Electricity and Magnetism II

1. 10.1 Electricity and Magnetism II Griffiths Chapter 10 Potentials & Fields Clicker Questions

2. 10.2 If I tell you , what can you conclude about F?

3. 10.3 If I tell you , what can you conclude about F?

4. 10.4 If I tell you , what can you conclude about f?

5. 10.5 If I change gauge, soI claim B is unaffected. But, what about E?A) Looks like E is also unaffectedB) Looks like we changed E, but that’s okC) Looks like we changed E, that doesn’t seem acceptableD) What are we doing?

6. Why can’t we use a scalar potential to find the magnetic field, as we have done with the electric field, i.e., why can’t we use 10.6 • Because the divergence of B is always zero • Because only either E or B can be described with a scalar potential, not both • Because B can have a non-zero curl • I don’t know/remember • None of the above (but I know the right reason!)

7. 10.7 How have we been “setting the gauge” so far in this class? • Requiring A and V go to 0 at infinity • Putting a condition on A like: • Putting a condition on • Choosing the function “f” in the formula • Something else, none of these, MORE than one, not really sure…

8. 10.8 In Coulomb’s gauge (CG): In Lorentz’ gauge: These look subtly different, which is correct? • CG is unphysical and incorrect, it violates relativity • CG result is only correct for time independent problems, LG is what you want for time dependent problems • They only LOOK different, but in fact they give the same result for V when you work them out • Both are equally “correct”, it’s just a gauge choice. You can use either one in any situation • Something else is going on, none of the above articulates my opinion very well here!

9. 10.9 How do you interpret • is the actual time of observation at point r. • is the time light needs to travel from r’ to r. • is a time in the future when light emitted from point rat time tarrives at point r’. • is a time in the past such that light emitted from point r’arrives at r at time t. • None of these.

10. 10.10 At what time, t, does an observer at s first know the current was turned on? I=0 t<0 I0 up t>0 • t=0 • t=c s • t=s/c • Other!! • Not sure about this? s

11. 10.11 At what time t, after s/c, does an observer at s see current from the entire wire? • Immediately • Never • Something else

12. 10.12 At time t = 0, a current I0 in an infinitely long neutral wire is turned on. When does observer at s first know the current was turned on?(We are working in the Lorentz gauge.) Is there a non-zero potential V in the space around the wire at anytime after t = 0? A) Yes B) No I=0 t<0 I =I0 t>0 s Is there a non-zero electric field E in the space around the wire at anytime after t=0? A) Yes B) No

13. 10.13 What is I(z,tR)?

14. 10.14 What should the limits of integration be?

15. 10.15 What is the direction of A near the wire? • +y B) –y C) +y on left , -y on right D) Toward wire E) away from wire What is the direction of B at point x? • A) up B) down C) in • D) out E) other A A x B What is the direction of E at point x? • A) up B) down C) in D) out E) other

16. 10.16 A neutral, infinite current sheet, K, flows in the x-y plane, in the +ydirection. To the right of the x-y plane, according to what you know from Phys 3310, the E and B field directions are: X K E along z-axis, B is zero B along z-axis, E is zero B along y, E along z B along x, E along y None of these Y -Z Z -X

17. 10.17 A neutral infinite current sheet, K, is turned on at t=0, flows in the x-y plane, in the +ydirection. Very shortly after t=0, the E and B fields: X K Remain zero. Immediately appear with their static values in all space. Appear only near K Appear everywhere, but exponentially suppressed as you move farther away from K. None of these Y Z -Z -X

18. 10.18 A neutral infinite current sheet, K, is turned on at t=0, flows in the x-y plane, in the +ydirection. Shortly afterwards, the B field near the sheet: X K B=0 out here for awhile is in the z-direction is in the x-direction is in the y-direction is actually zero close to K. None of these Y Z -Z Front moves at v. -X

19. 10.19 A neutral infinite current sheet, K, is turned on at t=0, flows in the x-y plane, in the +y-axis. Shortly afterwards, the E field near the wavefront (but not past it): X K B is in the –z direction is in the –x direction is in the –y direction is actually zero close to the front. None of these Y Z -Z B=0 out here for awhile Front moves at v. -X

20. 10.20 A neutral infinite current sheet, K, is turned on at t=0, flows in the x-y plane, in the +ydirection. Shortly afterwards, the E field very near the sheet: X K B is in the -z-direction is in the -x-direction is in the -y-direction is actually zero close to K. None of these Y Z B=0 out here for awhile -Z Front moves at v. -X