Question

Explain why the potential energy decreases when a bubble rises up?

Answer 1

As a bubble rises through a fluid three forces act upon it as shown in the figure below:

For an air bubble, the interface is water only, as such drag is zero. As the bubble is rising there is a net upward force acting on the bubble. It may be seen that gravitational potential energy increases as the bubble rises, or height increases. Also, there is the upward motion of velocity v because of which the bubble attains kinetic energy. As the bubble rises, pressure on the bubble decreases, which leads to expansion of bubble and thereby increase of force due to buoyancy, as such kinetic energy increases. Another energy which is associated with the bubble is its buoyancy potential energy which is directly proportional to the depth of the bubble. As the bubble rises its depth decreases. Hence, buoyancy potential energy decreases.