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Showing posts from October, 2008

Amateur Radio License Exam Practice Applications

The amateur radio license exam practice applications on this site offer two features. First, they provide practice exams randomly constructed from the actual examination question pools. Second, as each exam question is displayed, study materials related to the question are displayed as well.

The practice exams can be found at:
Technician Class Exam
General Class Exam
Extra Class Exam

Exam Practice Instructions

The exam practice screen is shown above. A question is presented along with four multiple choice answers. A correct answer will be highlighted in green. If the wrong answer is selected, it will be highlighted in red. Additional answers can be selected to determine the correct answer, but only the first selected answer will count in the test score. If exam question requires a figure, it will be shown below the multiple choice answers.

Study Material Instructions
As each question is displayed, related study material may be displayed in the 'Available Study Material section of t…

A Query Tester for the Google Visualization API

One of the first things I always find myself doing when writing an application using the Google Visualization API Query Language is writing a simple application framework to test out my data models and queries. Rather than re-inventing the wheel on every project, I decided to just implement a simple tester here where, hopefully, everyone else can benefit from it as well.

To use it, simply place the datasource URL you intend to use as well as the query string in the appropriate blanks below. Then click, 'Test Query' to see the results of the query. For example, you can try the datasource URL and a query string from the Hierarchical Chooser post.

datasource URL:
Query String: Select A, B, C
Datasource URLQuery String

A Hierarchical Chooser Using Google's Visualization API

Writing about physics and specifically electromagnetism got me to thinking about amateur radio. In the United States, any person can receive an amateur radio license that allows them to transmit and receive on a number of frequencies allocated by the FCC. Amateur radio is a great hobby. People involved in the hobby, (called amateur radio operators), get a chance to turn their EM knowledge into concrete experience by building usable radio transceivers and antenna structures.

To get a license, operators must first pass a written test on radio operations fundamentals, engineering, and FCC regulations that apply to amateur radio. The question pools used for the exam are available online as are practice tests. I thought it would be fun to build an application that would construct practice tests on this blog. The final application will provide practice tests similar to the ones at but, with a bit more interactivity. This series of articles will cover the construction of the ap…

Electric Fields, Charge Distributions, and the GRE

Reviewing for the GRE physics subject test, I've frequently come across questions like:

Given a distribution of discrete charges, (as shown in the diagram), determine the electric field at a given point r, (also shown in the diagram).

Formula for the Electric Field from a Distribution of Charges
The distribution of charges I've seen in sample questions have, so far, involved only finite numbers of point charges, so the electric field equation can be written as a sum:


is the distance from a charge contributing to the sum to the location where the electric field is being measured, q is the value of the particular charge, and u is the unit vector pointing from the contributing charge to the field measurement point.

This can be separated into x and y components:

where theta is the angle between the particular charge and the point where the field is being measured with respect to the x axis. Using these formulas, it's just a matter of summing up the contribution of each point c…

Fourth of July Trailhead on the Fourteenth of October

Snow is already coming down all over Colorado! In fact, they opened a few of the ski runs today! I traveled up to Diamond Lake via the Fourth of July trailhead yesterday where there's just a little bit of snow in the shady spots. The hike is beautiful, it crosses two very nice waterfalls and Boulder Creek on the way up. I ran into three other groups making the trek on a Tuesday afternoon. For more information on the trail and the Forest Service regulations for the area check:

Diamond Lake [pdf]

Back down the valley toward Eldora.

Middle Boulder Creek

Campsites available!

Diamond Lake
Trail: Diamond Lake Trail #975 from Fourth of July Trailhead
Forest Service description and regulations [pdf]
Roundtrip distance on trail: 5.2 miles
Transit time from Boulder to trailhead: 55 minutes
Hike time to lake: 2 hours
Hike time back: 1 hour


View Larger Map Widgets

It's Obvious... Not: Charged Particle Motion in a Magnetic Field and Third Order Homgeneous Differential Equations

This installment of “It’s Obvious. Not!” looks at: Book: “Classical Dynamics of Particles and Systems”Edition: thirdAuthors: Jerry B. Marion and Stephen T. ThorntonPublisher: Harcourt Brace JovanovichPage: 68This post looks at Example 2.10 that investigates the motion of a charged particle in a magnetic field.The example is fairly straightforward with one exception.When determining the equations of motion, the authors propose a solution for the system of differential equations discussed below and reference example C.2 of Appendix C.It’s not immediately apparent how to use Example C.2 to arrive at the authors’ solution, so the steps are outlined in detail here.If you have questions, or suggestions, all comments are always welcome!The original system of coupled differential equations is:First, the authors’ differentiate both equations and then substitute the results into the other:at this point, the book suggests using the technique of example C.2, (finding the roots of the auxiliary …