User:John R. Brews/CZ psychology authors: Difference between revisions

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==CZ psychology authors==
{{TOC|right}}
*[[User:John_Calvin_Moore]]
{{Image|JFET.png|right|150px|JFET with n-type semiconductor body and p-type gate. Ohmic contacts are source (S), drain (D), and gate (G)}}
*[[User:Douglas_Griffith]]
*[[User:Michael J. Formica]]
*[[User:Patrick Malone]]
*[[User:Richard Pettitt]]
*[[User:Michael S. Everhart]]
http://www.charlierose.com/view/interview/11891


A '''junction field-effect transistor''' or JFET is a three-terminal device that conducts current, made of two [[semiconductor]] layers. For a discussion of dopant impurities and the terminology ''p-'' and ''n-''type. see [[Semiconductor#Dopant_impurities|dopant impurities]].


Before we conclude that clocks we perceive as moving actually run slower than clocks that appear stationary, so they count two events as taking less time if the clocks are moving, there is a wrinkle to examine. On the train, the light is sent and received at the same location, so the same clock records departure time and arrival time. On the track, though, the location of the flashlight changes with time, so two clocks are needed, one at the sending location and one at the receiving location. Can we be sure the two clocks are synchronized?
==Operation==
The figure shows a JFET with an n-type body and a p-type gate region. The dark-colored portion of the body is conducting, due to the electrons in this region, and is called the ''channel''. The light-colored portion of the body is depleted of electrons and is not electrically conducting.  


To synchronize the clocks on the ground, a light beam is sent from one location to the other. Because light travels at speed ''c'', it will arrive at the other location a time ''L/c'' later. Thus, we set the distant clock to time ''L/c'' when the light arrives. That is the method for synchronizing separated clocks.
The n-type body in the figure conducts electricity when a voltage drop is applied between source and drain. The amount of current depends, among other things, upon the cross-section of the device that is conducting. This cross section is controlled by the gate, which makes ohmic contact to a p-type region. If the gate-source [[pn diode|pn-junction]] is reverse biased by applying a negative voltage to the gate and holding the source at ground, the depletion region within the pn-junction widens, restricting the cross section of the conducting channel.


From the train however, these clocks are not synchronized. The distance ''L=vt'' used by the track observers is not the correct separation, which is a shorter distance ''vt' ''. Consequently, the distant clock is set to the wrong time of arrival for the synchronizing light, namely ''vt/c'' instead of ''vt'/c''=''vt √(1-''v''<sup>2</sup>/''c''<sup>2</sup>)/c''. Instead of ''L'', the track observers should have used ''L''√(1-''v''<sup>2</sup>/''c''<sup>2</sup>), the so-called ''Lorentz contraction'' of ''L''.
The insulating portion of the body varies in width from source to drain because the current through the body causes a voltage drop as suggested by [[Ohm's law]] that varies with distance and increases the amount of reverse bias between the gate and body as a function of distance down the channel.
 
This example can be converted to a comparison of clocks. Suppose that a unit of time is that for the light to go from the flashlight to the mirror and back. The flashlight and mirror become a clock. The same clock is used on the train and on the track. Both the clock on the track and that on the train count one unit of time as ''t = 2h/c''. But to those on the track, the traveling clock on the train counts one unit of time as the longer time ''t' ''= 2√(''h''<sup>2</sup>+(''L''/2)<sup>2</sup>)/''c''. Substituting ''L''=''vt'' and ''h''=''ct''/2, we find:
:<math>t^\prime = \frac{2\sqrt{(ct)^2/4+(vt)^2/4}}{c} =t \sqrt{1+v^2/c^2} \ . </math>

Revision as of 12:20, 26 March 2012

(PD) Image: John R. Brews
JFET with n-type semiconductor body and p-type gate. Ohmic contacts are source (S), drain (D), and gate (G)

A junction field-effect transistor or JFET is a three-terminal device that conducts current, made of two semiconductor layers. For a discussion of dopant impurities and the terminology p- and n-type. see dopant impurities.

Operation

The figure shows a JFET with an n-type body and a p-type gate region. The dark-colored portion of the body is conducting, due to the electrons in this region, and is called the channel. The light-colored portion of the body is depleted of electrons and is not electrically conducting.

The n-type body in the figure conducts electricity when a voltage drop is applied between source and drain. The amount of current depends, among other things, upon the cross-section of the device that is conducting. This cross section is controlled by the gate, which makes ohmic contact to a p-type region. If the gate-source pn-junction is reverse biased by applying a negative voltage to the gate and holding the source at ground, the depletion region within the pn-junction widens, restricting the cross section of the conducting channel.

The insulating portion of the body varies in width from source to drain because the current through the body causes a voltage drop as suggested by Ohm's law that varies with distance and increases the amount of reverse bias between the gate and body as a function of distance down the channel.

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