Hello:
Thank you in advance for responding to my question. One of my students has completed the Science Buddies Experiment: Measuring the Surface Tension of Water. She is all set to do the equation for measuring the force, but we are both unsure of how to do the calculations. Can you plug in some sample numbers in the equation to show us how to do it, along with the proper units, and also how to solve for surface tension as well.
We would appreciate your help. We have Internet Explorer 6.0, Windows 98.
Thank you,
Denise
Math Equation
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deniserochelle
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barretttomlinson
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Re: Math Equation
Hi,
This resource (in the bibliography of the project) has some approximate numbers and the equation:
http://www.newton.dep.anl.gov/askasci/p ... y00455.htm
I assume this is the project you mean:
https://www.sciencebuddies.org/science- ... p012.shtml
The calculation is explained in step 4 of the experimental proceedure:
“Measuring surface tension.
Hang the pan from one end of the beam and the needle from the other. Use a small piece of modeling clay as a counterbalance (as shown in Figure 1) to balance the needle and empty pan.
Place your container of water so that the needle (or wire), still hanging horizontally, is submerged in the water.
You will add small amounts of weight to the pan, and measure the force needed to pull the needle (or wire) free from the surface of the water.
It will not take much weight, so you need to add it in small increments. Here are two different methods you could try.
Use common pins as your weights, adding them one at a time. Calibrate them by weighing a bunch of pins on a postal scale, and dividing by the number of pins to get the weight per pin.
Use drops of water from an eyedropper or plastic transfer pipette. You can calibrate the water drops by counting how many drops are needed to make, say, 5 ml. Each ml of water weighs 1 g, so with your count you can calculate how much each drop weighs.
Try both methods and see how your results compare!
Repeat the measurement (steps 1–3) at least 5 times (more is better), to assure consistent results. If something goes wrong (e.g., you accidentally tap the pan and pull the needle out of the water), repeat the trial from the beginning.
Average your results.
The force you will be measuring can be expressed by the equation:
F = 2sd, where
F is the force, in newtons (N),
the factor of 2 is because the film of water pulled up by the needle (or wire) has 2 surfaces,
s is the surface tension per unit length, in units of newtons/meter (N/m), and
d is the length of the needle (or wire), in units of meters (m).
To convert grams to the force, F, you have to account for gravity pulling down on the mass in the pan. Do this by multiplying the mass (in grams) by 9.83×10-3 N/g (for more information, see the link on "Gravitational Force" in the Bibliography).
You can rearrange the equation above to solve for s, the surface tension of water. Measure the length of the needle (or wire), and you'll have all the information you need to calculate the surface tension of water.
How do you know that you are measuring surface tension, and not an attractive force between the needle (or wire) and the water? Here's a good tip from Robert Gardner's book (Gardner, 2004). Surface tension is the cohesive force between water molecules. Observe the needle (or wire) carefully after it is pulled out of the water. If it remains wet, then it must be the water that pulled apart, and this is the force (surface tension) that you measured. If it is dry, then the adhesive force between the water and the needle broke first, and this is what you measured, not surface tension.
Measuring surface tension.
Hang the pan from one end of the beam and the needle from the other. Use a small piece of modeling clay as a counterbalance (as shown in Figure 1) to balance the needle and empty pan.
Place your container of water so that the needle (or wire), still hanging horizontally, is submerged in the water.
You will add small amounts of weight to the pan, and measure the force needed to pull the needle (or wire) free from the surface of the water.
It will not take much weight, so you need to add it in small increments. Here are two different methods you could try.
Use common pins as your weights, adding them one at a time. Calibrate them by weighing a bunch of pins on a postal scale, and dividing by the number of pins to get the weight per pin.
Use drops of water from an eyedropper or plastic transfer pipette. You can calibrate the water drops by counting how many drops are needed to make, say, 5 ml. Each ml of water weighs 1 g, so with your count you can calculate how much each drop weighs.
Try both methods and see how your results compare!
Repeat the measurement (steps 1–3) at least 5 times (more is better), to assure consistent results. If something goes wrong (e.g., you accidentally tap the pan and pull the needle out of the water), repeat the trial from the beginning.
Average your results.
The force you will be measuring can be expressed by the equation:
F = 2sd, where
F is the force, in newtons (N),
the factor of 2 is because the film of water pulled up by the needle (or wire) has 2 surfaces,
s is the surface tension per unit length, in units of newtons/meter (N/m), and
d is the length of the needle (or wire), in units of meters (m).
To convert grams to the force, F, you have to account for gravity pulling down on the mass in the pan. Do this by multiplying the mass (in grams) by 9.83×10-3 N/g (for more information, see the link on "Gravitational Force" in the Bibliography).
You can rearrange the equation above to solve for s, the surface tension of water. Measure the length of the needle (or wire), and you'll have all the information you need to calculate the surface tension of water.
How do you know that you are measuring surface tension, and not an attractive force between the needle (or wire) and the water? Here's a good tip from Robert Gardner's book (Gardner, 2004). Surface tension is the cohesive force between water molecules. Observe the needle (or wire) carefully after it is pulled out of the water. If it remains wet, then it must be the water that pulled apart, and this is the force (surface tension) that you measured. If it is dry, then the adhesive force between the water and the needle broke first, and this is what you measured, not surface tension. “
Hope this helps.
