PHY 2092 Distance Learning Experiment Guide 01 Electrostatics YouTube Video #1 This 5:37 video is fun to watch and the background music can be used to help you go to sleep! But don’t nod off. After watching the video, choose two of the demonstrations, describe them in writing and then explain the physics in the discussion section of your report. https://www.youtube.com/watch?v=ViZNgU-Yt-Y Photographs The GSA making the videos listed below also took many still photos of the pieces of equipment used in this experiment. These photographs can be found in Canvas > Files > Experiments > 01 Electrostatics. In the data analysis section of your report, create a table. In one column, list the file name of each photograph and in the next column, write the technical name of each piece of equipment. Videos All of these videos (except the YouTube video) have been intentionally recorded withOUT any narration or explanation. Part 1 Watch the videos with these file names by going to the lab’s Canvas course and Click on Panopto Recordings > Exp 01 Electrostatics. Read the procedure for Part 1 and watch these videos in the alphabetical order listed. You may need to watch a video more than once. Determine which procedure number corresponds to each video. In your discussion section, create a table that lists both the file names and the procedure number. Pt 1 a.mp4 Pt 1 b.mp4 Pt 1 c.mp4 Pt 1 d.mp4 Pt 1 d.mp4 The data to take for this experiment is the readings of the electrometer shown in the videos. This is not a very quantitative experiment. Use the following and draw conclusions from the brief videos. Color of Pad Trial 1 (Volts) Trial 2 (Volts) Trial 3 (Volts) Trial 4 (Volts) White 3 3 2 2 Blue -4 -4 -4 -4 Both -1 -1 -1 -1 Both (touching) 0 0 0 0 Part 2 Watch the videos with these file names in Panopto Recordings > Exp 01 Pt 2 a.mp4 Pt 2 b.mp4 Charge source Electrometer Reading (Volts) Side near to first sphere -15 Side far from first sphere 15 Side far from first sphere after ground 0 Again, determine which procedure number corresponds to each video and list both the file names and the procedure number in your discussion. Additional Question: For Pt 2 b.mp4, what could the experimenter have done to see a larger deflection of the LED in the electrometer? YouTube Video #2 It is recognized that these brief videos may be too limited to give the beginning Lab 2 student sufficient clues to draw conclusions. Therefore the link to this YouTube video is provided so the student can pull these ideas together in Part 2. https://www.youtube.com/watch?v=ed62gIH1dos Part 3 Go to the website mentioned in the procedure and complete the activities in Part 3. White a brief description of each activity. Include a screen capture for each activity. Place these in the Discussion section of your report. Data Analysis Sample Calculations: These are good examples of how to set up your sample calculations in the Data Analysis section of your lab reports. These are single calculations from various students’ reports. The reports should include sample calculations, set up like these ones, for every type of calculation done during an experiment. These calculations would all get full credit but if you are looking to have outstanding calculations you should also discuss how these equations relate back to the physics concepts of the lab and why they are essential to the experiment. Example 1: The slope of the best-fit line on the range vs. velocity graph represents the function of the time of flight in seconds. The following equation uses the height from which the ball falls from, h, and the local acceleration of gravity, g, to solve for the theoretical slope, or theoretical time, xt. 2h xt = √ g 2(0.97m) xt = √9.81 m/s2 = 0.445 s Example 2: The equation [the equation for projectile motion previously stated] can be rearranged to solve for the time, t, by multiplying both sides by two, dividing both sides by the gravitational acceleration, g, and taking the square root of both sides. t=√ 2h g Formula 5. Formula 4 changed to solve for the time. By replacing the values with the measured values of the height, h, from the floor to the end of the track and of the gravitational acceleration in Melbourne, FL, we can get the ideal time. 2 ∗ 0.961 m t= √ = 0.443 seconds 9.792 m/s2 Formula 6. Theoretical time for the metal ball to get in contact with the floor. Example 3: This is a sample calculation for the theoretical time (tth) using the height of the ramp (h) and the gravitational acceleration (g). Slope = √(2*h/g) Slope = √(2*.98/9.7929) = .447 (s) Example 4: 2ℎ Sample calculation of time. The time can be calculated by 𝑡 = √ 𝑔 , where h is the height and g is the gravitational acceleration. By using the theoretical value of gravity, this will calculate the theoretical value of time. This will later be compared to the experimental value of time. 2ℎ 𝑡=√ 𝑔 2(0.9560 𝑚) 𝑡=√ 𝑚 = 0.442𝑠 9.792 2 𝑠 Example 5: Determining The Theoretical Value of Time The theoretical value of time is found by rearranging an equation that uses constants in the experiment. For the distance in the Y direction this formula, derived from the projectile motion equation, is used and it can be rearranged to find the theoretical time. ℎ = ℎ𝑒𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑡𝑒𝑟𝑠, 𝑔 = 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑑𝑢𝑒 𝑡𝑜 𝑔𝑟𝑎𝑣𝑖𝑡𝑦 𝑖𝑛 𝑚𝑒𝑙𝑏𝑜𝑢𝑟𝑛 𝑖𝑛 𝑚𝑒𝑡𝑒𝑟𝑠 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑 𝑡𝑝 = 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑡𝑖𝑚𝑒 𝑖𝑛 𝑠𝑒𝑐𝑜𝑛𝑑𝑠 1 ℎ = ( ) ∗ 𝑔 ∗ (𝑡 2 ) 2 𝑡𝑝 = √2ℎ/𝑔 The values from the experiment are plugged in and used to solve time. 