ECE3300 Lecture 2-4 Electric Flux Density

Uploaded by cfurse on 24.08.2009

There's one more topic we should talk about
while we're talking about electric fields. Let's talk
about these electric field lines again. Remember how
the lines go radially out away from our electric charge
like this? And remember how the electric field is
stronger closer to the charge? So my electric field is
strong here, getting a little weaker, and then weakest
out here around the outside? The electric field is
always strongest near the charge and weaker as you
get away, but remember the electric field is the charge
divided by four pi epsilon distance squared, right here,
distance squared.
As our distance -- as the distance increases
that means that the electric field always decreases,
and as the permittivity increases that also means that
the electric field decreases. So this means that if I
had a given charge that the electric field in air, which
has an epsilon R of one, is going to always, always,
always be greater than the electric field in any other
material such as water, which has an epsilon R of 80
magnitude of these electric fields. The direction is
going to stay the same.
Sometimes it's useful for us to just describe
the density of the lines that you see here in this
picture, these arrows, so we use another variable. The
variable that we use is called electric flux or electric
flux density. A flux is just something that's moving or
a flux line. This is a flux line. Here's another flux line.
Each of these arrows is a flux line so the electric flux
density is given by D, it is a vector, and it is epsilon
times E so the electric flux is proportional to the
charge inversely proportional to four pi and the
distance squared. The epsilon value, this epsilon value,
has been normalized out. This means that the density
of the field lines or the shape -- I'm sorry, the density
of the flux lines or the shape of the flux lines is the
same regardless of the material that you are in so the
electric flux in air is equal to the electric flux in water.
electric flux in air is equal to the electric flux in water.