Lab 2: Electric Interactions II
LEARNING GOALS OF THE LAB:1. Learn what an electroscope is and how it works.
2. Determine whether the human body is a conductor or an insulator. 3. Learn to analyze phenomena using videos.
In this lab you will be working with several video experiments. After you finish analyzing each video experiment, you need to perform it using your own equipment. (Electroscope experiments are sensitive to atmospheric conditions so we made videos for consistency. Do not be discouraged if the experiments with real electroscopes do not work as well as they do in the videos.)
Available equipment for all experiments: Electroscope, plastic rod, foam rod, rubbing materials, Styrofoam square A note about the electroscope: There is a continuous conducting path between the plate at the top and the moving leaf in the middle.
I. OBSERVATION EXPERIMENT: CHARGED RODS BROUGHT NEAR THE ELECTROSCOPE: Watch the video at: https://youtu.be/TVa43DOtRXE
a) Describe in words what happened in the first part of the experiment (with a positively charged clear rod). b) Explain the first part of the experiment using your knowledge of the electric interaction and the
internal structure of conductors. Use a charge diagram similar to the one shown here to indicate what happens when the rod approaches the plate. Include the position of the “leaf” (the moving part of the electroscope in the middle) and the position of any unbalanced charges.
c) Repeat the above steps for the second part of the experiment (with a negatively charged foam pipe).
d) Explain why in both experiments the leaf of the electroscope goes back to the original position. e) According to your explanations, what will happen if each rod has a larger charge? Smaller charge? f) Repeat the experiments with real equipment.
II. OBSERVATION EXPERIMENT: TOUCHING ELECTROSCOPE WITH CHARGED PIPE Go to https://youtu.be/B2rK_5yzv5E
a) Observe the experiment. Draw a picture representing the charges on the pipe and on the electroscope before and after Alan touches it with the pipe. b) Explain in detail why the leaf of the electroscope remains deflected even
after the charged rod is moved far away.
c) Explain in detail why the leaf goes back to the vertical position after Alan touched the electroscope.
d) What do you need to assume about the structure of the human body in order to explain the observations?
e) Repeat the experiment with real materials. Also, in addition to touching the electroscope with the bare skin of your hand, repeat the experiment but with a slab of Styrofoam between your hand and the electroscope. Record your observations. Suggest an explanation for what happened.
III. APPLICATION EXPERIMENT: THE SIGN OF THE CHARGE
Imagine that somebody charged an electroscope and left. How can you find out what charge (positive or negative) is on the electroscope? Devise and describe two methods to determine the sign of the charge of the electroscope (you do not have to use the methods at this point, but you will in part IV below). You have
access to a single charged rod and you know the sign of its charge (you choose). Hint: you can bring the charged rod close to different parts of the electroscope.
Explain your reasoning to convince others that the methods are reliable. Draw charge diagrams for all relevant situations.
IV. APPLICATION EXPERIMENT: CHARGING.
The goal of this experiment is to devise two distinctly different methods of charging an electroscope. Hint: you can watch the videos at https://youtu.be/PwJG7i_f4Xw for ideas about how you might do this. Note: In the video neither the plastic nor foam rod ever touches the electroscope, only her hand does.
a) Describe the two ways of charging the electroscope. Explain why each method will work. Include sketches and charge diagrams with your explanation.
b) Charge an electroscope using each of the two methods. Use the methods that you devised in experiment III to determine the sign of the electroscope’s charge. In order to do this you will need an object with a known charge. Use this: the foam tube rubbed with fur will become negatively charged. Is the outcome of the experiment reasonable? Describe anything unexpected that might have occurred…
Continue… Rubrics you’re being scored on in this experiment:
B5, B9, B10
Rubrics attached at end of lab
Rubrics you’re being scored on in this experiment:
D2, F1
V. APPLICATION EXPERIMENT: “ELECTROPHORUS” In this experiment you will apply your knowledge of
electric charge and conductors to explain new phenomena. Additional available equipment: The “electrophorus”
apparatus shown in the photo. The aluminum foil ball is hanging from the wood dowel by a thin thread (which is hard to see in the photo).
a) Rub the upward facing side of the Styrofoam base with fur. Holding the rest of the apparatus from above by the cup, place it on top of the Styrofoam base as shown. Make sure the aluminum ball is hanging freely outside the aluminum plate. Next, using the thread to move the aluminum ball, make the ball touch the aluminum plate. Be sure not to touch the ball or plate with your hand. Take your hand away from the thread.
b) Next, very slowly bring your forefinger closer to the ball again from the left (according to the perspective shown in the picture) until something begins happening between your finger, the ball, and the plate.
c) Record and describe your observations.
d) Using your understanding of conductors, insulators, electron transfer, and the electrical properties of the human body, explain your observations. Include charge diagrams in your explanations.
VI. WHY DID WE DO THIS LAB?
1. Why is it important to know that a human body is a conductor and not an insulator? Give an example from everyday life or your future work that illustrates that knowing this is important.
