To which extent cathode rays can be considered as current in a wire

In summary, the conversation discusses the presence of a magnetic field around a cathode ray tube and its potential use for producing Lorentz force on a moving charge particle. The experts conclude that while a cathode ray can produce magnetic fields, it can easily be reshaped by external fields. They also mention that the magnetic force and electric force are already given by Maxwell's equations, and a cathode ray is strongly charged while a current in a wire is electrically neutral.
  • #1
universal_101
325
3
I want to know if there is magnetic field around a cathode ray tube. That is, does magnetic compass deflect when brought close to a CRT ? If so, can this magnetic field (produced by cathode rays) be used to have Lorentz force on a moving charge particle ?

Presently, what I think is,

Maxwell's equation, [itex]c^{2}\nabla \times B = \frac{j}{\epsilon_{o}} + \frac{\partial E}{\partial t}[/itex]

Demands the need of Magnetic field for cathode rays, and the Lorentz force then should put some force on the nearby moving charge. But, due to relativity this magnetic force is balanced by extra electric force due length contraction.

Is that so ?
 
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  • #2
A cathode ray is a current, but it isn't a current in a wire. The current can do all the things you would expect of a current in EM, including producing magnetic fields.

The only thing is that if there are any external fields it will pretty easily re-shape the current, that is the big difference from a current in a wire which tends to stay in the shape of the wire.
 
  • #3
Note that devices such as an Intergrated Charge Transformer are often used in accelerators to quantify the amount of charge moving in an accelerator beampipe. This is nothing more than a coil of wire to pick up the magnetic field from the moving charges.

So the properties are practically similar.

Zz.
 
  • #4
The first point has already been answered. The remaining point:
universal_101 said:
[..] due to relativity this magnetic force is balanced by extra electric force due length contraction.
Is that so ?
I don't think so: Maxwell's description is supposed to be valid in the "stationary" system to which you refer. "length contraction" suggests the use of another inertial system that is in motion relative to your "stationary" system, but to which you not refer.
 
  • #5
harrylin said:
The first point has already been answered. The remaining point:

I don't think so: Maxwell's description is supposed to be valid in the "stationary" system to which you refer. "length contraction" suggests the use of another inertial system that is in motion relative to your "stationary" system, but to which you not refer.

The frame in which length contracts is the one in which there should be magnetic force due to motion of the charge particles of cathode rays.
 
  • #6
ZapperZ said:
Note that devices such as an Intergrated Charge Transformer are often used in accelerators to quantify the amount of charge moving in an accelerator beampipe. This is nothing more than a coil of wire to pick up the magnetic field from the moving charges.

So the properties are practically similar.

Zz.

Thanks, and for using the great example.
 
  • #7
universal_101 said:
The frame in which length contracts is the one in which there should be magnetic force due to motion of the charge particles of cathode rays.
I would not know what length would contract: in your description I discern no object of fixed length on which that concept can be applied.
And independent of such considerations, the magnetic force and electric force are already given by the equations of Maxwell.

PS. concerning the question in the title: an important difference is that a cathode ray is strongly charged while a current in a wire is electrically neutral.
 
Last edited:

1. What are cathode rays?

Cathode rays are streams of electrons that are emitted from the cathode (negatively charged electrode) in a vacuum tube.

2. How are cathode rays related to electric current?

Cathode rays can be considered as a form of electric current, as they are made up of charged particles (electrons) that are moving through a conductor.

3. Can cathode rays be measured as current in a wire?

Yes, cathode rays can be measured as current in a wire. However, their movement is not as consistent or predictable as traditional electric current, as they can be affected by external factors such as magnetic fields.

4. What is the extent to which cathode rays can be considered as current in a wire?

This is still a topic of debate and ongoing research. While cathode rays do exhibit some properties of current in a wire, they also have unique characteristics that make them distinct from traditional electric current.

5. How are cathode rays used in modern technology?

Cathode rays, or more specifically their modern counterpart, electron beams, are used in a variety of technologies such as cathode ray tube (CRT) televisions, computer monitors, and medical imaging devices. They are also crucial in the development of electronic devices and semiconductor technology.

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