A hollow bubble has surface tension acting on the internal and external surfaces of the bubble. Which formula is correct for the total surface tension acting on a bubble, where:

T = total surface tension acting on the bubble P = pressure difference between the inside and outside of the bubble r = radius of the bubble  
  1. T = Pr/2
  2. T = Pr/4
  3. T = P/2r
  4. T = P/r2