Electron kinetic energy, movement

[UltrafastPED]

It's simply a method (Kelvin connection) to eliminate the effect of sensor wiring/contact resistance (causing a voltage drop from the voltage source to the material at the probing points) changing the probed materials voltage reading with very low resistance measurements that need significance current levels to generate a measurable and stable voltage potential.

I think you mean sheet resistance instead of 'Transresistance' as sheet resistance is what we normally use when measuring doping/implant levels in semiconductor wafers with very low resistance levels.

http://en.wikipedia.org/wiki/Sheet_resistance

I agree with everything you say here; I have even used this technique for measuring the sheet resistance of ultrathin films.

I only posted the link because of the picture: it shows two contacts for measuring current, and two for measuring voltage ... thus four contacts. Our other correspondent was objecting to the requirement for four contacts to obtain an V/I curve as mentioned in the reference for transresistance.

 

[sophiecentaur]

You can certainly do what you suggest ... anything in the box will have some V/I curve.

But resistivity is a material property: http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity


And you keep asking where I learned this. Some as an undergraduate, and more in my graduate studies. You are welcome to look at my public profile; it lists my general qualifications and background.

So you are telling me that you learned that measuring a two terminal device, turns it into a four terminal device? That is bizarre. Does any two terminal device actually exist in your approach?

 

[nsaspook]

Your classic 'Fluke type' two terminal meter uses the constant current method to measure resistance.

The four terminal method can be used when lead resistance values become significant because of higher sense currents for better accuracy measurements of the DUT.

Transresistance is a term normally used with a 4 port device to describe a transfer function (a change in output voltage to a change in input current) in a current source to voltage source device (amplifier).

 

[UltrafastPED]

So you are telling me that you learned that measuring a two terminal device, turns it into a four terminal device? That is bizarre. Does any two terminal device actually exist in your approach?

I'm not clear what you are saying here. How would making a measurement change the nature of a device?

But if I were to use a voltmeter to measure the voltage drop across a resistor - then I need to make two connections, one on each end. If I were to use an ammeter to measure the current flowing through the resistor I would also need to make a connection at each end.

The connections can be shared if you don't require great precision; to get more accurate readings use the Kelvin technique.

But in either case you have four physical connections. The consolidation of wires into nodes implicitly assumes no resistance in the wires.

Is this your point? Nodes on a diagram are logical; connections are physical. The map is not the territory, but it is good enough for an initial analysis.

 

[Salvador]

But if I were to use a voltmeter to measure the voltage drop across a resistor - then I need to make two connections, one on each end. If I were to use an ammeter to measure the current flowing through the resistor I would also need to make a connection at each end.

But you need a series connection for an ammeter not a connection at each end?

I appreciate the discussion but I still don't know why there is resistance for current in a medium made up entirely out of electrons like in a glass tube filled with electron gas?
In a vacuum at first there is infinite resistance right? and when the E field is strong enough a current path out of cathode electrons can be formed and resistance drops to whatever resistance the electron beam has but why it then has a resistance at all ? if there is vacuum and nothing in the way for the electrons? Not to account for the resistance of the hot cathode , just the electron beam itself.

 

[sophiecentaur]

I'm not clear what you are saying here. How would making a measurement change the nature of a device?

But if I were to use a voltmeter to measure the voltage drop across a resistor - then I need to make two connections, one on each end. If I were to use an ammeter to measure the current flowing through the resistor I would also need to make a connection at each end.

The connections can be shared if you don't require great precision; to get more accurate readings use the Kelvin technique.

But in either case you have four physical connections. The consolidation of wires into nodes implicitly assumes no resistance in the wires.

Is this your point? Nodes on a diagram are logical; connections are physical. The map is not the territory, but it is good enough for an initial analysis.

I learned the basics of Resistance measurement at School; nothing has changed about that, since, afaik. NAspook's two diagrams say all that's needed.

My "point" is that you introduced this "transresistance" as if it somehow distinguishes a 'resistor' resistor (Ohmic?) from any other conducting device (the electrons in a vacuum tube, for instance). For some reason, you introduced the 'four terminal' measurement to justify the 'transresistor' term to justify it. But the term is a complete red herring and refers to a four terminal device. It would surely have helped if you had just admitted that the term is not relevant to this thread.
My initial point was that Ohm's Law is not synonymous with the expression R = V/I and that people who know better should really be using the right terminology, in the interests of those who are trying to find out about it.

 

[sophiecentaur]

But you need a series connection for an ammeter not a connection at each end?

I appreciate the discussion but I still don't know why there is resistance for current in a medium made up entirely out of electrons like in a glass tube filled with electron gas?
In a vacuum at first there is infinite resistance right? and when the E field is strong enough a current path out of cathode electrons can be formed and resistance drops to whatever resistance the electron beam has but why it then has a resistance at all ? if there is vacuum and nothing in the way for the electrons? Not to account for the resistance of the hot cathode , just the electron beam itself.

