# Electric field

## Homework Statement

A charge q is placed at point O (please refer diagram in attachment) in the cavity in a spherical uncharged conductor. Point S is outside. If q is displaced from O towards S(q still remains inside the cavity) then which of the following is correct:
Electric field at S will:
1. increase
2. Decrease
3. First increaseand then decrease
4. will not change

## The Attempt at a Solution

How to do this....please help me to get started.....

#### Attachments

• ooo.bmp
238.1 KB · Views: 382

Andrew Mason
Science Advisor
Homework Helper

## Homework Statement

A charge q is placed at point O (please refer diagram in attachment) in the cavity in a spherical uncharged conductor. Point S is outside. If q is displaced from O towards S(q still remains inside the cavity) then which of the following is correct:
Electric field at S will:
1. increase
2. Decrease
3. First increaseand then decrease
4. will not change
Can you state the law that applies?

The drawing/question does not make it clear what "the cavity" is. Can you explain further?

AM

This is the exact question.....What exactly is not clear? The cavity they are talking about is inside the spherical conductor completely ....thats it....

Andrew Mason
Science Advisor
Homework Helper
This is the exact question.....What exactly is not clear? The cavity they are talking about is inside the spherical conductor completely ....thats it....
I was interpreting the outer line as a spherical? (ellipsoidal?) conducting shell. I gather that the white part in between the two circles (ellipses) is solid conducting material and the white part inside the inner one is empty. It would help if the drawing showed that.

So what law applies?

AM

All of them are spheres and ya the inner sphere is empty and outside it its solid conducting material....

And i think gauss law applies.....not sure though because there is no symetry....will gauss law help.....
Basically how to do it?

Andrew Mason
Science Advisor
Homework Helper
All of them are spheres and ya the inner sphere is empty and outside it its solid conducting material....

And i think gauss law applies.....not sure though because there is no symetry....will gauss law help.....
Basically how to do it?
Gauss' law applies.

I am not clear on the shape of the "spheres". Are they spheres or ellipsoids? The question says spheres but the drawing does not show spheres. It shows non-spherical ellipsoids.

AM

Andrew Mason
Science Advisor
Homework Helper
Let's assume the drawing is wrong and they are perfect spheres. I don't think it really should matter since we are only asked for a qualitative analysis.

What can you say about the field inside the conductor?

What does that say about the charge density on the inside surface of the conductor on a line between O and S as the charge approaches the right side of the cavity?

What does that arrangement of charge on the inside surface do to the charge distribution on the outside surface on a line between O and S?

AM

Hi...i am really sorry all af them are perfect spheres....actually i drew this using paint so this is what i managed ....

What can you say about the field inside the conductor?

What does that say about the charge density on the inside surface of the conductor on a line between O and S as the charge approaches the right side of the cavity?

What does that arrangement of charge on the inside surface do to the charge distribution on the outside surface on a line between O and S?

AM

Field in side the conductor is zero ....But i am not getting what you meant by charge density/distribution on a line between O and S.....

Why would the field outside change if the field inside the conductor is zero?