𝑡𝑝 = √2 ∗ 0.92𝑚/(9.792𝑚/𝑠 2 ) 𝑡𝑝 = 0.433𝑠 Example 6: The experimental slope, or time, is gotten from the equation that we got from the graph of the Range vs. Velocity which was graphed and calculated by excel. The experimental slope is the multiple of the x in our equation. Equation of the slope: y=0.3974x + 0.0184. As we know the equation of a slope is y=ax + b, 0.0184 is our b that is known as y-intercept which is the initial position and 0.3974 is the slope, a, the experimental flight time. R2=0.99855. As much as R2 is closer to 1, it is better. Our R2 is very close to one which shows our good accuracy in doing the experiment. t’= √2ℎ/𝑔 t’= theoretical slope h= average hight h=0.9563 g=9.792 t’= √(2(0.9563))/9.792 = 0.442 g= local acceleration due to gravity Discussion of Errors: These are examples of how to completely discuss the errors in your experiment. Each paragraph here discusses a single error, your lab reports should discuss all errors in your experiment (hint: there is always more than 2). In both these paragraphs the error is described and categorized, and how the error affects the results/other parts of the experiment is discussed. Example 1: The primary source of error was intrinsic systematic error in the shaping of the track, as it was slightly sloped in the final stretch right before the photogate, so index cards were needed to make this part of the track level. This paper may not have been completely level, resulting in instability in the track and inaccurate measurements of the ball’s range. A more stable track setup could be used that lacked this instability in order to procure more accurate results and measurements. Example 2: There were sources of error that took place in the experiment that caused inaccuracies in the results. The angle measurement was based on the location of the angle scale. It was hard to align the scale with the protractor perfectly. Because of this, it was difficult to get a good accurate measurement of the angle. This is a systematic error in the measurement because there was a flaw in the tool process. This error impacted results and calculations because the angles were very important when it came to calculating the x and y components of each force. Also, the angles were very important when it came to constructing the vector diagrams. Experiment 01 Electrostatics Introduction This experiment is largely qualitative and offers the student a shallow learning curve to introduce triboelectric charging and charging by induction. These activities provide numerical results but no theoretical values for comparisons. Concepts Electric charge is a property of objects that was first discovered and recorded by the Greek philosopher Thales around 600 B.C. He found that by rubbing pieces of amber (petrified pine tree sap) with a cloth he could make other small objects move or experience a force. The Greek word for amber is elektron. This is where we get the terms electron and electricity. So objects can become electrically charged by rubbing them against one another. This phenomenon is called triboelectrification. The Greek word for rubbing is tribos. As we saw in Physics 1, rubbing generates a frictional force which usually does negative work on the object it acts upon. Work is a form of energy. So by rubbing we are transforming kinetic energy into thermal energy through friction. This thermal energy elevates the temperature of the object but it can also remove electrons from the molecules making up the object. As you know from chemistry, some elements, compounds and materials ‘want’ more electrons and others readily give them up. So by picking the right two materials to rub together, we can move electrons from the first object to the second. This leaves the first object positively charged because the rubbing does not move the protons in the atoms’ nuclei. Likewise, rubbing leaves the second object negatively charged because it now contains more electrons than it did when it was electrically neutral. Neutral means the number of positively charged particles in an object equals the number of negatively charged particles. In 1733 the French scientist DuFay discovered the “likes repel and unlike attract” nature of what was then thought of as two different types of ‘electric fluid’. In letters written in 1747, Benjamin Franklin described the results of his own experiments. It was Franklin who first coined the terms positive and negative and hypothesized that there was