What is the total charge for this distribution?
$$\rho(\mathbf{r}) = \sum_{k=0}^2 (1+k)\,q\,\delta^3(\mathbf{r}-k\mathbf{a})$$
1. q
2. 2 q
3. 4 q
4. 6 q
5. Something else
Note:
* Correct Answer: D (write it out)
* Follow up what does it look like?
## Announcements
* As requested, Homework 2 grading rubric posted
* Exam 1 is coming up! October 4th (More details next week!)
A Gaussian surface which is *not* a sphere has a single charge (q) inside it, *not* at the center. There are more charges outside. What can we say about total electric flux through this surface $\oint_S \mathbf{E} \cdot d\mathbf{A}$?
1. It is $q/\varepsilon_0$.
2. We know what it is, but it is NOT $q/\varepsilon_0$.
3. Need more info/details to figure it out.
Note:
* CORRECT ANSWER: A
<img src ="./images/dipole_gauss.png" align="right" style="width: 300px";/>
An electric dipole ($+q$ and $–q$, small distance $d$ apart) sits centered in a Gaussian sphere.
What can you say about the flux of $\mathbf{E}$ through the sphere, and $|\mathbf{E}|$ on the sphere?
1. Flux = 0, E = 0 everywhere on sphere surface
2. Flux = 0, E need not be zero *everywhere* on sphere
3. Flux is not zero, E = 0 everywhere on sphere
4. Flux is not zero, E need not be zero...
Note:
* CORRECT ANSWER: B
* Think about Q enclosed; what can we say about E though?