Best regards,
Barrett L. Tomlinson
This resource (in the bibliography of the project) has some approximate numbers and the equation:
http://www.newton.dep.anl.gov/askasci/p ... y00455.htm
I assume this is the project you mean:
https://www.sciencebuddies.org/science- ... p012.shtml
The calculation is explained in step 4 of the experimental proceedure:
“Measuring surface tension.
Hang the pan from one end of the beam and the needle from the other. Use a small piece of modeling clay as a counterbalance (as shown in Figure 1) to balance the needle and empty pan.
Place your container of water so that the needle (or wire), still hanging horizontally, is submerged in the water.
You will add small amounts of weight to the pan, and measure the force needed to pull the needle (or wire) free from the surface of the water.
It will not take much weight, so you need to add it in small increments. Here are two different methods you could try.
Use common pins as your weights, adding them one at a time. Calibrate them by weighing a bunch of pins on a postal scale, and dividing by the number of pins to get the weight per pin.
Use drops of water from an eyedropper or plastic transfer pipette. You can calibrate the water drops by counting how many drops are needed to make, say, 5 ml. Each ml of water weighs 1 g, so with your count you can calculate how much each drop weighs.
Try both methods and see how your results compare!
Repeat the measurement (steps 1–3) at least 5 times (more is better), to assure consistent results. If something goes wrong (e.g., you accidentally tap the pan and pull the needle out of the water), repeat the trial from the beginning.
Average your results.
The force you will be measuring can be expressed by the equation:
F = 2sd, where
F is the force, in newtons (N),
the factor of 2 is because the film of water pulled up by the needle (or wire) has 2 surfaces,
s is the surface tension per unit length, in units of newtons/meter (N/m), and
d is the length of the needle (or wire), in units of meters (m).
To convert grams to the force, F, you have to account for gravity pulling down on the mass in the pan. Do this by multiplying the mass (in grams) by 9.83×10-3 N/g (for more information, see the link on "Gravitational Force" in the Bibliography).
You can rearrange the equation above to solve for s, the surface tension of water. Measure the length of the needle (or wire), and you'll have all the information you need to calculate the surface tension of water.
How do you know that you are measuring surface tension, and not an attractive force between the needle (or wire) and the water? Here's a good tip from Robert Gardner's book (Gardner, 2004). Surface tension is the cohesive force between water molecules. Observe the needle (or wire) carefully after it is pulled out of the water. If it remains wet, then it must be the water that pulled apart, and this is the force (surface tension) that you measured. If it is dry, then the adhesive force between the water and the needle broke first, and this is what you measured, not surface tension.
Measuring surface tension.
Hang the pan from one end of the beam and the needle from the other. Use a small piece of modeling clay as a counterbalance (as shown in Figure 1) to balance the needle and empty pan.
Place your container of water so that the needle (or wire), still hanging horizontally, is submerged in the water.
You will add small amounts of weight to the pan, and measure the force needed to pull the needle (or wire) free from the surface of the water.
It will not take much weight, so you need to add it in small increments. Here are two different methods you could try.
Use common pins as your weights, adding them one at a time. Calibrate them by weighing a bunch of pins on a postal scale, and dividing by the number of pins to get the weight per pin.
Use drops of water from an eyedropper or plastic transfer pipette. You can calibrate the water drops by counting how many drops are needed to make, say, 5 ml. Each ml of water weighs 1 g, so with your count you can calculate how much each drop weighs.
Try both methods and see how your results compare!
Repeat the measurement (steps 1–3) at least 5 times (more is better), to assure consistent results. If something goes wrong (e.g., you accidentally tap the pan and pull the needle out of the water), repeat the trial from the beginning.
Average your results.
The force you will be measuring can be expressed by the equation:
F = 2sd, where
F is the force, in newtons (N),
the factor of 2 is because the film of water pulled up by the needle (or wire) has 2 surfaces,
s is the surface tension per unit length, in units of newtons/meter (N/m), and
d is the length of the needle (or wire), in units of meters (m).
To convert grams to the force, F, you have to account for gravity pulling down on the mass in the pan. Do this by multiplying the mass (in grams) by 9.83×10-3 N/g (for more information, see the link on "Gravitational Force" in the Bibliography).
You can rearrange the equation above to solve for s, the surface tension of water. Measure the length of the needle (or wire), and you'll have all the information you need to calculate the surface tension of water.
How do you know that you are measuring surface tension, and not an attractive force between the needle (or wire) and the water? Here's a good tip from Robert Gardner's book (Gardner, 2004). Surface tension is the cohesive force between water molecules. Observe the needle (or wire) carefully after it is pulled out of the water. If it remains wet, then it must be the water that pulled apart, and this is the force (surface tension) that you measured. If it is dry, then the adhesive force between the water and the needle broke first, and this is what you measured, not surface tension. “
Hope this helps.
Best regards,
Barrett L. Tomlinson