**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 * Exam 2 is coming up (2 weeks from today) * BPS 1415 (this room), 7pm-9pm, Nov 7th * 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