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

can anyone help me in the best way to work out potential difference and current through resistors in parallel???? :smile:

Reply 181

Dale12

Original post by shyro1000

doing june 2010 paper, anyone know how to do 1ciii? :smile:

did that peper this morning :smile:

refering to I=naVe
V=I/nae
that means V is proprtional to 1/a
since from x to y the area is halving, that means that V will be increasing by a factor of 2

Reply 182

Toshiya

1

I just did a practice paper, got 62/100, was really downhearted before I checked the grade boundaries and saw for that season 56/100 was an A...
Those thresholds are insanely low, I'm just gonna have to hope I can get around 65 and get a low A out of it >_>

Anyway, what I wanted to ask was, can anyone give a list (and brief answers) of any define and explain questions we might get asked? There was a 'explain what the mean drift velocity is' question on the paper I just did and I was completely stumped... want to make sure that doesn't happen tomorrow. I need all the marks I can get!

Reply 183

JazzyM94

Can someone help me real quick please?
im stuck on the photoelectric effect.. i thought it was the gold leaf experiment, but in the mark scheme for jan 2011, it doesn't even mention the gold leaf experiment and its a 6 mark question
the markscheme just says - a photon is absorbed by an electron (in a metal surface); causing electron to be emitted (from surface)., and a whole lot of stuff about the equation hf = workfunction + KEmax

what would i write?

Reply 184

Dale12

Original post by JazzyM94

Can someone help me real quick please?
im stuck on the photoelectric effect.. i thought it was the gold leaf experiment, but in the mark scheme for jan 2011, it doesn't even mention the gold leaf experiment and its a 6 mark question
the markscheme just says - a photon is absorbed by an electron (in a metal surface); causing electron to be emitted (from surface)., and a whole lot of stuff about the equation hf = workfunction + KEmax

what would i write?

The gold leaf electroscope is just for visual purposes so u can see the leaf becoming less negatively charged... you need to mention E=hf and hf=(phi) +KEmax.... you need to say how 1 photon interacts with 1 electron transfering all its energy to escape the surface of the metal.. if the wave has a frequency lower than the threshold frequency, then E=hf cant be achieved and the work function cant be obtain (energy needed to remove electron form metal surface).... then also mention the intensity... that even if the intesity is increased, if the wave doesnt have a high enough frequency, then more per second, still wont remove the electron from the form the metal.. sorry its abit wordy, but hope it helps and can help with any more questions

Reply 185

Rothers

Ah i remember this! I'm doing the A2 of it now, when/if you go on to do the A2, you'll look back at this and smile at how much more simple life was!

Good luck everyone! :smile:

Reply 186

JazzyM94

Original post by ebmaj7

•

Energy from photons (eg .UV photons) is absorbed by a metal (work function).

•

When a photon enters the metal, it gets absorbed by an electron.

•

The electron then uses this energy as kinetic energy in order to release itself (eg. jump over a vacuum) which then creates a current.

•

The more intense the UV light, the higher the current will be. (More UV photons being absorbed by more electrons.)

•

One to one ratio. One photon is absorbed by one electron.

That's what I would write. And I'd state the formula and what all the symbols meant.

Thanks alot! :smile:

Reply 187

Pangol

15

Original post by JazzyM94

Can someone help me real quick please?
im stuck on the photoelectric effect.. i thought it was the gold leaf experiment, but in the mark scheme for jan 2011, it doesn't even mention the gold leaf experiment and its a 6 mark question
the markscheme just says - a photon is absorbed by an electron (in a metal surface); causing electron to be emitted (from surface)., and a whole lot of stuff about the equation hf = workfunction + KEmax

what would i write?

It entirely depends on how the question is asked. On the Jan 2011 paper, it asks what the photoelectric effect is and how you can explain it, making particular reference to the conservation of energy. On the Jun 2009 paper, it asks you to show with the aid of a diagram an experiment to demonstrate the photoelectric effect. The electroscope setup would be appropriate for the Jun 2009 question, but not for the Jan 2011 question; that one only needs a defintion of the effect and a theoretical explanation.

