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
## 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
<img src="./images/v_at_an_angle_to_B.png" align="right" style="width: 300px";/>
A proton (speed $v$) enters a region of uniform $\mathbf{B}$. $v$ makes an angle $\theta$ with $\mathbf{B}$. What is the subsequent path of the proton?
1. Helical
2. Straight line
3. Circular motion, $\perp$ to page. (plane of circle is $\perp$ to $\mathbf{B}$)
4. Circular motion, $\perp$ to page. (plane of circle at angle $\theta$ w.r.t. $\mathbf{B}$)
5. Impossible. $\mathbf{v}$ should always be $\perp$ to $\mathbf{B}$
Note:
* CORRECT ANSWER: A