mouth. An electric bell is suspended inside a bell jar. Two leads from the electric bell pass through the stopper and they are connected to a battery. The bell jar is placed on the disc of a vacuum pump.

Working: Set the bell ringing. Initially when jar has normal air inside it, sound waves produced by the ringing bell are heard outside the jar. The vacuum pump is started and the air from the jar is gradually drawn out. As air inside the jar decreases, the sound becomes feebler. After sometime no sound is heard though the hammer is still striking the bell.