Department of Physics and Astronomy, Stony Brook University
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PHY in the Studio Version
Alternate Lab
TABLE OF CONTENTS
Introduction
Equipment
Background
Procedure
Analysis
Questions
References and Tools
Introduction

In this section you will find a very brief statement of what the lab is about and the way you will collect and analyze the data.

More detail will be found in subsequent sections. There will be footnotes. Hoveroverthese!

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Equipment
  • 1 Object
  • 2 Pieces of equipment
  • 1 Platform
  • 2 Other things
  • 1 Pile of stuff
  • 1 Meter Stick
  • Record data in a Google Sheets data table
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Background

Valuable Material

This lab manual will have necessary physics background for the experiment presented here. Here is a footnote.1

Field Lines & Equipotentials

While an explicit calculation of electric field is excellent, we are often more concerned about getting a picture of electric field than getting an exact description. A good tool to get this qualitative picture is electric field lines.

Physics Behind Electric Fields

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Procedure

The lab procedure will be here.

Setup

Measuring Voltages

Measuring Equipotentials

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Analysis

Analysis of Part I

Here will be guidance on how to analyzed the data you collected..

Analysis of Part Deux

More guidance! SO very much guidance.

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Discussion Topics

Your TA will ask you to discuss some of the following topics (they will tell you which ones):

  • Parallel Plate Linearity: Your plot of voltage vs. position for the parallel plates should have come out looking reasonably linear. What does this tell us, physically, about the electric field?
  • Dipole Field Strength: Based on the ways we qualitatively understand "electric field strength" using equipotentials and field lines, circle the place on your dipole configuration where the electric field is the strongest.2 (It may be handy to do this in a different color than your field lines/equipotentials, to prevent your sketch from getting too jumbled.)
  • Systemic Errors: What errors could have impacted the qualitative behavior (i.e., the shape) of our electric field lines, compared to the ideal field lines for a dipole/plates? (Hint: what makes electric fields? How could such things have been present in the ambient environment?) Note: we're looking for things that could have impacted our actual, physical field lines here, not just altered our measurement of them.
  • Alternative Configurations:
    • Suppose you swapped positive and negative charges in the dipole arrangement. What would happen to the electric field lines (both shape and direction)? Would they be the same or different?
    • What if you made both charges positive, or both negative? Would the electric field line configuration be the same or different (and if different, how so)?
  • Deviations from Expected Behavior:
    • Conductors:We expect an ideal conductor to be a constant voltage throughout. The conductive surface (silver marks) is mostly a constant voltage, but sometimes you may observe a millivolts-ish difference between different points on it. Explain, if you can.
    • Third Dimension: We call the first configuration a "parallel plates" arrangement, but technically, they're not "plates," because they don't extend vertically, in the third dimension. But if it's not parallel plates of charge, we don't expect the field lines to all be parallel, which is what we observe. What is going on here?3
References and Tools

Guide to Uncertainty Propagation & Error Analysis (Quick Reference)

Spare Parallel Plates Sketch Paper

Spare Dipole Sketch Paper

Hovering over these bubbles will make a footnote pop up. Gray footnotes are citations and links to outside references.

Blue footnotes are discussions of general physics material that would break up the flow of explanation to include directly. These can be important subtleties, advanced material, historical asides, hints for questions, etc.

Yellow footnotes are details about experimental procedure or analysis. These can be reminders about how to use equipment, explanations of how to get good results, troubleshooting tips, or clarifications on details of frequent confusion.

For a review of electric field, see Katz, Chapter 24 or Ginacoli, Chapter 21.

Really, this is where the voltage flips from positive to negative - you aren't going to be able to get an actual reading of zero (our hands aren't as precise as our voltmeter is, from this perspective). Moreover, the imprecision in drawing from copying the points from your carbon sheet to the printout is far higher than this imprecision if you take even a modicum of care here.

On the equipotentials, you have to be a little careful: "spacing of equipotentials" is only a good measurement of electric field strength if they're equally spaced in voltage, which ours are not (they're equally spread in "position-down-the-center" rather than voltage).

To be completely accurate: this is the average value of that component of electric field on the line between \(\vec{x}_1\) and \(\vec{x}_2\)

Technically, the voltmeter doesn't directly measure the voltage. Instead, it measures the current flowing through a very high-resistance resistor in the voltmeter. It then uses the amount of current that flows to determine the voltage. This is why it doesn't read anything when you don't have it touching the paper, despite there (theoretically) being voltage there: there's no current, since that requires a source of charges. When you touch the carbon paper, the voltmeter gets a (very small) part of the (already small) current that is flowing through the carbon paper.

Actually, this issue of "not being parallel plates" can get pretty complicated if you go deep enough down this rabbit hole. But there's a relatively simple solution that solves the immediate concern that this question asks. (Hint: read the title of this sub-point again; it kind of has a double-meaning.)