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