activity-1-2

Activity 1 Electric Charge Purpose The purpose of this lab is to gain experience with electric charge and to make observations that will later be explained by work in class. Introduce Yourself to Your Lab Partners Your best resource for help when you need it is other students in the class. So exchange phone numbers and e-mail addresses with your lab group. 1.1 The Golf Tube - The Nuclear Weapon of Electrostatics 1.1.1 Try This Rub the golf tube (actually, a golf club sleeve—$3.99 from Dick’s online) with the oven roasting bag (2 for $1.47). Bring the tube close to the hair on your arm. To see a maximum effect, we have to avoid getting oils from our hands on the tube and the oven bag. To minimize this hold the oven bag from the inside and the golf tube at the end. This is the first alert box in the activity guide. I know students read the activity carefully before they come to lab because they know it gets them through lab faster, prevents waiting to be checked out at the end of lab, and lets them learn more from the lab. I will be placing important points in boxes like this in future labs. Observe-1.1:What happens when you bring the golf tube near your arm? 1.1.2 Forces We can roughly divide the forces we observe in everyday life into two categories: (1) Contactforces , forces exerted by one object on another object when in contact, (2) Forcesthatactatadistance , forces that act without contact between two objects. Analysis-1.2:Record some examples of contact forces from everyday life. Analysis-1.3:Record some examples of forces that act at a distance from your everyday life. Analysis-1.4:Is the force exerted by the golf tube a contact force or a force that acts at a distance? 1

1.1.3 Forces that Act at a Distance Forces that act at a distance play a very important role in the universe and there are only four (assuming we don’t need another force to take care of dark energy). The one you observed on your arm is by far the most important and the most powerful over long distances. The four forces that do all the work in the universe are in order of decreasing strength. • Strong Force (Holds Protons, Neutrons, and Nuclei Together) Very Short Range. (Strength 1 ) • Electromagnetism - Infinite Range (Strength 1 / 137 ) • Weak Force (Changes one quark into another, responsible for radioactive decay) Very short range. (Strength 1 × 10 - 5 ) • Gravity - Infinite Range (Strength 6 × 10 - 39 ) (Naturally, the world is a bit more complex. Forces that act at a distance do so by exchanging quanta (like photons for the electric force) and contact forces are actually forces that act a distance applied on a microscopic level.) 1.2 Forces Between Charged Rods (A Classic) We have learned that rubbing one substance against another produces an object that exerts a force that acts at a distance on another object. In class this week, we will learn of the existence of electric charge and that it is conserved. The force you felt is the electric force and is the result of the net electric charge of the golf tube. The golf tube is a little unwieldy, so we will work with somewhat smaller plastic objects for a while. There are two types of plastic rods at your station. One set of rods is nylon and the other teflon. Unfortunately, both rods are white. The teflon rod is a bright white while the nylon rod is a milkly white often with blue lettering. If you can’t tell which rod is which, ask your TA. If you rub the teflon rod with a vinyl cloth or the nylon rod with the vinyl cloth, they become charged. Experiment with the rods and record your observations below. • To develop enough charge to work with on the objects, they must be rubbed vigorously. • Try to handle the objects by the same end so that oils from your hands do not get on them. • Use a different piece of cloth or a different part of the same piece of cloth with each object. • Keep track of which end is charged. 1.2.1 Force Between Two Nylon Rods Observe the force between two nylon rods that have been charged by rubbing with the brown vinyl cloth at your station (make sure the brown side makes contact with the rod). Since the force is not very big, you have to place one of the rods on a low friction rotating stand. One member of the lab group should rub one of the rods and place it on the stand while another member charges the other rod. Observe-1.5:The force between the two charged nylon rods is (Circle One): Attractive or Repulsive . Play with the separation between the two rods and determine whether the force between the rods becomes larger or smaller as the separation between the rods increases. Observe-1.6:Select the statement that describes your observations (Circle One): The force between the rods increases with increasing distance, decreases with increasing distance, or does not change with increasing distance. 2

