Static Electricity (page 2)
Try New Approaches
How would using a positively charged object affect the results? Since the balloon stripped away electrons from the wool scarf, the scarf becomes positively charged. Repeat the original experiment, again rubbing the balloon with the scarf, but this time using the scarf instead of the balloon.
Design Your Own Experiment
- Two charged objects that have different charges attract each other, while two charged objects that have the same charge repel each other. So if a negatively charged object is brought near a neutral, nonpolarized solid, will the solid be polarized by induction because of the separation of its positive and negative charges? Design a way to determine if neutral solids can be polarized by electrostatic induction. One way is to prepare a pendulum using a bob (the suspended object on a pendulum) made of aluminum foil. Crush a small piece of aluminum foil to make the bob, then tie it to a 12-inch (30-cm) piece of string. Using tape, secure the free end of the string to the edge of a table. Charge a balloon as in the original experiment by rubbing it on a wool scarf. Hold the charged balloon under the hanging aluminum bob so that the charged balloon is near but not touching it. Slowly move the charged balloon to the left, then move it to the right. Observe the motion of the hanging bob.
- Electrical conduction is the movement of electric charges through a substance. Electrical conductivity is the measure of the ability of a substance to conduct an electric current (the flow of electric charges). Conductors or electrical conductors, such as metals, are substances with a high concentration of free electrons and are good conductors of electric charges. Insulators or electrical insulators are substances with a low concentration of free electrons and are poor conductors of electric charges. Charging by conduction is the method of charging a neutral object by touching it with a charged object. Design an experiment to determine which would hold a static charge longer, a conductor such as aluminum, or an insulator such as paper. One way is to prepare two sets of pendulums: set one with two aluminum bobs, and set two with two paper bobs equal in size to the aluminum bobs. Use the procedure in the previous experiment to make the four bobs. First hang the pendulums from set one by taping the free ends of their strings to a table so that the bobs are slightly separated. Charge the aluminum bobs by conduction. Do this by touching them simultaneously for 1 to 2 seconds with a charged balloon. The bobs will have like charges, so they will separate. Measure the length of time of separation between the bobs. Make note of how many times you rub the balloon against the cloth when charging it. Then repeat the procedure with the paper bobs from set two.
- Electric discharge is the loss of static electricity. Does the magnitude of the charge on the balloon affect electric discharge? Repeat the experiment twice, first rubbing the balloon more times, then rubbing it fewer times against the cloth. Make sure that all materials start with a neutral charge. Allowing the bobs to hang undisturbed and away from charged materials for 10 or more minutes should give them time to electrically discharge. This happens as excess electrons on the bobs are picked up by molecules in the air, especially water molecules.
- What effect would bobs made of insulating material have on the results? Repeat the experiment using insulating materials, such as paper and cotton. Be sure that the only variable you change is the type of material used; keep the size and shape of the bobs as similar as possible.
- An instrument called an electroscope is used to determine the presence of an electric charge on an object. As shown in Figure 47.3, an electroscope has a conducting metal rod with two thin metal leaves attached. If the rod is charged by conduction, the leaves separate. Design a way to determine any difference in the effect of charging the electroscope with a positively or negatively charged object. One way is to build an electroscope and test it first with a positively charged object (wool scarf after it has been rubbed with a balloon) then with a negatively charged object (balloon after it has been rubbed on a wool scarf). The electroscope can be made using a glass jar with a circle of cardboard to cover its top. Use pliers to reshape a large paper clip into a U-shaped loop. Punch two holes in the center of the cardboard circle about 1⁄2 inch (1.25 cm) apart. Push the ends of the loop of wire through the holes and mold a small piece of modeling clay around the base of the loop to hold the wire in place. The bent section of the loop should stand up above the cardboard about 1⁄2 inch (1.25 cm). Bend both ends of the wire loop outward to form hooks. Hang a small light-weight aluminum foil strip on each wire hook. The strips should jiggle back and forth freely. If they do not, enlarge their holes. Place the cardboard circle over the mouth of the jar with thefoil strips hanging down inside the jar. Tape the cardboard cover to the jar. Charge the foil strips by conduction (touching the metal top with a charged object). Variables that could affect the results of your experiment include size and weight of the aluminum foil strips, humidity, and separation of the foil strips. Electroscopes can also be charged by induction. See a physics book for information information on this method of charging as well as instructions on how to charge an electroscope by induction.