AP Physics help--Emergency

[brandon749]

My AP physics teacher sometimes gives us extremely difficult worksheets, and with the AP test at hand, it's really picked up. I'm having trouble with one problem in particular, and I need your help.


A small object with a mass of 350 mg carries a charge of 30.0 nC and is suspended by a thread of negligable mass between vertical plates of a parallel-plate conductor. The plates are seperated by 4.00 cm. If the thread makes an angle of 15.0 degrees with the vertical, what is the potential difference between the plates?

 

[CivCube]

Ahhh, I feel for you! I'm studying for that as well this Monday. In fact, I'm studying at this moment. I don't know the formulas needed for that quite yet, let me check my sheet and I'll get back to you.

...

OK, I think I have it. First you want to find out the energy of the capacitor with E = F / q. The force should be gravitational. Next, find the voltage with E = - V / d. The potential difference should be the sine of the angle times the voltage.

Edit: Hmm...actually, I don't know whether to use the cosine or the sine of the angle.

 

[Azadre]

Vectors... tap tap

 

[col]

Throw in that

Tan(theta) = Horiz force / vert force

= Vq/d / mg

and you know everything but V (assuming a value for g)

 

[MummyMan]

speaking of physics, col, your avatar looks exactly like my physics teacher mr. hopkins.

 

[CivCube]

Or one could just skip this portion of the test.

 

[Azadre]

BTW, I am only a CP Physics student..

 

[brandon749]

Make sure I did this right: It all seemed a lot easier after a good 7 hours of Civ3 and your help...

F=tan(15deg)*(350_mg*_g)
F=0.0009196894_N

F=qE; E=V/d

V=F*d/q

V=0.0009196894_N*4.00_cm/30_nC
V=1226.252509_V

 

[Goober]

SigFigs, SigFigs!!!

Watch those, in that answer, they look atrocious. According to the question, there should be three, so the answer should be, V=1.23X10^3 V (Yes, V=V, great, ain't it?)

I do not know how picky the AP is about SigFigs, in regular Physics, 2 or 3 is good. I have no clue as to how to do the question, btw . . .

 

[brandon749]

Originally posted by RealGoober
SigFigs, SigFigs!!!

Watch those, in that answer, they look atrocious. According to the question, there should be three, so the answer should be, V=1.23X10^3 V (Yes, V=V, great, ain't it?)

I do not know how picky the AP is about SigFigs, in regular Physics, 2 or 3 is good. I have no clue as to how to do the question, btw . . .

After the first semester, my physics teacher stopped carring about sig figs, and instead wants us to submit our work and answers with all decimal places so he can better understand what we did to solve the problem. On the AP Test itself, however, sig figs can be an issue.

 

[Goober]

Well, my Physics teacher enjoys putting the little "-1" beside the question for not having 2 or 3 SigFigs.

He is a rather harsh marker, and I am very surprised that I am getting 85.7% in that class . . . very surprised . . .

 

[col]

I take off 1 mark every time - and I dont stop when I reach zero. I gave one student -15 out of 10 once. That can really ruin your average

Originally posted by MummyMan
speaking of physics, col, your avatar looks exactly like my physics teacher mr. hopkins.

If he's half as good as my avatar then he would be outstanding

 

[brandon749]

Originally posted by col
I take off 1 mark every time - and I dont stop when I reach zero. I gave one student -15 out of 10 once. That can really ruin your average

That's a little harsh. When a student would have a better grade had he done nothing at all than when he applies at least some effort, that might be going a little far.

 

[Moss]

Brandon does have a point, but then again I can see things the other way to. If you don't show any work at all then you just automatically fail the class...or test...or wksht...

 

[brandon749]

Originally posted by Moss321
Brandon does have a point, but then again I can see things the other way to. If you don't show any work at all then you just automatically fail the class...or test...or wksht...

As a student, I can't sympathize with that at all. If you get the right answer, regardless of lack of shown work, that should entitle you to--at the very least--a hefty portion of partial credit. Maybe you shouldn't be able to get a 100% without thoroughly documenting your work, but you shouldn't get an F if you get the right answers.

 

[Goober]

Originally posted by col
I take off 1 mark every time - and I dont stop when I reach zero. I gave one student -15 out of 10 once. That can really ruin your average

WHAT?!?! This is ridiculous!!! Come on, col, you want to give those poor Physics students a bit of a chance, dont'cha? I thought it was impossible to get worse for handing something in . . .

Are you a Physics teachers, btw?

 

[Mise]

sig figs are dead important. I mean, you can't possibly claim that your figures are THAT accurate, considering all the assumptions that were made in constructing the question. And showing your working shows an understanding of the physics.