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Re: [Phys-l] Which one is positive?

What follows is trivial and idealized (not practical).

1) I have a charged object; its net charge is positive by definition. Where does it come from? From an experiment in which a glass rod is rubbed with silk. By an old definition, the charge that appears on glass is positive while the charge that appears on silk is negative.

2) Now I am able to distinguish positive charges from negative charges. A charge is positive if it is attracted toward silk; it is negative is it is attracted toward the glass.

3) How to decide which terminal of the dc power supply is positive. First use the power supply to charge a capacitor. Then disconnect the capacitor and separate its plates from each other. Determine which plate is positive. The terminal to which the positive plate was connected is positive. This is possible but not practical.

4) A definition of positive based on rubbing glass with silk can be challenged; there are many kinds of glass and many kinds of silk. For that reason the definition can be changed; we can say that all protons are positive. (We have no doubt that protons would be attracted by silk and repelled by glass.)

5) Accepting the idea of conventional current (opposite to the flow of electrons, real or imagined) we are ready to talk about the anode and the cathode. Suppose we accept the definition suggested earlier by J.D. "The anode of a device is the terminal where current flows in from outside. The cathode of a device is the terminal where current flows out." Let me show that this is consistent with common usage.

Consider a typical X-rays tube. The electrode that is heated (provider of electrons) is called cathode. The other electrode, toward which electrons are accelerated, is called anode. Outside the device (in wires forming a closed circuit) electrons flow away from what is commonly called anode, and they flow toward what is commonly called cathode. Using the idea of conventional current we say that outside the device positive (fictitious) charges flow toward the anode and away from the cathode. This does coincide with John's definition, except I am using the word "toward" rather that the word "where."

Ludwik
= = = = = =




On Jun 17, 2010, at 12:19 PM, Pete Lohstreter wrote:

Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:

There are other problems, including abuse of the terms
"anode" and "cathode".
http://www.av8n.com/physics/anode-cathode.htm

The anode of a device is the terminal where current flows in from outside.
The cathode of a device is the terminal where current flows out. This is
illustrated in [
http://www.av8n.com/physics/anode-cathode.htm#fig-def-anode-cathode
]figure 1. (shows positive charge leaving the cathode)

How does this set with 'cathode rays'? Aren't cathode rays negative? If
cathode rays come from the cathode, something's backwards.

Cathode rays (also called an electron beam or e-beam) are streams of [
http://en.wikipedia.org/wiki/Electron ]electrons observed in [
http://en.wikipedia.org/wiki/Vacuum_tube ]vacuum tubes, i.e. [
http://en.wikipedia.org/wiki/Vacuum ]evacuated glass tubes that are
equipped with at least two metal [ http://en.wikipedia.org/wiki/Electrode
]electrodes to which a [ http://en.wikipedia.org/wiki/ Voltage ]voltage is
applied, a [ http://en.wikipedia.org/wiki/Cathode ]cathode or negative
electrode and an [ http://en.wikipedia.org/wiki/Anode ]anode or positive
electrode. Electrons were first discovered as the constituents of cathode
rays. In 1897 British physicist [
http://en.wikipedia.org/wiki/J._J._Thomson ]J. J. Thomson showed the rays
were composed of a previously unknown negatively charged particle, which
was later named the electron. [
http://en.wikipedia.org/wiki/Cathode_ray_tube ]Cathode ray tubes (CRTs)
create the image in a classic [ http://en.wikipedia.org/wiki/ Television
]television set.

I'm really confused now. How can positive charges leave the negative
electrode? Shouldn't they attract?


From the keen-grasp-of-the-obvious department: If you
want to know which is positive, use a voltmeter.

If there is some heretofore-unmentioned constraint that
prevents you from using a voltmeter, you need to ask a
different question, being careful to say what the
constraints are.


Pete Lohstreter "Happy is he who gets to know
The Hockaday School the reasons for things. "
11600 Welch Rd Virgil (70-19 BCE) Roman poet.
Dallas, TX 75229

214-360-6389

plohstreter@mail.hockaday.org

See what our students are doing......
http://home.hockaday.org/physics/index.html


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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Ludwik

http://csam.montclair.edu/~kowalski/life/intro.html