Summary-1.13:Your Law for the Way the Electric Force Changes with the Distance Between the Charged Objects: Experimental Support: 1.3 The Golf Tube and Charged Rods 1.3.1 Think About What Could Happen The golf tube also becomes charged. There are two possibilities: (1) The charge placed on the tube is the same kind as the charge placed on either the nylon or teflon plastic rod, (2) The charge placed on the golf tube is different than the charge placed on either the nylon or teflon rod. Predict-1.14:Suppose the golf tube has the same kind of net charge as the teflon rod. What would you observe as the golf tube interacts with the teflon rod? What about the nylon rod? Implications of Prediction-1.15:If the above prediction turns out to be correct experimentally, which of the following hypothesis about the total number of types of charge in the universe will be supported? Only one type of charge, Only two types of charge, At least two types of charge, At least three types of charge Predict-1.16:Suppose the golf tube has the same kind of net charge as the nylon rod. What would you observe as the golf tube interacts with the teflon rod? What about the nylon rod? Implications of Prediction-1.17:If the above prediction turns out to be correct experimentally, which of the following hypothesis about the total number of types of charge in the universe will be supported? Only one type of charge, Only two types of charge, At least two types of charge, At least three types of charge Predict-1.18:What would you observe if the golf tube had a different type of charge than EITHER the charged nylon or teflon rod? Implications of Prediction-1.19:If the above prediction turns out to be correct experimentally, which of the following hypothesis about the total number of types of charge in the universe will be supported? Only one type of charge, Only two types of charge, At least two types of charge, At least three types of charge 4

1.3.2 Charged Golf Tube and Charged Nylon Rod Now charge the golf tube using the oven bag and the nylon rod using the vinyl cloth. Place the nylon rod on the rotating stand. Observe-1.20:The force between the charged golf tube and the charged nylon rod is (Circle One): Attractive or Repulsive 1.3.3 Charged Golf Tube and Charged Teflon Plastic Rod Now observe the behavior of the charged golf tube and the charged teflon plastic rod. Observe-1.21:The force between the charged golf tube and the charged teflon rod is (Circle One): Attractive or Repulsive 1.3.4 Write a Hypothesis for the Number of Kinds of Charge A hypothesis is a tentative explanation of some observation. You have only enough data to form a hypothesis, not a theory. A theory is based on a large number of observations. Summary-1.22:Write a hypothesis for the number of kinds of electric charge in the universe: Experimental Support: Analysis-1.23:A good hypothesis is testable by further experimentation. What would be required to further support your hypothesis about the number of kinds of charge in the universe and turn it into a theory? 1.4 Charged Objects and Uncharged Objects 1.4.1 Now Let’s Try Something Stupid The previous observations involve forces between charged objects, but what happens if the objects aren’t charged. Predict-1.24:What will happen when the charged golf tube is brought near an UNCHARGED wood rod? 5

1.5.2 Golf Tube and Plastic Soda Can Charge up the golf tube really well. Use it to roll an empty plastic soda bottle across the counter without touching the bottle with the tube. The plastic soda bottle is an insulator. Observe-1.29:The force between the plastic bottle and the charged golf tube is (Circle One): Attractive or Repulsive 1.6 Building Your Own Electrophorus Producing a net charge by rubbing two plastics together is fine (but possibly not dignified), but it would be more useful if we could place that net charge on a conductor so that it was easier to move around. A conductor is a material, often a metal, through which charge can move large distances. An electrophorus is a flat metal plate that develops a large static charge when charged by induction over a charged insulator. The electrophorus was invented by Volta. The original electrophorus used a mixture of paraffin wax and turpentine as the insulator. We will use Teflon(because Volta’s mix didn’t work for me and it had a disgusting, rank smell). (Note from Mrs. Dr. Stewart - ask him about the gas grill...) Reply: I cooked the electrophorus on the grill (I turned off the flame when the grill was hot, before I placed the mixture on the grill). It didn’t blow up. 1.6.1 Build an Electrophorus An electrophorus is a flat metal plate with an insulating handle. We will use a pie pan for the flat metal plate and a Styrofoam cup as an insulating handle. Styrofoam is a great insulator. Obtain the required materials from the front of the room. Build one electrophorus per lab group by taping a Styrofoam cup (the insulating handle) to the center of a pie pan. The tape is not nearly as good an insulator as the Styrofoam, so you don’t want to have the tape where you will touch it. There is an example of an electrophorus at the front of the room. A picture of the electrophorus over its charged board is shown to the right. _ _ _ _ _ _ _ Charged Board Electrophorus Far Away Pie Pan Cup 1.6.2 Try This Charge the Teflon board by rubbing vigorously with the oven bag. Test that its charged by bringing your arm close to the Teflon. The hairs on your arm should stand on end. Now do the following: Observe-1.30: • Grab the pie pan by its metal edge. • Holding the edge place the pie pan on the charged Teflon board. • Release the metal edge and without touching any metal grasp the insulating cup and lift the pie pan off the table. • Bring the edge of the pie pan close to the knuckle of the lab partner who has most annoyed your TA. Record what happened. 7

