Lab 1: Electroscope and electrophorus
Learn how an electroscope works.
Determine whether the human body is a conductor or an insulator.
Learn to analyze videotaped phenomena.
In this lab you will be working first with videotaped experiments. After you finish analyzing a videotaped experiment, you need to repeat it with the equipment on the instructor’s desk. The purpose of repeating the experiment is to give you an opportunity to work with real electroscopes. However, electroscope experiments are weather dependent, thus we videotaped them for the consistency of results. Do not be discouraged if the experiments with real electroscopes do not work perfectly. For the video experiments access the folder LAB2 on your computer and proceed working with the experiments in this folder.
Available equipment: rods, fur, electrophorus, foil bits.
Open this file.
Observe the first part of the experiment (with a clear rod), record your observation. Make sure that you record what happens when the rod approaches and when it moves away from the plate.
Explain the observation using your knowledge of electric interactions and the internal structure of conductors. Draw a picture to clarify your reasoning, decide how you will represent electric charges inside the conductor.
Observe the second part of the experiment (with a foam pipe) and record your observations.
Explain—use a picture to clarify.
Explain why in both experiments the leaf of the electroscope goes back to the original position.
What do you think will happen if each rod has a larger charge? Smaller charge?
Open this file.
Observe the experiment. Draw a picture representing the charges on the pipe and on the electroscope before and after Alan touches it with the rod.
Explain why the leaf of the electroscope deflects after it was touched with the charged rod.
Explain why the leaf goes back to the vertical position after Alan touched the electroscope.
What do you need to assume about the structure of the human body in order to explain the observations?
Briefly describe how you can test these assumptions.
Open this file. In the video there are 5 experiments. Pause the video after you view each one and answer two questions corresponding to this experiment. Make sure that you understand the difference between the "description" and the "explanation".
Describe what happens to the leaf of the indicator mechanism after the plate has been touched with a negatively charged rod. Explain why it happens.
Describe what happens to the leaf of the indicator when the negatively charged rod is brought close (without touching) to the indicator.Explain why it happens.
Describe what happens to the leaf of the indicator when the negatively charged rod is brought close (without touching) to the plate. Explain why it happens.
Describe what happens to the indicator when a positively charged rod is brought close (without touching) to the plate. Explain why it happens.
Describe what happens to the indicator when a positively charged rod is brought close (without touching) to leaf of the the indicator.Explain why it happens.
IV. Testing experiment: Charging by induction
Open this file. Do not run the video yet. In the video you will first observe Sahana bringing a positively charged rod close to the plate of the electroscope (without touching). With her other hand she will touch the electroscope on the side opposite to the side where the rod is, she then releases her hand, and finally she moves the rod away.
Use your knowledge of electric charge, conductors and electroscopes to predict what will happen to the electroscope when Sahana brings a positively charged rod near it without touching it. Record your prediction and explain it.
Predict what will happen if Sahana touches the top of the electroscope with her hand on the side opposite to the side where the rod is while holding the rod near by. Record your prediction and explain it.
Predict what charge (positive or negative) will be on the plate after Sahana's hand and the rod have been removed. Record your prediction and explain it.
Now watch the video in the file *** and record the observations. Reconcile all of the discrepancies with your preditions. Can you say now whether the charge on the electroscope was positive (the same as the charge of the rod) or negative (opposite to the charge of the rod)?
Repeat the experiment with real materials. Were you able to charge the electroscope without touching it with the rod?
Explain how one can charge an electroscope without touching it with a charged object.
In this experiment you will apply your knowledge of electric charge and conductors to explain new phenomena.
Available equipment: A foil bit (crumpled aluminum foil) attached to an insulating string; a copper sphere with insulating stand (or brass mass on a base), a plastic rod, rubbing material.
Hang the foil bit from the wood dowel. Rub the rod as usual. Predict what will happen if you move the rod very slowly towards to the bit (but do not touch it). Explain your prediction. Use a sketch to show the charge distribution.
Now perform the experiment and record what you observe. Did the outcome match the prediction? If not, reconcile your reasoning with the experiment. What is the net charge on the bit as the rod approaches it (but does not touch)?
Now take the copper sphere with insulating stand and touch it lightly with your finger to take away any net charge close to the foil bit. Hold it only by its base and avoid touching the plastic stand so that it remains an effective conductor. Bring it close to the foil bit (even closer than in the photo above, but do not let them touch).
What do you observe?
Explain whether your observation is consistent with zero net charge on both the copper sphere and the foil bit?
Predict what will happen if you, keeping the foil bit and copper sphere where they are, again charge the rod by rubbing and bring it close to the copper sphere, opposite the side of the foil bit. Explain your prediction.
Now perform the experiment and note the outcome. Reconcile the outcome with your prediction.
Maintaining the rod close to the sphere, slowly bring your finger towards the foil bit and gently make contact with it. Note the outcome. Explain.
Now explain the whole sequence of events in f)-h) in terms of charge sign and transfer.
Finally, while keeping your finger where it is, move the charged rod away from the sphere.
Record the outcome. How do you account for the observed motion, even though the rod is far away from the sphere or foil bit?
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 whether a human body is a conductor or an insulator is important.
Sometimes we cannot design and perform our own experiment to test an idea and need to use an experiment that was already perfromed by somebody else. Why is it still important to make a prediction based on the idea under test before we acquire data from that experiment?
This fall, 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 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 could you explain using the newspapers' need for filler material?