OCR Physics B G495 Field and Particle Pictures June 21st 2011 Exam Thread

Original post by wki

Can anyone explain capacitor to me please? CGP revision guide p8 I dun understand how repel charges n plates stuff like that =o

Capacitors store charge. In order to does this, a potential difference must be applied across the component, therefore, an electrical current must flow. Electrons travel in the opposite direction to conventional current which is (positive to negative). The accumulation of electrons on one side of the capacitor results in an equal and opposite charge building up on the other plate (seperated by dielectric). As more and more electrons are collected they start to repel each other (Coloumb's Law - F=(kQq)/r^2) because they are both carrying the charge of an electron (-1.6x10^-19) a repulsive force exists between the two point charges. This repulsive charge means that it takes and increasing amount of work (E=QV) to add to the exisitng charge on the plate. Over time, the capacitor reaches its maximum charge storage (determined by Q=CV) and the current drops to zero and there is no longer a rate of flow of charge.

Hope this helps. If it doesn't - ask again.

Reply 942

arianex

9

Original post by Summerdays

I pretty much said the same thing. For the last one I aid that the bar is more likely to sag under its own weight, and thu the length would tend to decrease.

For the temperature one I said that the length would increase because length increases with temperature :biggrin:

What did you say about why platinum-iridium wa used instead of just platinum?

For the platinum-iridium Q, I said something with reference to the properties of alloys (y'know, the whole diff sized atoms thing where it's less likely to slip and slide :tongue:

) Not sure if I've shot myself in the foot there...

Reply 943

Original post by arianex

For the platinum-iridium Q, I said something with reference to the properties of alloys (y'know, the whole diff sized atoms thing where it's less likely to slip and slide :tongue:

) Not sure if I've shot myself in the foot there...

That's perfect!

I said something silly like less chance of rust, so less chance of mass change and thu les chance of length change :s-smilie:

Reply 944

5liam5

Any body know if the answers have been worked out and published anywhere by a teacher or something? :smile:

Reply 945

reallyoriginalusername

Original post by Summerdays

I know what you mean. I found the article interesting because I didn't know how the metre was defined :smile:

But, it made preperation just about impossible - which annoyed me. They could literally had asked anything from that article they gave us - which annoyed me. Preperation would have been 10^6 times easier if it wasn't for section C. And I hated some of the questions they asked us.

BTW, what did you put as what would happen if the temperature, atmospheric pressure, if the distance of the cylinder were too high/big of a distance apart?

Yeah, out of all the past papers I've looked at, I couldn't find any nice advance notice articles. What I find ridiculous about the questions is that, they try to make us think beyond A-level standard for some of the more obscure questions. At the same time, most teachers are just worried about getting the content taught with what little(and mainly useless) resources there are for OCR B. It's all a big mess, I don't know why the IOP would endorse this specification(and they wander why there aren't enough people studying Physics). I would have answered your question, but it's already been done (better than I would have done, at that). Anyway, we don't have to worry about OCR B ever again! So well done all and I hope we all get good results :smile:

Reply 946

Unkempt_One

13

Not even that, the advance notice article is just an excuse for them to ask brainstorming-type question with little actual Physics content, which judging from the textbook is how OCR B think Physics should be learnt. I'd be fine with difficult questions if they revolved around challenging problem-solving rather than playing mind games with the examiner. I honestly think I might have enjoyed Physics A-level if it hadn't been for this horrible course.

Reply 947

reallyoriginalusername

Original post by Unkempt_One

Not even that, the advance notice article is just an excuse for them to ask brainstorming-type question with little actual Physics content, which judging from the textbook is how OCR B think Physics should be learnt. I'd be fine with difficult questions if they revolved around challenging problem-solving rather than playing mind games with the examiner. I honestly think I might have enjoyed Physics A-level if it hadn't been for this horrible course.

Yeah, fair enough :/ A purely academic Physics A-level course would be awesome. Also, I never quite understood why they ask questions like 'Show that such and such is approximately/about x value'. If you're assessing the method used to reach a value, you might as well give the precise value. It would just avoid confusion.

Reply 948

scraggyorcore12

3

Please can someone post the answers online or post the paper online!! I'm dying to know whether i've got into uni!!!!

Reply 949

scraggyorcore12

3

CAN SOMEONE POST THE ANSWERS ONLINE OR THE PAPER ONLINE :mad:

!!! I'M DYING TO FIND OUT WHETHER I'VE GOT INTO UNI!!! :frown:

Reply 950

Gadwin

I flunked the **** out of this :P

Reply 951

Original post by scraggyorcore12

CAN SOMEONE POST THE ANSWERS ONLINE OR THE PAPER ONLINE :mad:

!!! I'M DYING TO FIND OUT WHETHER I'VE GOT INTO UNI!!! :frown:

No one has it.

Reply 952

Original post by Gadwin

I flunked the **** out of this :P

What does flunked mean ? Does this mean you blagged it and did well or did poorly ?

Reply 953

Original post by Summerdays

I pretty much said the same thing. For the last one I aid that the bar is more likely to sag under its own weight, and thu the length would tend to decrease.

For the temperature one I said that the length would increase because length increases with temperature :biggrin:

What did you say about why platinum-iridium wa used instead of just platinum?

If the bar bent - then wouldn't one side's length be shortened and one side's length be longer due to relative extension - that's what i put :s

Reply 954

Original post by foxymoron

If the bar bent - then wouldn't one side's length be shortened and one side's length be longer due to relative extension - that's what i put :s

No clue.

Reply 955

boba

Original post by reallyoriginalusername

Yeah, fair enough :/ A purely academic Physics A-level course would be awesome. Also, I never quite understood why they ask questions like 'Show that such and such is approximately/about x value'. If you're assessing the method used to reach a value, you might as well give the precise value. It would just avoid confusion.

thats just so they can check for the exact value at the end to give you the marks rather than spending ages working through every single persons method it avoids confusion in marking because there might be debates about whether some methods are questionable. them questions arn't exactly to test the method exactly but more so that if you get them wrong you can still do later parts of the question with they given value, or thats what we have always been told.

Reply 956

boba

maths exams always seem to be up with answers by now why doesn't anyone have access to the physic paper :frown:

Reply 957

Original post by boba

maths exams always seem to be up with answers by now why doesn't anyone have access to the physic paper :frown:

I know how you feel :frown:

Reply 958

arianex

9

Original post by Summerdays

I know how you feel :frown:

SD, I spoke to my physics teacher this morning - shame he doesn't have answers that I can scan and UL for you guys, but he did say that the bit about the vacuum + phasors in Section C really was a bit weird/off-putting. :tongue:

I'll see if I can pester him about it, but I can't promise anything yet...

PS// Have just discovered that I only need 92 UMS out of 150 (was it?) to get the minimum needed for an A. :biggrin:

Reply 959