A negative charge ($-q$) is moving in the $+x$ direction when it encounters a region of constant magnetic field pointing in the $-y$ direction. Which is the direction of the initial net force on the charge? 1. $+y$ 2. $-y$ 3. $+z$ 4. $-z$ 5. ??? Note: * CORRECT ANSWER: C * Make sure to take into account the sign change

In the boundary condition for $E_{\perp}$, the difference across the boundary is $\dfrac{\sigma}{\varepsilon_0}.$ What does that $\sigma$ contain? 1. Just $\sigma_{f}$ 2. Just $\sigma_{b}$ 3. Both 4. Something else Note: Correct answer C

Let's assume the normal vector at the material boundary points upward. <img src="./images/cap_w_2_dielectrics.png" align="center" style="width: 300px";/> What the sign of the dot product of the electric field and this normal vector above and below the boundary (i.e., $\mathbf{E}\cdot\mathbf{n}$)? 1. Above is positive; below is negative 2. Above is negative; below is negative 3. Above is negative; below is positive 4. Above is positive; below is positive Note: Correct answer B

## Magnetostatics <img src="./images/death-magnetic.jpg" align="center" style="width: 500px";/>

A proton ($q=+e$) is released from rest in a uniform $\mathbf{E}$ and uniform $\mathbf{B}$. $\mathbf{E}$ points up, $\mathbf{B}$ points into the page. Which of the paths will the proton initially follow? <img src="./images/proton-in-EandB.png" align="center" style="width: 800px";/> Note: * CORRECT ANSWER: C

A + charged particle moving up (speed $v$) enters a region with uniform $\mathbf{B}$ (left) and uniform $\mathbf{E}$ (into page). What's the direction of $\mathbf{F}_{net}$ on the particle, at the instant it enters the region? <img src="./images/charge_enters_EandB.png" align="right" style="width: 400px";/> 1. To the left 2. Into the page 3. Out of the page 4. No net force 5. Not enough information Note: * CORRECT ANSWER: E * The forces point in opposite directions, but not sure of their size