only one electric fluid. Franklin correctly reasoned that objects became oppositely charged by gaining a surplus or deficit of this one electrical fluid. In Franklin’s day, the existence of atoms, electrons and protons were unknown. So when Franklin rubbed a glass rod with a silk cloth he arbitrarily decided to call the charged state of the glass positive. Later, the idea of an electrical fluid that flowed between objects was discarded. When two plastic rods are rubbed with a piece of animal fur, the rods become positively charged and the fur negatively charged. When suspended from threads, the two rods will push against each other but will be pulled toward the fur. So we say two objects push (repel) each 1-1 other when they have the same electric polarity and conversely two objects will pull on each other (attract) if they have opposite electric polarity. In writing the previous paragraphs, care was taken to refer to charged objects and not electric charge as if it was an object itself. Again, charge is a characteristic of something, not a material thing. Most books and many instructors do not emphasize this distinction. This plants a misconception that electric charge is like peanut butter: something that can be spread onto other objects. The better analogy is to think of electric charge like color or taste or odor (properties of an object). So if a textbook uses red for positive and blue for negative (as ours does) then it is best to think of more positive charge as losing electrons and becoming redder. The Earth (both the planet and the ground beneath our feet) is a source of a vast amount of free charge. Here free means free to move. In reality this means electrons can easily move up or down a wire that is connected to a metal post drilled into the Earth/ground. By default we define the electric potential energy of the Earth/ground as zero. This is analogous to the worldwide average sea-level being defined as zero topographic elevation. In this way, we use the word ground to mean a value of zero Joules per Coulomb of charge. Any good conductor of electricity (a piece of metal or a wire) can be made to have an electric potential energy of zero Joules / Coulomb (which are called Volts) by connecting it to the Earth/ground. So we often refer to a piece of metal or wire connected to the Earth as ground in an electric circuit. If a positively charged object is insulated and isolated from its environment (tables, chairs, people) the object will remain charged for quite a while. However water is a moderately good conductor of electricity. And humidity in the air and the movement of the air will provide a way for charged particles to travel from the object to the Earth/ground. This slow discharge of the object increases its speed in humid environments like sunny Florida. You will have to take this into account when doing these demonstrations. Water is good conductor due to the dissolved ions usually found in it. Purified, de-ionized water is actually a very good insulator. Cold, dry air has low humidity and doesn’t move much. This is why it is easier to charge yourself and feel a shock on dry days. We are constantly separating charge by rubbing parts of our clothes together or against other objects like carpeting or automobile seats. Teachers want to induce their students to study. Police want to induce people to obey the speed limit. Telecom companies want to induce their customers to upgrade their service. So the word induce means “to cause to happen”. Therefore, charging by induction is to cause something to become charged but not by a direct method. Rubbing is one direct approach to create a charged object. Another is bombarding the object with a beam of electrons, which is how a CRT television screen becomes charged during use. Charging by induction uses the Coulomb force to pull excess charged particles from the ground onto an object. Humidity and air currents tend to rapidly discharge isolated, insulated, charged objects. On the other hand, on a dry winter day, just a little rubbing will charge a balloon or soda bottle (for example) to a potential of several thousand volts. This is much more than the electrometer can measure so be sure not to “peg” the meter. NEVER allow the needle to quickly snap back 1-2 and forth and bang off of the clear plastic pegs seen at the bottom of the electrometer’s window. On dry days you may have to wait a while or blow moist breath onto an object to lower its potential before using the electrometer. Method ! Figure 1. The Pasco Electrostatics System In this experiment, you will use the PASCO Basic Electrostatics System to make discoveries about non moving (static) electrical charge, electrical charging, grounding, induced charging and electrical polarization. The apparatus consists of two conducting, metallic spheres mounted on insulating stands, two wire cages one inside the other, three wands with black plastic handles, a high voltage / low current power supply and a special meter for measuring voltage levels (which will give an