Question

A balloon filled with helium gas floats in a big closed jar which is connected to an evacuation pump. What will be your observation, if air from the jar is pumped out?
A. The balloon will sink.
B. The balloon will expand.
C. The balloon remains the same.
D. None of the above are correct.

Answer 1

Hint: Buoyant force helps the balloon to rise up in the air. When air is less then density will also be less. Hence Balloon will change its position. Force of gravity will always act on the balloon with or without air.

 Complete step-by-step solution:
The balloon filled with helium gas will float in the jar filled with air since helium is lighter than air. The buoyancy force acting on the balloon filled with helium due to air in the jar will be greater than the weight of the helium filled balloon. If air is pumped out of the jar then density of air in the jar will decrease and there is no buoyancy force acting on the helium filled balloon which leads to decrease in upthrust force on the balloon and only force of gravity acts on it. Therefore, the weight of the balloon will exceed the upthrust force on it so it will sink in the jar.

i) Helium balloon inside jar with air.

ii) Helium balloon inside jar without air.

Note:- Buoyant Force works against the force of gravity and helps the balloon to rise up. Do not use rubber balloons with helium because they will expand and explode in the bell-jar when air is pumped out. Use only self-sealing foil balloons.