If you walk across a carpet on a dry day and then stretch out your hand to a metal door knob, a spark may jump across between your fingers and the knob. You can get a fatter and longer spark if you shuffle as you walk. Your hair may crackle as you comb it. You have probably noticed other examples of the electrical effect of rubbing two objects together.
Small pieces of paper are attracted to a plastic comb or ruler that has been rubbed on a piece of cloth. If someone in your group has a comb, try it. The attractive force is often large enough to lift the scrap of paper off the table and thus must be greater than the weight of the paper! The force between the rubbed plastic and the paper is an electrical force, one of the four basic forces of nature. In this lab you will perform some simple experiments and make some observations of the nature of the electrical force.
Your equipment consists of acetate and vinyl strips, silk and wool cloths, a graphite coated pith ball, an electroscope and a glass rod. The vinyl strip has a white edge. To charge the vinyl strip rub it with the wool cloth. To charge the acetate strip rub it with the silk cloth. Begin by charging the acetate strip, then touch it to the pith ball. Do this several times and describe what happens in your notebook. (If nothing happens, consult your instructor.) Now touch the pith ball with your hand to remove any charge that may be on it, then repeat the experiment using the vinyl strip. Now charge the vinyl strip and touch it to the pith ball. Rub the acetate strip and bring it near the pith ball. What happens? Can you now draw any conclusions about the charges on the two strips?
Once more rub the acetate strip and charge pith ball. Bring the silk near the pith ball, but don't touch it. What do you observe? Touch the pith ball to remove its charge. Repeat with the vinyl strip and wool. Identify which of your objects have the same charge and which have different charges. Using the method you learned in lab last semester, try to determine which objects are positively charged and which are negatively charged.
Now you will do a few simple experiments to study how charges move from place to place. The electroscope consists of a metal plate and rod to which is attached an aluminum coated straw. The rod is supported by a plastic insulator, which acts to prevent the charges from leaking onto the case. The case protects the straw from air currents. Charge the acetate strip and rub it along the top of the electroscope. Describe what happens and try to explain in terms of what you have already learned. If you are not sure, ask your instructor to discuss it with you. Touch the top of the electroscope and describe what happens. Place the glass rod on top of the electroscope then touch the rod with the charged strip. What happens? Try it using a wooden pencil. What conclusion can you draw about the motion of the electrons through metal, glass, and wood?
You may have noticed that the electroscope reacted before you actually touched it. Try it again. Describe what happens as you bring the strip near the electroscope and then remove it without touching the plate. Explain this behavior in terms of the motion of the electrons (assume your strip is negative). Repeat the first experiment you did with the pith ball. Notice that it is initially attracted to the charged strip. Using the same basic ideas you just used with the electroscope, explain this attraction between the charged strip and the uncharged pith ball.
It is possible to charge the electroscope without touching it with the charged strip. This is called "charging by induction." Hold a charged strip 2 or 3 cm from the metal plate. Now touch the plate with your other hand. This allows charge to move from the plate to your body or vice versa. Then remove your hand while still holding the charged rod in place. Finally, take away the rod. Is the charge on the electroscope now the same or opposite that on the strip? How could you check? Describe the motion of the straw during this charging process. Describe the motion of electrons from place to place throughout this procedure assuming the strip is negatively charged. Repeat this assuming the strip is positively charged.
Web Author: Beverley Taylor, MUH