# Homework Help: Electrical field lines

1. Sep 19, 2007

### tongpu

1. The problem statement, all variables and given/known data
Sketch the electrical field lines of a uniformly charged insulating disk. Show field lines close and far away.

2. Relevant equations
none, conceptual

3. The attempt at a solution
For close to the surface of disk near the center the field lines are almost perpendicular to surface, as you move away from center the field line curves away. For the curve part of the disk the field lines point outwards.

For far away the disk is like a pint charge so the field lines point away from the disk.

No charge is given so how can i tell, and how does knowing in an insulator the electrons do not move freely help? What if it is a conductor?

2. Sep 19, 2007

### dynamicsolo

Although you have no picture here to help see what you mean, your verbal description sounds basically correct.

In electrostatics problems, when no charge is specified, the convention is to describe the behavior for positive charge. (It is understood that the field direction would be reversed for negative charge.)

I suspect that, since the problem specifies an *insulating* disk, they are asking you to treat the charge as being on only *one* side of the disk. (For an "ideal" insulator, it would all *stay* there.) For an "ideal" conductor, the charge would spread itself over the entire surface as uniformly as possibly, so (nearly) half the total charge would be on either side. (We can neglect the thickness of the disk in an introductory physics problem.)

3. Sep 19, 2007

### learningphysics

Is this over 2D or 3D? If the charge is uniformly distributed... then the field lines are going to be radially outward from the center of the disk.