- #1
brotherbobby
- 618
- 153
I mention the details in the book (verbatim) in the form of a paragraph in green below. Later I ask my questions in blue font for better reading.
"Surface tension also explains why hot, soapy water is used for washing. To wash clothing thoroughly, water must be forced through the tiny spaces between the fibers (Fig. 12.16). To do so requires increasing the surface area of the water, which is difficult to achieve because of surface tension. The job is made easier by increasing the temperature of the water and adding soap, both of which decrease the surface tension." (University Physics, 13th Edition)
- Let's begin with the diagram. I know that the excess pressure within a (spherical) water molecule with a single surface is ##p=\frac{2T}{R}##. Assuming hot water and soap bring about a reduction in T, it will reduce p. In which direction is water (with soap) supposed to travel in the diagram? Shouldn't air and water be on one side and the clothing and its inner fibers with dirt on the other?
- Let's see the text above in green from the book. To force water through the fiber spaces in the clothing, it says the surface area of water needs to be increased! Increased? Shouldn't the surface area of water be decreased so that the water, with the same volume, can move through the clothing fiber spaces?
In order to force the soapy water through the fiber spacings, their surface area need to be reduced. A sphere is known to have the maximum surface area for a given volume. A decrease in the surface tension of water using soap and heat effects a decline in its surface area with the same volume of water. This reduced size of the soap water helps it make its way into the clothing. (Please note that nowhere in my explanation have I used the concept of excess pressure).
A help would be welcome.