Monthly Archives: April 2007

A response to the problem of two moving electrons


The following physics problem has been proposed on Momes (via  Perfectly Reasonable Deviations):

Two adjacent electrons move parallel to each other, neuropathist in the same direction in a vacuum. What happens? Consider the situation at varying speeds from 0 to c.

The most intuitive response is to say that they attract since it has been taught to us that two parallel wires carrying electric current in the same direction will attract each-other. The question is somewhat ill-posed until the frames of reference are defined. Do the electrons move at the same speed? What speed does the observer move? If we assume that the electrons are stationary with a fixed distance between then and the reference frame of the observer moves at some v that varies from 0 to c, we can write the Lorentz force equation to describe the force that one electron exerts on the other as observed in the moving reference frame. The thing to remember here is that the magnetic field is the Lorentz transform of the electric field, or rather that it is the result of a charge moving with respect to the observer. Since the Lorentz force equation, F=qE + v x B, has both an electric and magnetic component, it can be shown that the force is repulsive when the observer is stationary with respect to the electrons and becomes attractive after the speed (I know this should be velocity, but we are assuming that the direction is away from the electrons) becomes a certain v’ such that F = 0. This is different from parallel cables that attract for any measurable current in the same direction because the metal cables are electrically neutral for any macroscopic volume. That is, for every electron, there is a proton, so there is no net radial electric field and the total force component is due to the magnetic field only.

One final thought to keep in mind is that two electrons stationary in vacuum without external fields is nothing more than a mathematical construct. Like plane waves, this configuration would not be static in time without some external force keeping the electrons a fixed distance from each-other. The reason that I bring this up is that, although they are good exercises, one must not try to infer laws of nature from mathematical constructs.

[Image is of density measurements of an electron cloud from Physics Central.]

IC Friday: Analog Devices’ ADXL150


This IC Friday installment sets two technological records: first MEMS IC imaged and first time oil immersion was used to allow 100x magnification (versus a maximum of 20x with air). This chip is a single axis Micro Electro-Mechanical System accelerometer which has a set of finger in the middle of the die that sense acceleration by measuring capacitance between the fingers. It is hard to tell from the pictures, buy but the fingers are not attached to the substrate where they interleave, allowing them to move with applied acceleration and thereby change their inter-finger capacitance. Hopefully, I will image the ADXL311 in the upcoming weeks, a more modern dual axis accelerometer.

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( adxl150.pdf )

Another piece of hardware to hack: your iPod


I have been thinking about integrating my iPod into other systems for some time now, info
and only recently decided to look into how exactly that would work. I am not interested in altering the software/hardware of my device in any way, health
just to be able to interface it from some external source. The key here would be understanding how to use the 30 pin connector on the bottom of the device. The first piece of information comes from the great people at where the docking connector for the device is described. The important bits are that the iPod provides +3.3V power on this connector and that it has a serial communication interface. The same people have also outlined the Apple Accessory Protocol which describes the necessary data transfers to gain access to the functionality of the iPod through the bottom connector. Finally, there are also pins that bring out the left and right audio signals as well as take in power to charge the device. Now I just need to come up with an application to integrate iPod into. Maybe replace the cassette deck on an old boombox with an iPod dock?

( 2$ iPod dock connector from Spark Fun )

Exclusive: leaked images of the second generation Sony PSP production

A story recently hit the internet waves on Next Generation (ang Gizmodo) confirming the forthcoming update to the PlayStation Portable line from Sony:

Still, allergy
there will surely be some consumers upset that a sleeker, significantly more capable PSP may be arriving so soon. The original launched in North America on March 24, 2005 for $250.

While it is true that the release date has come and gone, there system is indeed in late stages of production. The key features that Sony will be focusing their efforts are increased memory, better wifi, a slightly higher resolution display, some faster processing and better media capabilities to further competition with portable music players. Through sources that request confidentiality, I was able to obtain these images directly from one of the prototype labs. I apologize in advance for the low resolution of the images as they were taken with a camera phone.

psp-wifi.jpg psp-svga.gif psp-rtc.jpg

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