Question on Gauss law & charged disks

1. Nov 10, 2008

mazen_zone

hey everyone...

i want to know why can't we use Gauss law to calculate the electric flux density of a charged disk?

and how to calculate it "everywhere"? as coulomb's law calculates it at a certain point only...

2. Nov 10, 2008

ZapperZ

Staff Emeritus
You need to realize when Gauss's Law are typically used - in highly-symmetric situation where you can construct a Gaussian surface that will have an electric flux that is either a constant, or zero. When you can do that, then using Gauss's Law will be quite easy. If not, it is seldom solvable in closed form mathematically.

That's not right. You CAN use Coulomb's law to calculate the field at any point - it doesn't mean you can solve it EASILY. Again, because of the lack of symmetry, calculating at field points off the axis of symmetry will make the solution more involved (you will end up with an infinite series since there may be no closed form of the solution). When you get to a graduate level E&M course using texts such as Jackson, you WILL learn how to solve this using "Coulomb's Law", i.e. using Green's function technique to solve the Poisson's equation.

Zz.

3. Nov 10, 2008

mazen_zone

yeah i know coulombs law can calculate the electric flux at any point, i meant i can't use it to derive a general formula to calculate the electric flux density at any point near the disk like Gauss law, but i have to remake the calculations at every point that i want the flux at...

correct me if I'm wrong...

4. Nov 10, 2008

ZapperZ

Staff Emeritus
I have no idea what you just said.

Coulomb's Law IS a "general formula" to calculate E-field.

Zz.