2-5.1 Mini-Experiment: Charges and Interactions Physics 244 The Charge Model Two objects with mass always feel a force towards one another known as gravitational force. The gravitational force is very small and is only an attractive force. This semester, we introduce a new set of physical interactions based on electric charges. The picture we want to develop is called the charge model of electricity. As you learned last semester, all matter has a property called mass. Matter also has a fundamental property called charge. All large material objects in nature contain some amount of discrete positive charges and some amount of discrete negative charges. The sum total of the positive and negative charges is call the net charge of the object, and a large object can have a positive net charge, a negative net charge, or be electrically neutral (if the total number of positive and negative charges are equal). We now know a lot about where charge resides – it resides in the atoms that make up every molecule and object. By convention, or just the way folks decided to label it, protons are said to have a positive charge of +e = 1.6 x 10-19 C (unit C = Coulombs), electrons have a negative charge of equal to –e = -1.6 x 10-19 C, and neutrons have no net charge. Protons are very, very hard to remove from atoms or materials – it takes a nuclear reaction to do that! But electrons can be removed fairly easily from some materials or added to others. Whenever an object is disturbed in some way, such as when it is rubbed or peeled, the amount of charge can be modified. This disturbing process is called charging. Two charged objects exert forces on one another that are equal in magnitude and opposite in direction (Newton’s third law), and the interaction between two charged objects can be either attractive or repulsive. Whenever two objects have the same sign to their net charge (both positive or both negative) they will repel. Two objects with different signs to their net charge (one positive and one negative) will attract. Question 1: What’s wrong (if anything) with the following statement? If the statement is correct, explain why you think it is correct. If the statement is wrong, re-write it to be a correct statement and explain your change. “If an object has a net negative charge, that’s because it has more electrons than protons. This occurs because someone figured out a way to add some electrons to that object.” Question 2: What’s wrong (if anything) with the following statement? If the statement is correct, explain why you think it is correct. If the statement is wrong, re-write it to be a correct statement and explain your change. “If an object has a net positive charge, that’s because it has more protons than electrons. This occurs because someone has figured out a way to add some protons to that object.” Mini Experiment 1: Electric Scotch Tape Procedure Follow the procedure below and record answers to the prompts at the end a. Place a 3-4 inch of Scotch tape on a smooth dry surface (such as a mouse pad) with the sticky side down. The end of the tape should be curled over to make a non-stick handle as shown below. b. Label the tape with a “B” for bottom. Place another tape of the same size on top of the “B” tape and label it “T” for top. The end of this tape should be curled over to make a non-stick handle. c. Quickly peel the “T” tape off the “B” tape and hang it on a plastic dowel, as shown below. Next, rapidly pull the “B” tape from the mouse pad and hang it on the dowel. The non-stick handle should be near the plastic dowel (see diagram below). Note: While you are doing this, try to handle the tapes only by the non-stick end. d. Obtain a plastic rod and a piece of wool (black). Rub the plastic rod with the wool, and bring it near the hanging tape labeled T. Next bring the plastic rod near the tape labeled B. Do not touch the tapes with the plastic rod. Prompts: Observation A: Is there any interaction (i.e. –a force) between the plastic rod and the T-tape? If there is a force, how can you tell? Observation B: What kind of interaction (attractive or repelling) exists between the plastic rod and the B-tape? Observation C: How does the distance between the tapes and the rod affect the interaction between them? Does the interaction get stronger or weaker as they are brought closer to each other? (Note: the greater the deflection of the tape the greater the force acting on it) Observation D: Put the plastic rod away and now move your hand slowly towards the top tape. What type of interaction did you observe? Now place your hand near the bottom tape. What type of interaction did you observe now? (Note: If you observe no interaction repeat steps b and c in the procedure). What happens here should surprise you and will require some more deep thinking. What has been explained to you so far does not quite explain what happens in this observation! By convention, the plastic rod that you used in the previous activity is said to be positively charged when rubbed with wool. Based on this information and on the charge model described above, answer the following questions. Question 3: Which tape, T or B, had a positive charge? Explain your reasoning. Obtain a glass rod and a piece of fur or silk. Vigorously rub the glass rod with the fur, and bring it next to the charged hanging tape labeled T. Now bring the charged glass rod next to the tape labeled B. Question 4: Based on the observations above, what is the charge on the glass rod? Explain your reasoning. Question 5: If you repeated steps b and c of the procedure and then brought the two “B” tapes towards one another, would the tapes repel or attract? Predict the answer, and now test it.