1.6.3 Experiment With It I was pretty specific about what to do in the previous step. Now you play with it. Observe-1.31:Try three different ways of placing the electrophorus on the charged board and removing it from the charged board(different combinations of holding the cup and the metal rim. You may also try touching the pan while it sits on the board). Record the method and what you observe for each new method of placing and removing the electrophorus. 1. 2. 3. 1.6.4 My Charging Sequence I generally charge the electrophorus by (1) holding the insulating cup while the electrophorus is placed on the Teflon board, (2) Touching the rim of the pan with my knuckle (spark), then (3) Lifting the pan by the insulating handle. This should yield a charged electrophorus, which should shock you again if you bring it close to your body. Observe-1.32:If my charging sequence was not one you tried above, try it now to make sure it works for you. 1.6.5 A Mystery To Solve You may take it as an absolute law of the universe that charge is conserved. I’m afraid you have to take it as a law of the universe because I don’t have the equipment or time to let you prove it yourself. It is most convincingly shown in particle accelerators. Unfortunately, to prove a conservation law, you have to keep doing experiments and show it does not ever fail. Physicists have been doing this for 100 years with conservation of charge and it is probably the most strongly supported conservation law in nature. This leaves us with a mystery. We know the charge on the board came from rubbing with the oven bag, but where did the charge generating the spark on the electrophorus come from? I can think of three possible sources: 1. The Charged Board: The charge on the pie pan could have been directly transferred from the Teflon board. 2. The Air: The charge on the pie pan could have been drawn out of the air without coming from the charged board. 3. You: The charge on the pie pan could have been drawn out of you when you touched the metal pan. Let’s figure out where the charge came from. Observe-1.33:If the charge was transferred directly from the board to the electrophorus, then the charge should be used up as the electrophorus is charged (because charge is conserved). See how many times the electrophorus can be charged without recharging the board. If this number is much more than one, then the charge is not coming from the board. 8

Step II: Electrophorus Sitting on Charged Board Next, place the electrophorus on the charged board using the insulating han- dle. Draw the location of any charge on the electrophorus for this step. Make sure the number of charges drawn correctly repre- sent the location of net charge on the electrophorus and the total charge of the electrophorus. Draw-1.37:Draw the electrophorus, showing the location of all charge, as it is first placed on the charged board. Analysis-1.38:What is the total charge of the electrophorus in this step? Step III: Touch the Electrophorus This step involves a charge transfer. The touching of the electrophorus with your finger elec- trically connects it to a large conductor (yourself). This is repre- sented using the ground symbol in the figure at the right. Charge is transferred when the pie pan is touched. Draw-1.39:Draw the transfer by indicating the direction of the transfer and the sign of the charge being transferred. Draw the location of the charge on the pie pan AFTER the transfer. It is unnecessary to draw the charge of the ground after the transfer. Analysis-1.40:What is the sign of the net charge of the electropho- rus after the charge transfer to ground? _ _ _ _ _ _ _ Charged Board Pie Pan Cup Ground Touch the Electrophorus Step IV: Disconnect the Ground Remove your finger from the pie pan, thus breaking the electrical connection. The electropho- rus is still sitting on the charged board. Draw-1.41:Draw the board and the electrophorus in the space at the right including the location of any net charge. Analysis-1.42:What is the sign of the net charge of the electropho- rus? 10

Step V: Remove the Electrophorus Using the insulating handle, remove the electrophorus from the charged board. Draw-1.43:Draw this situation at the right with the electrophorus far from the charged board. Draw the location of all net charge. Analysis-1.44:What is the sign of the net charge of the electropho- rus? Signature:To Be Completed Finished 11