Science Buddies: "Ask an Expert"

Earlier i posted this:
viewtopic.php?f=26&t=17074


I decided to change the project up because my teacher instructed me to do so. Instead of measuring the surface tension of water and the results when adding soap I am measuring the surface tension of water at different temperatures. 10 drops of water resulted in .0072667. 9 drops of water resulted in .00654. I checked online for the surface tension of water at different temperatures. (https://user.engineering.uiowa.edu/~cfd ... /1-41B.pdf) 1 drop less had a drastic change on the surface tension. Also for 60 degrees F it came up to .00218.How come the surface tension ranges around 70 and mine was way lower? What's my mistake?

Hi DiligentWorker,

The units for surface tension in the data table you provided are dynes/cm. But if you follow the project procedure the units are Newton/m (or N/m). And 1000 dyne/cm = 1 N/m. Thus the surface tension data from the table should be divided by 1000 to convert the units from dyne/cm to N/m. After making the unit conversion the surface tension is around 0.07 N/m.

But the values you found are about 0.007 or about 1/10 of what they should be. What units did you use for your needle (or wire)? The needle length should be in meters (m). Be sure to correctly convert the length to meters.

When I replied to your previous post, I checked your calculation steps and they are correct. You used 4.5 as the needle length and I should have asked about the units at that time. It’s unlikely that your needle length is 4.5 meters. So if your needle length was 4.5 cm you would use 0.045 m in the calculation. And the calculation result would be 0.072667 N/m. My apologies for not clarifying the units in my previous reply.

I hope this helps. Please post again if you have more questions.

A. Norman