Electric field at a conducting surface?

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Discussion Overview

The discussion centers around the behavior of electric fields at conducting surfaces, particularly in the context of the Rayleigh-Jeans calculation and the implications of boundary conditions imposed by metallic walls. Participants explore the relevance of these concepts to electromagnetic wave propagation in cavities.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • Some participants assert that a metallic wall cannot support an electric field parallel to its surface, as charges can flow to neutralize the field.
  • Others express confusion regarding the relevance of this statement to the Rayleigh-Jeans calculation and seek clarification on specific aspects of the problem.
  • One participant describes a metallic wall as a short circuit, stating it imposes a boundary condition that the electric field must be zero along the wall.
  • There are questions about the meaning of terms such as "short circuit" and how they relate to voltage and electric fields in circuits.
  • Some participants suggest that a foundational understanding of electricity and magnetism is necessary to grasp the concepts involved in the Rayleigh-Jeans calculation.
  • Recommendations for studying basic electricity concepts, including DC and AC circuits, electromagnetic waves, and resonant cavities, are provided by participants.
  • One participant mentions learning from a specific textbook (Purcell) and encourages others to explore the "Science and math textbooks" section of PhysicsForums for additional resources.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the implications of electric fields at conducting surfaces, with some agreeing on the fundamental principles while others remain confused about their application in the context of the Rayleigh-Jeans calculation. No consensus is reached on the clarity of the original statement or its relevance.

Contextual Notes

Some participants indicate that a deeper understanding of basic electricity concepts is necessary to engage with the advanced topics discussed, highlighting potential gaps in knowledge that may affect comprehension of the Rayleigh-Jeans calculation.

Who May Find This Useful

This discussion may be useful for individuals studying electricity and magnetism, particularly those interested in the behavior of electric fields in conducting materials and their implications for electromagnetic wave propagation.

bubblewrap
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"A metallic wall cannot, however, support an electric field parallel to the surface, since charges can always flow in such a way as to neutralize the electric field."

This is from a textbook and I'm not quite sure what this means.

The context is the process of calculating Rayleigh and Jeans calculation
 
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The statement is fine, but I am struggling to find the relevance of this is the calculation. Which part are you having an issue with?
 
I hope this helps:
A metallic wall is a short circuit. It does not permit a voltage (electric field) to exist along it. It therefore imposes a boundary condition which says that, along the wall, the electric field must be zero.
 
Jilang said:
The statement is fine, but I am struggling to find the relevance of this is the calculation. Which part are you having an issue with?
This was in part with the calculation of Rayleigh and Jeans. It was trying to solve for a simplified version with a cube cavity, and three separate axis perpendicular to each other with this case being the x axis. It was talking about the propagation direction being perpendicular to the electrical field vector and the propagation direction again being perpendicular to the metallic wall. And frankly I'm not quite sure what any of these mean
 
Alfred Cann said:
I hope this helps:
A metallic wall is a short circuit. It does not permit a voltage (electric field) to exist along it. It therefore imposes a boundary condition which says that, along the wall, the electric field must be zero.
What does 'short circuit does not permit voltage along it' mean? I mean a circuit has voltage along it, yes? In order for the electricity to flow?
 
bubblewrap said:
This was in part with the calculation of Rayleigh and Jeans. It was trying to solve for a simplified version with a cube cavity, and three separate axis perpendicular to each other with this case being the x axis. It was talking about the propagation direction being perpendicular to the electrical field vector and the propagation direction again being perpendicular to the metallic wall. And frankly I'm not quite sure what any of these mean
You're going to have to spend some quality time with an undergraduate E&M textbook to learn about these concepts before you'll be ready to take on Rayleigh-Jeans.
 
Nugatory said:
You're going to have to spend some quality time with an undergraduate E&M textbook to learn about these concepts before you'll be ready to take on Rayleigh-Jeans.
Can you recommend me a good one?
 
A circuit with ideal wires (zero resistance) may have voltages across certain components, like resistors or motors, but not across the wires. In real life, of course, wires have some resistance, but we try to keep it so small that the voltage drop is negligible. The term "short circuit" refers to an accidental contact between 2 wires that may result from mechanical injury to the insulation. Such a "short" may divert current from where it was supposed to go or may result in excessive current that can cause a fire or blow a fuse.

When I used the term "short circuit" to clarify things, I assumed that you knew all the above. Since you apparently didn't, it was not a clarification. So, let me approach your problem directly.

It sounds like you are trying to follow the calculation of propagating and standing electromagnetic waves in a cavity with conducting walls. This subject is a couple of years too advanced for you as you don't know basic electricity.You need to study at least:
1. DC (direct current) electric circuits, Ohm's law, Kirchoff's laws, Thevenin's theorem.
2. AC (alternating current) electric circuits, inductance, capacitance, transformers, resonant circuits.
3. Electromagnetic waves, antennas, transmission lines, waveguides, resonant cavities.
This was 3 years' work in college when I had it.
 
bubblewrap said:
Can you recommend me a good one?
Take a look at the "Science and math textbooks" section of PhysicsForums... I learned this stuff from Purcell, and I expect that I have a lot of company here.
 
  • #10
Thanks to all of you I didn't take a look at college level classical mechanics and dived straight into quantum physics. I guess that wasn't a very good idea. I'll have to take a look at them.
 

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