# Electric Flux Density / Electric Field Intensity

1. Sep 10, 2009

### zaydana

Hey people,

I'm struggling to understand exactly what Electric Flux Density D and Electric Field Intensity E are.

So far, my understanding is that D depends on relative permittivity, while E depends only on the permittivity of free space. Also, I understand that D reflects only the free charges, while E reflects both free and bound charges. Where I'm stuck though, is that my book also tells me that D is independent of the medium it is in, while E is dependent on the medium it is in. This just doesn't click with me, since the actual value of E doesn't change if you change the medium (and thus the relative permittivity), while the value of D does.

Can anyone help me out here?

Last edited: Sep 10, 2009
2. Sep 10, 2009

### Born2bwire

The vector quantity B is not the electric flux density, it is the magnetic flux density (or magnetic field depending upon your reference). D is the electric flux density and is related to the electric field by

$$\mathbf{D} = \epsilon \mathbf{E}$$

The boundary conditions for D and E are different. For example, the tangential E field is continuous across a boundary but the normal E field is discontinuous. The normal D across the boundary differs by the charge density that accumulates on the boundary so if we assume no bound charges then the normal D field is continuous, but the normal E field is still discontinuous if there is a change int he permittivity.

So both the D and E fields can change between media depending upon the properties of the media but most cases will assume that no charges will build up on the interface and thus the D field can be continuous in more cases.

Last edited: Sep 10, 2009