2. Sometimes we cannot design and perform our own experiment to test an idea and need to use an experiment that was already performed by somebody else. Why is it still important to make a prediction based on the idea being tested before we look at the data from that experiment?
VII. HOMEWORK (IMPORTANT! Do this homework AFTER Monday’s lecture but BEFORE Wednesday’s lecture.) This homework introduces you to an alternate way to represent the electric field. It is called the electric potential field (V-field for short). To help learn about it and visualize it we’ll use an online simulation. Go to
https://phet.colorado.edu/sims/html/charges-and-fields/latest/charges-and-fields_en.html
then click the image to start the simulation. Now, follow these instructions carefully answering the questions along the way.
• Make sure the “Voltage” is the only option checked at the top right of the screen.
• Drag a “1 nC” red positive charge from the bottom to the middle of the screen.
• Drag the rounded blue box with the crosshairs from the right to near the red positive charge. This box is a “V-field detector”. In the white box it shows the value of the V-“V-field in the middle of the crosshairs.
• Question 1: Drag the V-field detector in a circle around the positive charge keeping the crosshairs a constant distance from it. Describe the value of the V-field as you do this.
• Drag the positive charge back to the bottom. Drag a “1 nC” blue negative charge to the middle of the screen.
• Questions 3 and 4: Repeat questions 1 and 2 but with the negative charge present instead.
• Drag a positive charge to a location a moderate distance from the negative charge.
• Question 5: Use the detector to investigate the V-field produced by the two charges together. Describe your findings.
• Question 6: Investigate the region exactly halfway between the two charges. What is the value of the V-field in this region? Come up with a reason why this value makes sense.
POSTSCRIPT (OPTIONAL, AND REALLY JUST FOR YOUR AMUSEMENT): AUSSIE MENACED BY ELECTRIC JACKET
Several years ago, news outlets around the world all included some variety of this story in their "weird news" sections. The undisputed facts: Frank Clewer of Warrnambool, Australia, was wearing a nylon jacket. He entered the lobby of a local business. Something happened that ended up with the fire department on the scene.
Here is a selection of clips from various reputable newspapers:
"[He] accumulated an estimated 30,000 volts of static charge simply by walking around his home town in inadvisably large quantities of non-natural tailoring."
"A man left a trail of scorched carpet and melted plastic after static on his clothes built up to a 40,000 volt charge."
"A shaken Clewer said: "It sounded almost like a firecracker or something like that. It was at the reception area. Within say, around five minutes, the carpet started to erupt.""
"Fire official Henry Barton added: "He was one step shy of spontaneous combustion. His clothes would have self-ignited.""
"Scientist Karl Kruszelnicki later explained: "This poor guy has built up static electricity thanks to an unfortunate combination of insulating clothes that he's wearing, static, synthetic clothes, just walking along and he's just building up this static charge everywhere. I've read of it but I've never heard of it here in Australia.""
"The [fire authority] wisely impounded Clewer's jacket, which continues "to give off voltage.""
What do you think actually happened here? What could you explain using physics? What do you think is simply the newspapers' need for filler material?
RUBRICS
RUBRIC B: Ability to design and conduct an observational experiment
Scientific Ability Missing Inadequate Needs some
improvement Adequate
B5 Is able to describe what is observed without trying to explain, both in words and by means of a picture of the experimental set-up.
No description
is mentioned. A description is incomplete. No labeled picture is present. Or, observations are adjusted to fit expectations.
A description is complete, but mixed up with explanations or pattern.
Clearly describes what happens in the
experiments both verbally and by means of other
B9 Is able to devise an explanation for an observed pattern
No attempt is made to explain the observed pattern.
An explanation is vague, not testable, or contradicts the pattern. An explanation contradicts previous
knowledge or the reasoning is flawed.
A reasonable
explanation is made.
B10 Is able to identify the assumptions made in devising the
explanation
No attempt is made to identify any assumptions.
The assumptions are irrelevant or incorrect. Some significant assumptions are not mentioned. Most significant assumptions are correctly identified.
RUBRIC D: Ability to design and conduct an application experiment
Scientific Ability Missing Inadequate Needs some improvement Adequate
D2
Is able to design a reliable experiment that solves the problem The experiment does not solve the problem.
The experiment attempts to solve the problem but due to the nature of the design the data will not lead to a reliable solution.
The experiment attempts to solve the problem but due to the nature of the design there is a moderate chance the data will not lead to a reliable solution.
The experiment solves the problem and has a high likelihood of producing data that will lead to a reliable solution.
RUBRIC F: Ability to communicate scientific ideas
Scientific Ability Missing Inadequate Needs some improvement Adequate
F1
Is able to
communicate the details of an experimental procedure clearly and completely Diagrams are missing and/or experimental procedure is missing or extremely vague.
Diagrams are present but unclear and/or experimental procedure is present but important details are missing.
