Experiment I Electric Force

Transcription

1 Experiment I Electric Force Twenty-five hunre years ago, the Greek philosopher Thales foun that amber, the harene sap from a tree, attracte light objects when rubbe. Only twenty-four hunre years later, in the latter part of the nineteenth century, systematic investigations le to the formulation of a conceptual moel of electricity that explains Thales experiments. In this lab, we will evelop our own conceptual moel for electricity by analyzing a series of simple experiments. Although you have alreay learne a number of basic concepts in lecture, these rules, such as the one stating that objects with opposite electric charges attract one another, were state without proof. In this lab, you will conuct a series of simple experiments to convince yourself that your observations are consistent with these empirical rules. When performing the activities, be careful to avoi assumptions that might not be consistent with your observations. Construct explanations an conclusions from your observations alone. Activity 1 Remove two 10-cm long pieces of regular clear tape from a roll of tape. Curl the ens of the tape over to make hanles (see the sketch at the right). Press the sticky sies of the tape to the top of the lab table an rub them so that they make goo contact with the table. Then, quickly pull the strips of tape off the surface an bring the non-sticky sies of the tape near each other. What happens? Does it matter which sies of the strips face each other? How oes the istance between the strips affect what happens? Hanle Sticky sie of tape 10 cm Electric Force I - 1

2 Activity 2 Stick a new 10-cm strip of Scotch tape with a hanle to the lab table. Label the hanle of this strip with a b for bottom strip. Place a secon new strip on top of the first strip. Label the hanle of this strip with a t for the top strip. Rub the strips so they make goo contact with the table an with each other. Quickly pull the bottom ( b ) strip (with the t-strip still attache to it) off the table an then pull the two strips apart an bring their non-sticky sies near each other. What happens? Does it matter which sies of the strips face each other? How oes the istance between the strips affect what happens? Activity 3 Attach the two pieces of tape from the previous experiment to the woo owel so that the strips hang own from the owel. Repeat Activity 2, to create a secon b-strip an a secon t-strip. Hol each of the new strips first near the original b-strip an next near the original t-strip. If two similar objects are prepare in the same way (i.e. tapes that are pulle off a lab bench), it is reasonable to believe that the two objects have the same type of charge. What can you say about the force between two objects of the same type? About two objects of ifferent type? We will give the name charge to the properties you have just observe. We will fin later that objects with no electric charge can exert attractive forces on both the b-tape an the t-tape. Your boy is such an object. When performing the following experiments, hol the rubbe objects away from your boy so that you o not measure a force of your boy on the hanging tapes. Also, in humi conitions, the electric charge on the pieces of tape can leak off causing them to become ischarge. You may have to repeat Activity 2 from time to time to recharge your t- an b-strips. Electric Force I - 2

3 Activity 4 Rub a soft white cloth vigorously against a blue Styrofoam insulation boar. Hol the boar near the t-strip an then near the b-strip. Does the blue boar have the same electric charge as the t-strip or as the b-strip. Why? Activity 5 You have other objects at your lab table. Try rubbing some of these objects together (for example, a glass ro with silk or a plastic tube with wool) to etermine whether they have t-type electric charge or b-type electric charge after rubbing. List only those objects or types of material that are clearly t-type or b-type. Try to fin two objects of each type. t-type material b-type material Di any charge objects attract both t-strips an b-strips? Be sure to consier only objects that were charge sufficiently an that ha a clear attraction to both. What oes this imply (but not prove) about the number of ifferent types of electrical charge? You have charge objects electrically, by pulling tape off a lab table or by rubbing an object. You have observe two charge objects exert forces on each other. The electric force is similar to the force of gravity in that it is a non-contact force; the objects o not have to be in physical contact in orer to exert a force on each other. Being charge electrically is a property that an object may or may not have. The presence of this Electric Force I - 3

4 property is inferre from observations of forces that objects exert on other objects in experiments like those you have mae in this lab. For now, we will arbitrarily state that the b-tape pulle off the lab bench has a negative electric charge (-) an the t-tape pulle off the back of the tape has the opposite positive charge (). This labeling by () an (-) ates back to Benjamin Franklin. Keep in min, we coul have chosen other names for the charge; these signs are just wors to help us escribe a property of matter that prouces electric force. Activity 6 Question Be sure that you have charge b-type (- charge) an t-type ( charge) tapes hanging from the wooen owels. Hol an empty aluminum pop can first near the t-tape an then near the b-tape. This experiment requires careful observation. Does the can seem to have electric charge, - electric charge, no charge, or is some other explanation neee? Activity 7 Cut or tear a piece of paper into many small bits an lay them on the lab bench. Very vigorously rub the Styrofoam boar with a soft cloth or a wool cloth. Then, bring the charge foam boar near the paper but o not touch the boar to the paper. What happens? Using the table of the previous page, select an object that has an opposite charge to that of the foam boar. Try the same experiment with this new object. What happens when this oppositely-charge object is brought near the paper? Electric Force I - 4

5 Base on the outcome, ecie if the paper seems to have electric charge an if so, the sign of the charge. Or, is some other explanation neee for your observations? Here we have seen an example of a charge object attracting a neutral object. The attraction occurs whether the charge object is positively charge or negatively charge. The moel that explains this escribes a neutral object as a boy containing a very large number of charge particles, calle protons an electrons. In a conuctor, some of the electrons can move aroun. In an insulator, electrons cannot move, but they may shift so that their average position is not the same as the average position of the nucleus. We call this polarization. To moel what you have observe, raw the istribution of charges inuce in opposite sies of the conucting can by the charge ro. Draw the istribution of charges inuce within the insulating paper by the charge ro. Electric Force I - 5

6 Activity 8 Connect both aluminum cans to the same sie (either sie) of the Wimhurst machine (get your lab instructor s help). Arrange the cans so that they are very close to but not touching each other. As you crank on the Wimhurst machine, the cans will repel each other. (What oes this say about the charges on the two cans? What woul happen if we attache the cans to opposite sies of the machine?) Estimate the amount of charge on each can (assume that they have the same charge). Be sure you start with a free boy iagram. Instructor Initials: Date: Electric Force I - 6

7 Coulomb s Law (1) The figure shows three point charges positione in an equilateral triangle. q 1 = q 2 = 20.0 mc, q 3 = 30.0 mc an = 1.50 m. q 1 q 3 What is the force on q 3? (A harer problem woul be to fin the force on q 1. If you can work that, you know what you nee to know about vectors at this point in the class.) q 2-3q (2) A number of charges are place as shown in the figure, where q = 1.2 nc an the circle raius is 2.3 cm. What is the electrostatic force on the charge in the center? [Notation: 2q simply means a charge that is twice q where q can be any value an any sign. 2q means that the charge is efinitely positive.] q 2q -3q 10q 2q q (3) Two charges, q 1 an q 2, are fixe in place some istance apart. Both charges are negative, but the magnitue of charge q 2 is larger than the magnitue of q 1. Sketch a picture of the situation. Inicate on your sketch those places, if any, where a positive charge q 3 coul be place an experience zero electrostatic force. How oes your answer change if q 3 is negative? y (4) Point charges are positione as shown in the figure. What is the net force, F, on the Q charge ue to the other charges? Try to not perform calculations that aren t necessary. -2q -Q -2q - x -q - 2q Worksheets 1