# Classic E&M Problem: Analyzing Point Charge & Charged Sphere

• turnerre
In summary, when solving for potential or E-field at a point charge near a charged sphere, there are four possible methods of integration: using distances z and R, using distance x and integrating rings, using distance tau and integrating across the sphere, or integrating with respect to an angle. While the first method is the most common, it is not the easiest. The easiest and most efficient method is to use angles, as it involves a single variable and takes into account the symmetry of the problem.
turnerre
A general problem you run into during an E&M class is the one involving a point charge a distance from a charged sphere. In cases where the point is far enough away from the sphere, the system can be treated like two points.

In my case I have a point charge "q" a distance "z" away from the center of a sphere of radius "R." The distance is not big enough for the system to be considered as two points of charge.

When asked to solve for the potential or the E-field at the point charge usually one just integrates across the sphere while accounting for different distances (i.e. distance from point charge to dq on sphere) from the point charge to the "dq's" they are integrating.

My question is what form of integration would be easiest? I have it down to four types of integrating. It helps to use this picture to visualize each integration, there is an attached file/drawing of the sphere and the rings that are integrated.

The first would be to integrate using the distances z and R to solve for some "r" (the distance from point z to the dq) and then integrate across a ring and then the sphere.

The second involves integrating the rings using a distance "x" which is the distance from the center of the sphere to the height of the ring in the z direction. For example the distance x when the specific dq is on the point closest to point z would be z-R. Likewise the furthest dq would be z+R.

The third integration method would be to use a distance "tau" which is the distance from the z axis to the dq ring. In the picture that would be R*sin(phi). Using this tau one could integrate from -R to R to solve for the e-field.

The fourth form of integration I would believe to be the hardest... Integrate with respect to an angle. The angle would be the one formed by the point z and the line connecting point z to a charge dq. This would be hard because you would have to integrate to a point where the line between dq and point z is tangent to the sphere. Then you would have to integrate a second time for all the points after that tangent point.

So which integration is the easiest?... I was given a hint that the first one is not the easiest although it is the most common.

The idea of this problem is to make you attack a problem from all possible points so in the future you may understand the easier/possibly faster way of calculating things.

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If a sphere with radius "R" is a conductor then the charge is on the surface. The E field produced by the ring along the z axis is

$$dE_z=K\frac{\sigma{}2\pi{}R\sin{\phi}Rd\phi}{r^2}\cos{\theta}$$

where sigma is the surface charge density, Rsin(phi) is the ring radius, and Rd(phi) is the ring width. The distance, r2, can be expressed in terms of phi, z, and R using the law of cosines. Due to symmetry, only the z component of the field contributes so this component is determined using cosine of theta, which can be expressed in terms of theta, R and z.

$$\cos{\theta}=\frac{z-R\cos{\phi}}{r}$$

Then this becomes an intergral with a single variable phi with limits from zero to pi. So, using angles is straightforward.

## 1. What is the difference between a point charge and a charged sphere?

A point charge is a theoretical concept used to represent a single, infinitesimally small charged particle. On the other hand, a charged sphere is a physical object with a finite size that has a uniform distribution of charge on its surface or within its volume.

## 2. How is the electric field calculated for a point charge?

The electric field at a point due to a point charge is calculated using Coulomb's Law, which states that the magnitude of the electric field is directly proportional to the magnitude of the charge and inversely proportional to the square of the distance between the point and the charge.

## 3. Can the electric field of a charged sphere be calculated using Coulomb's Law?

No, Coulomb's Law can only be used to calculate the electric field for a point charge. To calculate the electric field for a charged sphere, we use Gauss's Law, which relates the electric field to the total charge enclosed by a surface surrounding the sphere.

## 4. How is the potential energy of a point charge related to its electric field?

The potential energy of a point charge is directly proportional to the electric field at any given point. This means that the higher the electric field, the higher the potential energy of the point charge.

## 5. Can the electric field of a charged sphere be zero at any point?

Yes, if the charged sphere has a uniform distribution of charge and the point in question is located at the center of the sphere, then the electric field will be zero. This is because the electric field vectors from all points on the sphere will cancel out at the center.

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