**True or False**: The electric potential of a pure dipole is given exactly by:
$$V(r) = \dfrac{\mathbf{p}\cdot\mathbf{r}}{4 \pi\varepsilon_0 r^3}$$
1. True
2. False
## Announcements
* Homework 7 Problem 5 graded as pure extra credit
* Exam 2 is coming up (2 weeks from today)
* BPS 1415 (this room), 7pm-9pm, Nov 8th
* Same format as Exam 1
* Details next week
$$\mathbf{p} = \sum_i q_i \mathbf{r}_i$$
What is the magnitude of the dipole moment of this charge distribution?
<img src="./images/2q_dipole.png" align="right" style="width: 200px";/>
1. qd
2. 2qd
3. 3qd
4. 4qd
5. It's not determined
Note:
* CORRECT ANSWER: B
$$\mathbf{p} = \sum_i q_i \mathbf{r}_i$$
<img src="./images/dipole_2q_and_q.png" align="right" style="width: 200px";/>
What is the dipole moment of this system?
(BTW, it is NOT overall neutral!)
1. $q\mathbf{d}$
2. $2q\mathbf{d}$
3. $\frac{3}{2}q\mathbf{d}$
4. $3q\mathbf{d}$
5. Someting else (or not defined)
Note:
* CORRECT ANSWER: B
$$\mathbf{p} = \sum_i q_i \mathbf{r}_i$$
<img src="./images/dipole_2q_and_q_shift.png" align="right" style="width: 200px";/>
What is the dipole moment of this system?
(Same as last question, just shifted in $z$.)
1. $q\mathbf{d}$
2. $2q\mathbf{d}$
3. $\frac{3}{2}q\mathbf{d}$
4. $3q\mathbf{d}$
5. Someting else (or not defined)
Note:
* CORRECT ANSWER: C
You have a physical dipole, $+q$ and $-q$ a finite distance $d$ apart. When can you use the expression:
$$V(\mathbf{r}) = \dfrac{1}{4 \pi \varepsilon_0}\dfrac{\mathbf{p}\cdot \hat{\mathbf{r}}}{r^2}$$
1. This is an exact expression everywhere.
2. It's valid for large $r$
3. It's valid for small $r$
4. No idea...
Note:
* CORRECT ANSWER: B
You have a physical dipole, $+q$ and $-q$ a finite distance $d$ apart. When can you use the expression:
$$V(\mathbf{r}) = \dfrac{1}{4 \pi \varepsilon_0}\sum_i \dfrac{q_i}{\mathfrak{R}_i}$$
1. This is an exact expression everywhere.
2. It's valid for large $r$
3. It's valid for small $r$
4. No idea...
Note:
* CORRECT ANSWER: A