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?