OCR Physics A G482, Electrons, Waves and Photons, 25th May 2012

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Oh I did it the other way around, going towards P. I assumed that's what it meant since the wave was moving at 0.50ms-1 to the right. :confused:

Yeah, I did it going towards P to, but there was a quarter of a metre gap.

Reply 1221

BaconFTW

Original post by wilky94

I did the same as you. Im not sure about the 'suitable length of wire' thing.

Ah well, probably/hopefully his imagination.

Reply 1222

bencwbrown

I think that as long as the diode, ammeter and resistor were in series, the voltmeter was parallel across the diode, the symbols were all correct, and if you drew the diode in the correct direction, you'd get all the marks. Never heard of something to do with a suitable length of wire

Reply 1223

HarryW95

Original post by BaconFTW

Does anyone remember what you were required to draw for the 'LED circuit' question? I added an LED with a voltmeter in parallel and an ammeter and resistor in series.

But now my friend mentioned that it specified that you should include a 'suitable length of wire', or something along those lines. I has confusion.

I did the same as you. LED with voltmeter in parallel, then 100 ohm resistor and ammeter in series.

Reply 1224

HarryW95

Original post by Malabarista

Yeah, I did it going towards P to, but there was a quarter of a metre gap.

Ah right maybe I didn't notice it then. Should only get penalised one mark out of the four possible.

Reply 1225

ahct_952

does anyone have the paper or an unofficial mark scheme yet?

Reply 1226

Malabarista

Original post by HarryW95

Ah right maybe I didn't notice it then. Should only get penalised one mark out of the four possible.

That's if I'm right of course, didn't sit well with me that one. However, I imagine there'll be marks for amplitude, wavelength and general shape, so what, 1 mark lost at worst?

Reply 1227

OllieReynolds

Original post by mora11

Anyone get the transition was from n=3 to n=2 on question 8?

also pretty standard paper tbh I would expect the A boundary to be around 70 marks and the full UMS boundary to be around 86-89ish.

I counted only 5 marks which I know I have dropped (missed a 2 marker :frown:

, didn;t even see it lol) so hoping for full UMS should be safe for an A in physics as now :smile:

if i find time within the next few days i will do a unofficial mark scheme but pretty busy atm

LOL No one will want to trust your mark scheme

Reply 1228

-Lychee

Anyone has the mark scheme yet?

Reply 1229

-Lychee

Original post by OllieReynolds

LOL No one will want to trust your mark scheme

yh ... i think the boundary wont be THAT high either... its a middleish paper..

Reply 1230

username916760

Original post by HarryW95

No, it doesn't! If R=V/I and V is reasonably constant (which it was, hence the steep gradient after 1.8V), and current is rapidly increasing, R decreases. Just substitute values into the equation and check for yourself.

Example from past paper, from what I remember this graph was very similar to ours. January 2010 if anybody wants to check.

http://gyazo.com/eafb7b433afb3946ae23333e57aa99f6

Answer to question: [markscheme]

i have done this paper (well i did all the papers), it says in the ms that r is steady after 1.8. check the ms. and thank u because if i had a smallest doubt you made me sure.

Reply 1231

Nick_

does anyone remember what they got for the length of the filament of the bulb?

Reply 1232

username916760

Original post by geditor

Oh well, I lost about 10-15 marks so I reckon I can still get a high B (around 100 UMS); really disappointed about the LED question. It's so obvious but just didn't give it two thoughts due to exam pressure. I did well in unit 1, so I might be able to scrap an A overall. I got 32 I think in the practicals - do you know roughly what UMS that would be?

if you have lost 10_15 sure it is an a. even with the highest boundary.

Reply 1233

HarryW95

Original post by mashmammad

i have done this paper (well i did all the papers), it says in the ms that r is steady after 1.8. check the ms. and thank u because if i had a smallest doubt you made me sure.

No, I'm afraid it doesn't. Resistance decreases.
Stop making stuff up.. :rolleyes:

http://gyazo.com/e5aea8bb9eaa4e9ff47195d1aaec73da

no current/no light/does not conduct until V is greater than 1.5 V
brightness/intensity of LED increases with current/voltage above 1.5 V
above 1.8 V current rises almost linearly with increase in p.d./AW
the LED does not obey Ohm’s law
as I is not proportional to V/AW
below 1.5 V, LED acts as an infinite R/ very high R/acts as open switch
above 1.5 V, LED resistance decreases (with increasing current/voltage)

Reply 1234

username916760

Original post by HarryW95

No, I'm afraid it doesn't. Resistance decreases.
Stop making stuff up.. :rolleyes:

http://gyazo.com/e5aea8bb9eaa4e9ff47195d1aaec73da

open your eyes please it says above 1.5. when it reaches 1.5 it decreases to allow current going through. but what i say is its steady from 1.8v

Reply 1235

HarryW95

Original post by mashmammad

open your eyes please it says above 1.5. when it reaches 1.5 it decreases to allow current going through. but what i say is its steady from 1.8v

It says:

above 1.5 V, LED resistance decreases (with increasing current/voltage)

It DIDN'T say:

above 1.5 V, LED resistance decreases (with increasing current/voltage) until 1.8 V

Read the markscheme. If you don't want to believe it, plug the figures in and see for yourself. Resistance decreases after 1.5V all the way to 2.0V.

I'm sorry but you got the answer wrong..

Reply 1236

username916760

Original post by mashmammad

open your eyes please it says above 1.5. when it reaches 1.5 it decreases to allow current going through. but what i say is its steady from 1.8v

i mean the third line of this very ms that you have put up.

Reply 1237

username916760

Original post by HarryW95

It says:

It DIDN'T say:

Read the markscheme. If you don't want to believe it, plug the figures in and see for yourself. Resistance decreases after 1.5V all the way to 2.0V.

I'm sorry but you got the answer wrong..

the third line. read it mate.

Reply 1238

HarryW95

Original post by mashmammad

i mean the third line of this very ms that you have put up.

Yes.. Current increases almost linearly with increase in P.D.
R=V/I If current is increasing almost linearly, resistance is decreasing. Go check it for yourself.
Why are you even telling me to look at that line? That doesn't suggest resistant is constant in any way what so ever.

If you gave yourself full marks for that question in a past paper, you're marking it wrong.

(edited 11 years ago)

Reply 1239

username916760

Original post by HarryW95

Yes.. Current increases almost linearly with increase in P.D.
R=V/I If current is increasing almost linearly, resistance is decreasing. Go check it for yourself.

what???? linear means constant instantaneous resistance. you may have a line that doesnt go through origin. but it doesnt mean the gradient is not constant.