If there is any finite Potential Drop across a device and there is finite current flowing then there must be a value for V/I (i.e what we call Resistance). There is, arguably, a problem in measuring what the Potential Drop is, between different points over the length of the beam but, if the field is linear, the PD would be proportional to the distance along the beam (potential divider equation).
I don't think there is a problem with the fact that introducing electrons into the vacuum would alter the measured resistance. A light sensitive resistor changes resistance when light falling on the LDR alters the number of free electrons. In the case of an electron tube of course, the V/I relationship will be highly non-linear but that doesn't alter the principle.

 

[Salvador]

ok, but apart from low temperature =low kinetic energy of electrons , what else makes a superconductors electrons so resistanceless?

 

[Drakkith]

ok, but apart from low temperature =low kinetic energy of electrons , what else makes a superconductors electrons so resistanceless?

See the following articles:

http://en.wikipedia.org/wiki/Cooper_pair
http://en.wikipedia.org/wiki/Bcs_theory

 

[UltrafastPED]

My initial point was that Ohm's Law is not synonymous with the expression R = V/I and that people who know better should really be using the right terminology, in the interests of those who are trying to find out about it.

So you disagree with this lesson from "All About Circuits"?
http://www.allaboutcircuits.com/vol_1/chpt_2/6.html

"Ohm's Law is not very useful for analyzing the behavior of components like these where resistance varies with voltage and current. Some have even suggested that "Ohm's Law" should be demoted from the status of a "Law" because it is not universal. It might be more accurate to call the equation (R=E/I) a definition of resistance, befitting of a certain class of materials under a narrow range of conditions."And for just for fun, read this long discussion on the same topic; all of the arguments are hashed over multiple times.

Conclusion: if you define resistance as whatever the V/I curve says for that (V,I) then you have an "Ohm's law" that is a tautology.

However if we consider only the linear behavior (regions of V/I curves of constant slope), then Ohm's law describes the behavior well within this region of applicability - it is an empirical law for which experimental results will be consistent with the predictions.

 

[Salvador]

If an electron beam in a glass tube much like in CRT would be put into extremely low temperature , would it then too become superconducting like various metals when under low temperatures?

Wikipedia states that...

In a normal conductor, an electric current may be visualized as a fluid of electrons moving across a heavy ionic lattice. The electrons are constantly colliding with the ions in the lattice, and during each collision some of the energy carried by the current is absorbed by the lattice and converted into heat, which is essentially the vibrational kinetic energy of the lattice ions. As a result, the energy carried by the current is constantly being dissipated. This is the phenomenon of electrical resistance.

in a vacuum once a current path has formed , the electrons flow but they have nothing in their way unlike in a metal conductor so what is the reason for them to experience resistance still?

 

[Drakkith]

If an electron beam in a glass tube much like in CRT would be put into extremely low temperature , would it then too become superconducting like various metals when under low temperatures?

The term "superconducting" doesn't apply to free particles traveling through space/vacuum. It specifically applies to the lack of electrical resistance in conductors. (A vacuum is not a conductor, it is actually an insulator)

 

[nsaspook]

Wikipedia states that...


in a vacuum once a current path has formed , the electrons flow but they have nothing in their way unlike in a metal conductor so what is the reason for them to experience resistance still?

The electrons in a vacuum can accelerate (in a specific direction instead of very fast random velocities with slow drift speeds like in conductors) to near light speed easily. (~10% of c in a small o-scope CRT with 2kv acceleration voltage) Electrons have a rest mass so it takes energy (work) to make them move faster. So we can think of electrical resistance in part as a indicator how much work is being used to accelerate them (along with the beam EM fields themselves ) to those speeds and the kinetic energy in each electron at that speed (2keV for the o-scope)
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ev.html

So we have a static beam with kinetic energy filled electrons moving at high speed, what do you think happens when we start switching that beam on/off, changing it's density and moving it around to display information? We then also have energy (a small amount usually) in EM waves that can radiate out and be detected from considerable distances.
EMR

 

[sophiecentaur]

So you disagree with this lesson from "All About Circuits"?
http://www.allaboutcircuits.com/vol_1/chpt_2/6.html

"Ohm's Law is not very useful for analyzing the behavior of components like these where resistance varies with voltage and current. Some have even suggested that "Ohm's Law" should be demoted from the status of a "Law" because it is not universal. It might be more accurate to call the equation (R=E/I) a definition of resistance, befitting of a certain class of materials under a narrow range of conditions."And for just for fun, read this long discussion on the same topic; all of the arguments are hashed over multiple times.

Conclusion: if you define resistance as whatever the V/I curve says for that (V,I) then you have an "Ohm's law" that is a tautology.

However if we consider only the linear behavior (regions of V/I curves of constant slope), then Ohm's law describes the behavior well within this region of applicability - it is an empirical law for which experimental results will be consistent with the predictions.

I would disagree with that lesson because it fails to include Temperature in its implied definition of Ohm's Law. To my mind that is sloppy. I agree that it is not a "Law" but a description of behaviour (I already wrote as much, earlier on).

You are not responding to my comments about that transresistance thing you introduced. Do you still think you are 'right' about it? I can't see how you can introduce four terminal devices in your view of resistance. The measurement method is not relevant here because you could produce a resistance of, say 1kΩ, from an appropriately shaped piece of conductor, without ever needing to connect it up to your four terminals. R doesn't care how it's measured.