Calling all Oxbridge Physicists!

help someone!

I have a signal filter designed using an LCR network and a large resistor (100kOhm)...put together in the form of a potential divider....R1 = 100k and R2=|Z| of LCR network. At the selected frequency the LCR network impedance resonates.....and hence an input signal is passed through the filter. At other frequencies, the input signal is lost over the large R1 resistor.

but what i am confused about is where the question asks "why would a small resistor, say, 1k, degrade the performance of the filter....remembering the positive and negative going transients and the effect of damping on resonance."

the positive and negative going transients are whent he input is a square wave, the positive going edge is heavily damped by the large resistor, whereas the negative going edge is not damped at all by the large resistor. So what i dont get is how that is relevant. What is the question asking? why isnt a small resistor suitable?

Reply 362

Hoofbeat

OP

14

I'm afraid I don't know Will...circuit theory really isn't my strong point and I avoid the questions wherever possible!

I have a nice mechanics question I'd appreciate help on. I have some scribblings from a tutorial last term, but I don't really understand it:

=====
Q. A space station is located in a gravity-free region of space. It consists of a large diameter, thin walled cylinder which is rotating freely. It is spinning at a speed such that the apparaent gravity on the inner surface is the same as that on earth. The cylinder is of radius r and mass M. Discuss the minimum total work which had to be done to get the cylinder spinning.
=====

I don't really get what apparent gravity is (I'm guessing it's the equivalent force produced by the spinning cylinder?) but I seem to remember it having the following formula:

Unparseable latex formula:

\fontsize{5} mg_{a} = mg + F_{c}

where

Unparseable latex formula:

\fontsize{5}mg_{a}

is the force due to apparent gravity and

F_{c}

is the centripetal force.

Therefore

Unparseable latex formula:

\fontsize{5}mg_{a} = 0 + m\omega^{2}\div r

As the station is in a gravity free region of space. If that's right, how do I progress to finding the minimum work done? Do I calculate

Unparseable latex formula:

\fontsize{5}E_{k}=I\omega^{2}\div 2

and get

Unparseable latex formula:

\fontsize{5}\omega

from the above equation and use the fact that

Unparseable latex formula:

\fontsize{5}I=mr ^{2}

EDIT: Go me with the latex :biggrin:

But how come the last code isn't working?!

EDIT2: Latext fixed :biggrin:

i wanna cookie!

OP

shut up shiny :p:

patience is a virtue...wait til Friday!

Reply 365

shiny

12

i'm clearly very organised :rolleyes:

Reply 366

have you tried solving the question yet cos i reckon you have the right method there

and your last tex things dont work cos you got carried away with your backslashes! it's not [\tex]

anyhoos....handed in my coursework and just did a physics test which i now realise i made loads of stupid mistakes on (like not ANSWERING THE DAMN QUESTION!) I'm such a fool! And physics is supposed to be what I'm good at!

Reply 367

shiny

12

the only method which has any relevance to me is one that involves a cookie landing in my pidgeon hole!

Reply 368

is someone giving out cookies!? I wish millies would do that....mmmmmmmm....coookies *goes out to buy some*

Reply 369

Willa

is someone giving out cookies!? I wish millies would do that....mmmmmmmm....coookies *goes out to buy some*

What is it with physicists and cookies?

Or cookies at all for the matter - just don't get the appeal. :confused:

Reply 370

mmmmmmmmm cooookies!

have you never even tried millies cookies....sooo yummy!?

Reply 371

~Raphael~

9

Willa

mmmmmmmmm cooookies!

have you never even tried millies cookies....sooo yummy!?

Dude one thing Oxford wings hands down - cookies! Ben's Cookies in the covered market is the best cookie shop in the world, they are fantastic in the extreme!!!!!!!!

Reply 372

Willa

mmmmmmmmm cooookies!

have you never even tried millies cookies....sooo yummy!?

is that some tab thing? :confused:

i think there's some place in oxford does good cookies but forget its name - have yet to "discover" their virtue - or sinfulness - whatever.

Reply 373

millies cookies is a nationwide chain that do cookies and cakes....and their deluxe gourmet chocolate cookie is the best cookie I've ever tasted! Bet when i go to oxford this term I'll make sure i try ben's cookies to compare!

Reply 374

don't do it! - a taste of Oxford's finest and millies'll never satisfy again

well, if they are really as good as some people say.

they may do mail order as well.

Reply 375

~Raphael~

9

Willa

millies cookies is a nationwide chain that do cookies and cakes....and their deluxe gourmet chocolate cookie is the best cookie I've ever tasted! Bet when i go to oxford this term I'll make sure i try ben's cookies to compare!

Yeah, Millie's is American in style and a bit meh, though still nice, but Ben's is almost home-made in style and really does destroy all competition.

My name is Raphael and I am a cookieholic.

Reply 376

LennonMcCartney

Willa

physicists of the world (or the bit that matters :p:

....now that was wrong of me)...what is a good program for drawing oscilloscope plots. Any ideas? I need to have some put in my report but I dont really want to draw them by hand...is there a nice program that will do scope plots according to a circuit anyone knows of?

Yes. I scoured for one. There is a text-based command line engine called SPICE, developed at Berkeley, many years ago. Thankfully, some clever bods have made a GUI out of it, and it's called SwitcherCAD III. It's actually quite powerful, and probably more complex than you/we need, which means that it's not necessarily that easy to use, at least to start with.

Look it up on google, I can't remember which site I got it from.

Reply 377

thanks i'll keep it in mind. Not much use to me now though, my deadline was today so I ended up drawing them out by hand. Ah well, the rest probably wasnt very good anyway!

Reply 378

LennonMcCartney

Hoofbeat

Q. A space station is located in a gravity-free region of space. It consists of a large diameter, thin walled cylinder which is rotating freely. It is spinning at a speed such that the apparaent gravity on the inner surface is the same as that on earth. The cylinder is of radius r and mass M. Discuss the minimum total work which had to be done to get the cylinder spinning.

I'm sorry, I'm not going to answer the question, because I'm well lazy. But apparent force, occurs in any non-inertial frame, and is due to the acceleration of the non-inertial frame with respect to some inertial frame. You can usually get some value for acceleration, all you have to do is multiply by m and stick that onto the end of Newton's second law.

Reply 379