Physical Science - MiniLab 1

 

 

"Each  man is a tool in his own hands.  Our greatest satisfaction doesn’t come from the rewards of our work, but from the working itself; and our greatest responsibility is to sharpen, and improve the tool that is ourselves so as to make it capable of tackling bigger jobs."
                                                                                  by Gordon R. Dickson

 Instructions:   This experiment is designed to be completed at home.  Neatly summarize all your information on notebook paper (front only).  Answer all questions (with complete sentences) as well.  You may wish to make drawings to complement your observations.   Be sure your name is at the top of the paper.

 

 1. Cohesion and Adhesion

          Find the tiny of clear plastic in the zip lock bag.  Place it over the typewritten instructions on this page.  Obtain a small amount of water in a small cup.  Use some kind of dropper to add 1 drop on top of the plastic.  Draw the shape of the convex droplet (use a side view).  Describe the appearance of the typeface beneath the plastic.  Try adding another drop of water to make your droplet larger.  What happens to the magnification?   The drop is acting as a lens.  What does a greater curvature of the droplet cause?  Is the water more attracted to itself (cohesion) or more attracted to the plastic film (adhesion)?

 How could you calibrate the dropper used?  What techniques worked to allow constant volume drops?  Why will the dropper not work if there is a small hole in the bulb?  Design an experiment to test your calibration ideas.

 Bonus:   Cut a clean straw nearly in half.  Extend one half into a cup of water vertically.  Holding the upper part of the straw at right angles to top of the first straw, blow through the horizontal straw.  Can you get water to move up the vertical straw and be expelled out, away from you?  What happens to the water as it is expelled?  What scientific principles are involved?  Make a scaled drawing, with measurements, of your final straw-setup. 

 

2. Magnifying Power of Water

We see images that are formed by the eye from light that comes from objects. Because light can be bent (refracted) by lenses the image we see can be different from the object that produced it. With a magnifying lens we can make an image seem larger than the object. The magnification is the ratio of real object size to the image size we see through the lens. Water droplets on a plastic surface act as a lens. Use graph paper and the clear plastic sheet to determine the magnification ability of one drop of water, several drops of water, and a drop of some other clear liquid.

Find the magnification of a lens:
1. Examine a section of graph paper with your water lens. Move the water droplet lens closer and farther away until you have the biggest squares you can still see clearly through the lens.
2. Count the number of squares it takes to cross the width of the droplet lens.
3. Set the lens back on the paper and measure the width of the droplet lens in graph paper squares.
4. The magnification is the image size divided by the object size. The image size is the width of the lens. The object size is the number of magnified squares you see in the lens.
5. Measure the distance between the lens and the graph paper (that gives you the maximum magnification). Record this on your data sheet. As you move the lens closer and farther away what happens to the magnification?
6. Describe the effect of adding extra drops of water. Did the magnification change? What other liquid did you use? Did it work the same as the water? The magnification does depend on the curvature of the droplet but over a small range of sizes the water droplets flatten out on top so the curvature does not change too much.

 3. Coins and Graphing

Obtain a penny, nickel, dime, and quarter. Clean and dry each coin, then complete this section without physically touching them.  Again using the cup of water and dropper, see how many drops you can place on each coin without them sliding off.    Remember to hold the dropper vertically and eliminate air bubbles (to have consistent drops).  Record these numbers.  You should repeat each coin three times to get an average number of drops held by each coin.

         Record the diameter of each coin (off chalkboard). Then find the surface area of each coin (using area = π r2 ). Next, calculate the number of drops per square centimeter for each coin (i.e.,  # drops/cm2).  Given the area of a half-dollar (area = 7.06 cm2 ), how would you calculate the number of drops it would hold?  What factors might cause this prediction to be incorrect?

Try graphing the number of drops on vertical axis (Y) versus area of coin on horizontal (X) axis.  Draw a best fit straight line through the data points.  Will this help you determine the drops that a half-dollar might hold?  What problems are encountered if you use soap on the coin before adding the drops (or touch the coin with the oils on your fingers)?  What shape did the water take on top of the coin?  Can you draw the shape?

 

 4.  Mirrors

          Hold a clean spoon about 25 cm (10 inches) away from your face.  Look into the concave surface.  You should see a rough image of your face.  Describe the image (i.e., upright, inverted, reversed left to right, enlarged, etc.)  Now flip the spoon and look at the convex surface and describe the image of your face.  Next move the concave and convex surface closer to and farther away from your face.  Describe your facial image each time (be sure to include a complete description; i.e., concave, farther away = …).  Try flipping the spoon handle up and seeing if it makes any difference.