A wire loop in a $B$ field has a current $I$. The B-field is localized, it's only in the hatched region, roughly zero elsewhere. Which way is $I$ flowing to hold the mass in place? <img src="./images/B_holding_mass.png" align="right" style="width: 350px";/> 1. Clockwise 2. Counter-clockwise 3. You cannot "levitate" like this! Note: * CORRECT ANSWER: A

I feel that Exam 2 was a fair assessment. 1. Strongly Agree 2. Agree 3. Neither Agree/Disagree 4. Disagree 5. Strongly Disagree

I feel that Exam 2 was aligned with what we have been doing (in class and on homework). 1. Strongly Agree 2. Agree 3. Neither Agree/Disagree 4. Disagree 5. Strongly Disagree

I felt better prepared for Exam 2 than Exam 1. 1. Strongly Agree 2. Agree 3. Neither Agree/Disagree 4. Disagree 5. Strongly Disagree

<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

In the first stage of the mass spectrometer, with $\mathbf{E} = E_0 \hat{z}$ (pointing upward) and $\mathbf{B} = B_0 \hat{x}$ (pointing out of the page), which particles travel through in a straight line? 1. All particles regardless of speed 2. Particles with speed $B_0/E_0$ 3. Particles with speed $E_0/B_0$ 4. Can't tell without knowing $q$ and/or $m$ *You may assume all particles move exclusively in the +y direction.* Note: * CORRECT ANSWER: C

If we place a physical filter (i.e., a piece of metal with a thin slot that is a bit larger than the beam width to avoid diffraction) at the end of the first stage, which particles (assume they are all positively charged) hit the upper-part of the filter? Which hit the lower part? 1. Fast moving particles hit the upper part; slow ones hit the lower part 2. Slow moving particles hit the upper part; fast ones hit the lower part 3. It's not possible to tell without $q$ and/or $m$ Note: * CORRECT ANSWER: B

Can we use the same mass spectrometer set up for negatively and positively charged particles? That is, will our set up distinguish between particles of a given mass and differently-signed charges? 1. Yes 2. No Note: * CORRECT ANSWER: A

For our velocity selector where $\mathbf{E}=E_0\hat{z}$ and $\mathbf{B} = B_0 \hat{x}$ and we start particles from rest, we end up with the following **coupled** equations of motion, $$m\dot{v}_y = q v_z B_0$$ $$m\dot{v}_z = q E_0 - q v_y B_0$$ How might we solve them for $y(t)$ and $z(t)$? 1. Just integrate the equations of motion 2. Guess the general solution 3. Take the time derivative of one and plug into the other 4. Give up???