# Electric field at a conducting surface?

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1. Jan 3, 2016

### bubblewrap

"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

Last edited by a moderator: Jan 3, 2016
2. Jan 3, 2016

### Jilang

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?

3. Jan 3, 2016

### Alfred Cann

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.

4. Jan 3, 2016

### bubblewrap

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 seperate 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

5. Jan 3, 2016

### bubblewrap

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?

6. Jan 3, 2016

### Staff: Mentor

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.

7. Jan 3, 2016

### bubblewrap

Can you recommend me a good one?

8. Jan 3, 2016

### Alfred Cann

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.

9. Jan 3, 2016

### Staff: Mentor

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. Jan 3, 2016

### bubblewrap

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.