Cancer's exam-grading brain damage poll #1

[Old Man]

The exam still hasn’t been posted here. 

[Cancer]

OK, here's problem 4, which is the last page of the exam.

 

Three points ( p1, p2, p3) make an equilateral triangle with sides of length S.  Initially there are no charges or fields near the triangle.

 

First a charge +Q is brought from infinity and put at point p1.  Then a charge -2Q is brought from infinity and put at p2.  Then, another charge -2Q is brought from infinity and put at point p3.

 

(a) How much work is done moving the first charge +Q to p1?  Is this work done by the electric force or against the electric force?

 

(b) How much work is done moving the first -2Q charge to p2?  Is this work done by the electric force or against the electric force?

 

(c) How much work is done moving the second -2Q charge to p3?  Is this work done by the electric force or against the electric force?

[Cancer]

Omitted in the final draft was: (d) What total work is done in assembling the triangle of charges?

[Pariah]

Or offspring to come in at a quarter to 5 in the morning.

 

Did that this past week.

[Pariah]

3 hours ago, Cancer said:

OK, here's problem 4, which is the last page of the exam.

 

Three points ( p1, p2, p3) make an equilateral triangle with sides of length S.  Initially there are no charges or fields near the triangle.

 

First a charge +Q is brought from infinity and put at point p1.  Then a charge -2Q is brought from infinity and put at p2.  Then, another charge -2Q is brought from infinity and put at point p3.

 

(a) How much work is done moving the first charge +Q to p1?  Is this work done by the electric force or against the electric force?

 

(b) How much work is done moving the first -2Q charge to p2?  Is this work done by the electric force or against the electric force?

 

(c) How much work is done moving the second -2Q charge to p3?  Is this work done by the electric force or against the electric force?

 

I'm reasonably certain I can still answer part (a). It'd take a little work to answer the other two, though.

[Badger]

4 hours ago, Cancer said:

OK, here's problem 4, which is the last page of the exam.

 

Three points ( p1, p2, p3) make an equilateral triangle with sides of length S.  Initially there are no charges or fields near the triangle.

 

First a charge +Q is brought from infinity and put at point p1.  Then a charge -2Q is brought from infinity and put at p2.  Then, another charge -2Q is brought from infinity and put at point p3.

 

(a) How much work is done moving the first charge +Q to p1?  Is this work done by the electric force or against the electric force?

 

(b) How much work is done moving the first -2Q charge to p2?  Is this work done by the electric force or against the electric force?

 

(c) How much work is done moving the second -2Q charge to p3?  Is this work done by the electric force or against the electric force?

 

Uhhhhhhhh (wipes away drool) Ummm, Rosebud?

[Old Man]

2 hours ago, Pariah said:

 

I'm reasonably certain I can still answer part (a). It'd take a little work to answer the other two, though.

 

Yes, but how much work?  That's the questions.

 

I was able to do a) and the second half of b and c.  I had to look up how to do the first part of b and c but I'm not having flashbacks to the GRE yet.

[Cancer]

It is relevant that at least some of the students taking my test will take the MCAT, and among my duties is try to make it so that doesn't nuke them.  And this is reminiscent of problems on the MCAT.

[Old Man]

I’m tempted to take the GRE again, 25 years later, just to see if I’d do any worse. 

[Cancer]

That way lies madness.

[Badger]

burgle smurgle   AHHHHHHHHHHHHH

 

Cthulhu is behind physics isn't he?

[Cancer]

Mmmmmmmmmmmmmmmmmmmmmmmmaybe..............

[Pariah]

4 minutes ago, Badger said:

burgle smurgle   AHHHHHHHHHHHHH

 

Cthulhu is behind physics isn't he?

 

Lemme post a few questions from last week's QM exam, and then you can decide.

[Cancer]

"Use perturbation theory to estimate the first-order change in the energy levels of the infinite square well if the well, rather than being square, is tilted by an amount  +/- delta E around its center."

[Old Man]

ASTROLOGY TEACHER FOUND BRUTALLY MURDERED IN CLASSROOM

Room Full of Students Swear They Didn't See Nothing

 

Details at 11

[Badger]

1 hour ago, Cancer said:

"Use perturbation theory to estimate the first-order change in the energy levels of the infinite square well if the well, rather than being square, is tilted by an amount  +/- delta E around its center."

 

Pie?

[Cancer]

25 minutes ago, Badger said:

 

Pie?

 

You are on the right track.

[Cancer]

59 minutes ago, Old Man said:

ASTROLOGY TEACHER FOUND BRUTALLY MURDERED IN CLASSROOM

Room Full of Students Swear They Didn't See Nothing

 

The irony in that subheading is superb.