# Electromagnetism: Trouble understanding the D field

1. Dec 16, 2013

### BeBattey

I've just finished my first term in an undergrad electromagnetism course using Wangsness as a textbook and we ended on the displacement field.

I feel like Wangsness doesn't give a very good explanation of what the whole point of it actually is, and I still don't quite understand how to use it to find the e field inside both a dielectric and a cavity within a dielectric.

If I could get a quick and easy explanation or a point in the right direction I'd greatly appreciate it, thanks!

2. Dec 16, 2013

### tiny-tim

Hi BeBattey!

The electric displacement field (D, measured in coulombs per square metre) is helpful if you know the surface charge distribution (because it's proportional to it).

So if you know the surface charge distribution, that gives you D, and you can immediately find E for different dielectric fillings, provided inserting the dielectric does not alter the surface charge distribution.

So it works eg for a parallel plate capacitor with dielectrics in series (and with edges parallel to the plates), but not for dielectrics in parallel (because the surface charge distribution will change).

3. Dec 16, 2013

### BeBattey

Thanks for the response!

So the surface charge density you're talking about, is that the bound charge mentioned in the text book? I know the bound surface charge is P dot n-hat, given the polarization of a dielectric, is this what you mean about the proportionality?

4. Dec 16, 2013

### tiny-tim

Hi BeBattey!

I was really thinking about those exam questions where you know the surface charge, either because the question specifically tells you, or because eg it's a sphere and you can use gauss, or it's a capacitor and you know C V and A (area), so you can find σ = Q/A (and D = σñ).