Re: [Phys-l] Test Bank question

[John Mallinckrodt <ajm@csupomona.edu>]

Bill Nettles wrote:

> [Bill Nettles] I simply want to make sure that I'm not having a brain freeze.  I was working through some MasteringPhysics test bank questions and came across a couple of simple ones that I disagree with their answers.
>
> Here are the paraphrased versions, with essential information:
>
> 1) An extensive sheet of a conductor carries charge density of 5 uC/mm^2 (u = micro). What is the electric field strength 9 mm from the conductor?
>
> I got 5.6 x 10^11 N/C. They got half of that. I used density/epsilon0 because it's a conductor. It looks like they used density/(2*epsilon0).

Agreed.  The wording is ambiguous as it doesn't specify that the value provided is the "surface charge density."  One COULD interpret it as the "amount of charge on each square millimeter of the sheet" (both sides).  A far bigger problem, IMO, is that the given data is totally absurd and leads to a similarly absurd answer.  How can we expect student to learn to evaluate their answers for reasonableness if we give them problems like this?

> 2) A uniform electric field is 5 N/C r-hat. How much work is done on an electron which moves 6 m along a line tilted 30 degrees wrt r-hat.
>
> I get +/- 26 eV depending on whether you move "with" or "against" the field. The question wasn't clear as to whether the field is doing the work, or another agent is moving the charge at a constant speed.  In any case, they got 10 eV.
>
> Who's wrong?

They are.  Despite the fact that the question is completely ambiguous as you say, the only interpretation that can lead to a defensible answer is the one you suggest resulting in the answer you gave.

John Mallinckrodt
Cal Poly Pomona