# E and M fields in a faraday cage

1. Jan 22, 2008

### Robismyname

I have a question regarding Faraday Cages.......

Faraday Cages blocks static electrical fields

I read somewhere that static magnetic fields can penetrate the faraday cage when changed slowly. Faster changes in static magnetic fields cannot penetrate a faraday cage, is this true?

Also does Magnetostatic = static magnetic fields?

Does Electrostatic = static electric fields?

2. Jan 23, 2008

### TVP45

Farady cages block static electric fields not static magnetic fields. Their effectiveness depends on the frequency of the changing magnetic field. Typically, one uses mu metal at low frequencies and cages at high frequencies and there is considerable overlap.

3. Jan 23, 2008

### Robismyname

If the magnetic field is static then that means it does't move. So if it doesn't move that means that it cant pass through the cage correct? Or does static just mean slow moving?

4. Jan 23, 2008

### TVP45

You should understand static to mean constant in time.

5. Jan 23, 2008

### Robismyname

Not to be a pain but I read a post from the Physics forum

( https://www.physicsforums.com/archive/index.php/t-12250.html) that suggest that

"Static electricity" can really only be defined one way: "charges that are not moving."

So I thought that Static Magnetic fields implied: "Magnetic fields that are not moving."

Thanks for the clarification.......

6. Jan 23, 2008

### TVP45

"constant in time" means dB/dt = 0. Saying that they do not change with respect to time is, IMHO, a little better way to say it than "not moving"

7. Jan 24, 2008

### Robismyname

When you say that "one uses mu metal at low frequencies" I assume you are talking about an inductor core made of mumetal powered by a low frequency power supply?

Would 60Hz Ac be considered a low frequency power supply?

8. Jan 24, 2008

### TVP45

No. I meant a mu metal shield. Google "mu metal shield" for lots of good info. 60 Hz is usually considered low frequency.