indication of how charged an object is). Many of these items are made of plastic but contain other parts whose function involves some very subtle physics. Read the following descriptions to learn of these subtleties. Applying these details will lead to correct observations and help dispel incorrect ideas you may have about electric charge. The Electrometer: At first glance, the electrometer looks like a cheap voltmeter. However ordinary voltmeters measure the electric potential energy of charged particles by siphoning off a small number of the charged particles being measured. This distorts the very thing you are trying to measure. One way to compensate is to siphon off as little as possible. To do this the instrument must have a very high internal resistance so very little electrical charge gets drained away from the object under study. Common voltmeters have an internal resistance of 107 Ohms. This sounds high but it isn’t high enough for electrostatics experiments. Electrometers have an internal resistance of 1014 Ohms. So they will siphon off a current of only pico-Amps instead of the micro-Amps an ordinary voltmeter will require. 1-3 Important: Throughout the experiment, the relative charged state of any object will be determined by the electrometer which measures the electric potential (energy per unit charge) of that object. The units of electric potential are the Joule / Coulomb which is called a Volt. In your report, try to distinguish between charge (which is properly measured in Coulombs) and the electrometer’s reading which will be proportional to the amount of charge. However, please recognize that electric potential is not the same thing as charge. Electrostatic Voltage Source (EVS): This is our ‘power supply’ for this experiment. But so little power is being supplied it is better to view it as a charging device. Through the marvels of modern semiconductor technology, this device can supply 1,000, 2,000 and 3,000 Volts (Joules of potential energy per Coulomb of charge) while limiting the maximum current to no more than 8.3 micro-Amps. Ordinarily thousands of Volts are extremely hazardous. But with this level of current limiting, you can touch bare wires energized to these voltages and not feel even a tingle. So this power supply is very safe despite its high voltage rating. The EVS has solid-state circuitry powered by four AA batteries. Be sure to turn it OFF at the end of the experiment. Also do not let the wires connected to the EVS come into contact as a small spark will result. Three Magic Wands: These have black plastic handles and a special white insulating neck near the disk. The white material is a polycarbonate with an electrical resistance of 1014 Ohms. When you rub the blue and white face of each disk against each other, the white disk becomes positively charged and the blue disk becomes negatively charged. The third wand has an aluminum covered disk. The disk below the aluminum is black polycarbonate mixed with carbon. This provides a moderate conductor (resistance is 1,000 Ohms) capable of storing charge with a very good conducting surface. The third wand is used for transferring charge. For good results you must keep all the disks clean. When not in use, place them on a new sheet of printer paper. This keeps grime and oils from the table top off the disks. You can clean the disks with alcohol and soft paper towel. When transferring charge from the spheres, always touch the conductive wand so the face of the disk is tangent to the sphere. In Franklin’s day, these wands would have had magical properties. Two Conducting Spheres: These are plastic spheres plated with layers of copper, non-sulfurous nickel and lastly chrome as the outer (shiny) layer. They have a jack for connecting a wire with a banana plug to the sphere. The support rod is a good insulator. The Faraday Ice Pail: This is the two cylindrical wire cages. The outer cage is a shield which prevents stray charges from affecting the charged inner cylinder. The inner cylinder is called the pail. It is your bucket for holding electrically charged particles. The pail is insulated from the base but the shield is in contact with the base. So the shield and base are effectively grounded due to contact with the table top. Michael Faraday used a metal ice pail for his experiments in electrostatics. A solid metal bucket would work here but the wire cages let you see inside. 