# Electric Fields and excess charge vs charge

• jlatshaw
In summary, when conducting hollow spheres are being discussed, it is important to understand that the field within the material is always zero. Any net charge on the sphere is distributed on its outer surface, and this charge adds to any other charges present. The concepts of point charge, excess charge, and charge on the surface all refer to different ways of thinking about and describing the distribution of charge on the sphere. A good rule to remember is that the field within a conducting material must always be zero.
jlatshaw
Hello,
I have a few questions (with regards to conducting hollow spheres) .

1: What is the difference between a charge in the sphere and an excess charge coming out of the sphere or the charge resting on the outside of a sphere?

2: Image a conducting sphere that has a shell of a small thickness R (like 1 cm for example, not infinitely small though). If there is a negative charge in the center of this sphere (-Qcenter)and a positive excess charge on the outside of the sphere(+Qexcess), then my E field at any given point outside or the R radius of the sphere should have an electric field of
E=(k*(Qexcess + Qcenter))/(r^2) where r is distance from center of the sphere. (r>R)

3: My question is why do we take add Qexcess + Qcenter for the total charge even though Qexcess is negative? And what is the difference between the excess charge on the surface and the charge at the center?

4: Also imagine two concentric conducing spheres that are hollow and the thickness of their shells is very very small.

The shell on the inside has an negative excess charge of -Qexcess, and we are given the value of the electric field at r distance away from the center of both of these shells (this r is greater than the radius of both the shells). We are then told to find the charge on the surface of the external shell. We can use:
E = (kq)/r^2
q = (E*r^2)/k
ok so that is the charge on the surface of the shell, however it is completely unrelated to the external charge on the interior of the shell, Why? What do they mean by "charge on the surface of the shell?"

5: I guess most of my questions are just really begging for a clear definition of (especially with regards to hollow sphere):
Point Charge:
Excess Charge:
Charge on the surface:

6: What is a good rule to remember to help me better understand the differences between these and what is going on in the equations?

Thank you for any help,
-James

The key is to realize that the field within the conducting material (at least in the electrostatic case, which is what concerns us here) must always be zero.

So, image a spherical conducting shell. If there is charge +Q placed within it, there must be a charge of -Q induced on its inner surface. And if the shell has no net charge, that means its outer surface has a charge of +Q. As far as the field outside the shell goes, it is the same as that of a charge of +Q at its center.

Any net charge on the shell just adds to the charge on its outer surface.

## What is an electric field?

An electric field is a region in which electrically charged particles experience a force. It is represented by a vector quantity that describes the direction and strength of the force.

## How do electric fields and excess charge differ from regular charge?

Excess charge refers to an imbalance of positive and negative charges in a material or object, while regular charge refers to the overall amount of positive or negative charge in an object. Electric fields are created by the presence of charge and can affect the movement of excess charge within a material.

## What is the relationship between electric fields and excess charge?

Electric fields and excess charge are closely related, as the presence of excess charge creates an electric field and the strength of the electric field is dependent on the amount of excess charge present. The direction of the electric field is also determined by the direction of the excess charge.

## How do you measure electric fields?

Electric fields can be measured using a device called an electric field meter or with a voltmeter and a known distance between two points. The electric field strength is measured in volts per meter (V/m).

## What are some examples of how electric fields and excess charge are used in everyday life?

Electric fields and excess charge are used in a variety of everyday applications, such as in batteries, electric motors, and electronic devices. They are also utilized in technologies like capacitors, which store excess charge, and electrostatic precipitators, which use electric fields to remove pollutants from the air.

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