What is the "direction" of excess pressure (or capillary pressure) inside an air bubble submerged in a fluid? Does it tend to act outwards towards fluid (from inside the bubble) or vice versa? My intuitions tell me it acts outwards, but from what I've seen in a solution to a question, it assumes the pressure to be acting inwards (from fluid to bubble). Or am I misunderstanding the question? For reference, the question
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Pressure is a scalar and doesn't have a direction. However the force generated by the pressure does have a direction because it is the product of the pressure and the vector area:
$$ d\mathbf F = P\,d\mathbf A $$
So the question is then about the direction of the force not the pressure, and that's simpler to understand. The pressure of the air inside the bubble exerts a force on the bubble surface that acts outwards, and the pressure of the water outside the bubble exerts a force on the bubble surface that acts inwards.
John Rennie
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