1-4 The terminology of electricity is old and varied. Most of these terms will be coined and defined as we come to them. However, here is a brief glossary to help you get started. plug - a protruding piece of bare metal used to make an electrical connection jack - insulated metal hole used to make a connection; plugs are pushed into jacks banana plug - a metal plug with an outer metal cover, the cover has slits that run the for most of its length, the slits act as springs and assure a good connection when this plug is inserted into a jack, banana plugs can be any color but the plastic insulator of ours just happens to be yellow piggy-back plug - a plug with a jack made into the rear of the plug, also called stacking plugs alligator clip - a spring loaded, pinching clip used to make temporary electrical connections, Americans (especially Floridians) also call them gator clips. The British (and British influenced) often call them crocodile clips test lead - insulated wire with a plug on one end and a prong on the other end mounted inside a pencil-like insulator, the pencil shape allows the user to conveniently grasp the working end and make brief contact to very small spots on a circuit board hook-up wire - insulated wires with banana plugs on each end which are used to build circuits terminal - another name for a jack, usually mounted inside an electrical measuring device such as a voltmeter or oscilloscope binding post - a special type of jack that often combines a banana jack with an insulating collar, the collar can be unscrewed to reveal a drilled hole for inserting a hard metal prong or bare wire, the collar is then screwed back down to squeeze the wire to insure a good connection stranded wire - hook-up wire that is made from a twisted bundle of very thin wires surrounded by insulation solid conductor wire - hook-up wire that is made from a single, thick bare metal wire, stranded wire is usually much more flexible than solid conductor wire solder - an alloy of lead and tin that melts at temperatures ranging from 300°F to 700°F, it is drawn into spools and used to permanently connect a wire (usually stranded) to some other metal contact in a circuit; it is pronounced sodder with a silent l. crimp - a method of making an electrical contact usually between a stranded wire and a solid metal piece, the wire is inserted into a split collar and the collar is squeezed very hard with pliers, the collar deforms and makes a secure physical and electrical contact with the wire spade lug - a two-pronged, fork with a collar; wire is placed into the collar which is crimped or soldered, the fork is then available to fit part-way around a binding post 1-5 Procedure Part 1 Triboelectric Charging 1) Examine the electrometer and make sure it is OFF and the needle reads zero. If it does not read zero, ask you instructor for help. He or she will have to mechanically adjust the needle to zero. Using a black wire, connect the jack labeled GROUND to the ground adapter plugged into the power strip on the table. Connect the coaxial cable with two alligator clips to the outer and inner wire cages. Connect the black clip to the outer cage (the shield) and the red clip to the inner cage (the pail). The shield will be your easily accessible ground for the experiment. Set the electrometer’s range to 30 Volts. This represents the maximum one-way deflection of the needle. Later, if this setting seems too high you may lower it. It is always safest to start by setting a meter’s range to its maximum value. Turn on the electrometer and press the zero button. 2) Remove any stray charged particles from the neck and handle of the two charging wands by touching these parts of the wand to the shield. Repeat this step often during the experiment for good ‘clean’ results. 3) Gently rub the blue and white disks together for a couple of seconds. Move one of them far away from Faraday’s ice pail (place it on the table). Then touch and continue touching the shield with your newly freed hand. This removes any stray charged particles from you and any charged particles you generate by moving. Next, lower the other wand into the pail without touching the inner wire cage. Do not switch the wand into the other hand. 4) Have you partner read and record the electrometer reading. 5) Remove the wand and again have your partner record the electrometer reading. Afterward, have your partner press the zero button on the electrometer. 6) Remove the wand and record the voltage with no wand in the pail. 7) Press the electrometer’s zero button and wave the wand in the air for 5 seconds. Then, reintroduce the wand into the pail. Are there any charged particles left on the wand? Remove your grounded hand from the shield. 8) Question: Can you explain the physics? Write an explanation of steps (3) through (7) in your report. 9) Repeat procedures (2) through (8) using the other wand. Question: What do you notice that is different and why? 1-6 10) After grounding yourself and the wands, repeat procedures (2) through (5) but this time modify procedure (3) by not putting both wands into the pail at the same time. Do not touch the pail with the wands and do not let the wands touch each other. Question: What physics does this exercise demonstrate? Answer with one or two full sentences, not just one or two words. 11) Repeat procedure (10) but this time touch both wands to each other when you place them in the pail. Question: Does this demonstrate the same physics as procedure (10)? Part 2 Charging by Induction 1) Connect the EVS to one of the conductive spheres and apply 2000 Volts. This charged sphere will be referred to as “the first sphere” in this part of the experiment. 2) Connect a common ground from the EVS to the electrometer. Connect the electrometer’s black wire to the Faraday Ice Pail’s shield and its red wire to the pail. Press the zero button to momentarily ground the shield. 3) Set the electrometer to the 30 Volt range. Place the second sphere 5 cm from the charged sphere and momentarily ground the second sphere by touching the ends of an extra wire between the sphere and the shield. This separation distance is surface to surface not center to center. As you proceed, keep an eye on the electrometer and do NOT allow it’s needle to exceed its maximum deflection. Reduce the range on the electrometer as needed for an accurate measurement (the needle should deflect into the upper half of its range). 4) Use the aluminum covered wand to collect charged particles from the sphere connected to the EVS and measure the relative magnitude of charge by placing the aluminum disk inside the pail. Do NOT touch the pail with the charged wand. Be sure to make contact with the sphere so the aluminum disk is always tangent to the sphere. Make note of the polarity of the charge on the sphere connected to the EVS. Repeat the measurements a few times at different locations. 5) After grounding the wand, sample the charge on the second sphere. First sample at points directly opposite (farthest from) the first sphere. Ground/zero the electrometer between measurements. Record the electrometer readings. 6) Next, sample at points on the second sphere that are closest to the sphere connected to the EVS. Questions: Is the polarity of the charge on the near and far sides of the second sphere the same? Is the charge density on the near and far sides of the second sphere constant? 1-7 7) Next, ground the wand with the aluminum disk by touching the shield. With the two spheres 5 cm apart, have your partner take a spare wire and connect one end of it to the ground (the shield). Then (handling the yellow, insulating plastic) touch the banana plug on the other end of the wire to the side of the second sphere farthest from the first sphere. 8) Remove the spare wire and use the wand with the aluminum disk to sample the charge on the second sphere at the point where you touched the spare wire. Place that wand in the Faraday ice pail and record the voltage. Question: Is the charge density on the second sphere the same as it was in procedure (5)? Explain your findings. 9) Turn off the EVS and allow the first sphere to discharge for 60 seconds. You can speed up the discharging by blowing on the sphere. Then, use a spare wire and ground the sphere. Move the spheres aside. 10) Turn OFF all equipment, disconnect all wires from the spheres and hang your red and black hook-up wires on the rack in the back of the laboratory. Leave the coaxial wires coiled up neatly at your station. 11) Remember to also turn in the post-lab worksheet when you turn in your lab report next week. Part 3 Visualizing Electric Fields 1) Start a web browser and enter this search term into Google: PhET Charges and Fields. Run the Java applet by clicking on the right pointing triangle. Note: Java must be updated and authorized to run by an administrator on the computer you are using. Windows based computers will put up many roadblocks to running Java so it is best to perform this part while you are still in the laboratory. For each of the following procedures make a hand-drawn sketch of the charge distribution you set-up and its electric field vector diagram. Notice the use of the phrase field vector diagram. You can include a field line diagram if you wish but you must include several field vectors too. 2) Play with the software and develop an idea of the two-dimensional shape of the electric field around a single, positively charged particle. 3) Then build a dipole on the screen and determine the shape of its electric field. Also, determine where the electric field is zero and label this point(s) on your sketch. 4) Next, build a linear quadrupole, determine the shape of its electric field and make the corresponding sketch. Note: A quick Internet search will describe how to arrange the charged particles. 1-8 5) Next, build a planar quadrupole, determine the shape of its electric field and make the corresponding sketch. 6) Make a long line of positively charged particles. Determine the shape of its electric field and make the corresponding sketch. Where is the electric field perpendicular to the line of positively charged particles? Label these points on the sketch. 7) Finally, make one long line of positively charged particles and a parallel second line of negatively charged particles. Keep the two lines separated by two centimeters. Determine the shape of this charge distribution’s electric field and make the corresponding sketch. 8) Your lab instructor will simply award 30 points automatically to your lab report score for successfully completing this part of the experiment and including all sketches in your lab report. 1-9
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