For the Jan 2011 question, I would suggest: "The photoelectric effect is the ejection of electrons from a metal when light of a sufficiently high frequency is shone on it.

One photon is absorbed by one electron, after which the photon no longer exists; its energy has been transferred to the electron. The work function

\phi

of the metal is the minimum energy required to release electrons, so by the conservation of energy,

hf = \phi + KE_{MAX}

, i.e. the energy of the incident photon is equal to the work function of the metal plus the maximum kinetic energy of the emitted electron. Photons with energy below

\phi

can never eject electrons; photons with energy above

\phi

can eject electrons, however low the intensity of the light."

Reply 188

the real viper

1

Original post by Abused Tampon

do we need to know and memorise all the wavelength ranges?
if yes, is there any quick way of doing it?

this is more of a short term memory thing i think.....so just memorise it 5 minutes before the exam tbh :wink:

or just memorise like an individual value for each.

Reply 189

demesto

Yeah, btw do we need to know the range for a member of the electromagnetic spectrum, or can we just memorise a value that falls into the range?

Reply 190

Dale12

Original post by demesto

Yeah, btw do we need to know the range for a member of the electromagnetic spectrum, or can we just memorise a value that falls into the range?

it would be better to memorise the ranges so if you get given a wavelength you can think about what part of the spectrum it is

Reply 191

Abused Tampon

6

Original post by Dale12

it would be better to memorise the ranges so if you get given a wavelength you can think about what part of the spectrum it is

some ranges overlap o.O
what do you do then?

Reply 192

Joseppea

OP

2

Original post by Abused Tampon

some ranges overlap o.O
what do you do then?

They'll make it obvious what you're supposed to do, e.g. they might say "A typical wavelength of radiowave" then it doesn't matter if it overlaps because it's specified as a radiowave. Or they'll have some ranges you have to match it to. :biggrin:

......................I hope

Reply 193

Dale12

Original post by Abused Tampon

some ranges overlap o.O
what do you do then?

sy if you have to find the wavelength of a wave with a certain frequency.. and it has a wavelngth of 1 x 10^-11 well that cud be x rays or gamma rays, but in the mark scheme they will accept both answers, however it is more likely they will give you a wavelength that goes in to a specific group such as visible light (700-400nm) or uv (4x10^-7 -- 10^-8)

Reply 194

sweetascandy

18

Original post by Dale12

you can use youngs double slit experiment, where a monochromatic light is shone through a double slit of roughly the same wavelength to cause diffraction of the two sources.. they then cause constructive and destructive interference depending on the path difference.. for sound/ microwave you use two coherence sources of sound/microwaves and you get alternating patterns of strong and weak signals

Original post by Pangol

Light: I'm sure you know this, it's effectively the Young double-slit experiment.

Sound: two loudspeakers, connected to the same signal generator set to ~250 Hz, side by side but separated by about two metres. Walk perpendicular to them about four metres away, and you hear loud and quiet points, analagous to the bright and dark fringes.

Microwaves: single microwave transmitter, directing microwaves through two ~3 cm slits in a piece of metal. Microwaves emerge, diffract and interfere. High ans low intensity points can be picked up with a microwave detector.

Original post by Joseppea

This is what we did in class to demonstrate it. We placed 2 speakers next to each other with a little bit of space between them. These transmitted a sound radiating out from the speakers. We then walked along the other side of the class room and listened and could hear it go quiet...then loud...then quiet again where the waves overlapped etc. :redface:

There's probably better ways of explaining it, but this is how we did it :tongue:

Edit: The above person's explanation is ALOT better than mine! :biggrin:

He must have wrote it at the same time :L

Thanks so much guys. But btw, is the 2source interference pattern for light waves EXACTLY the double slit experiment? What I mean is that does it literally initially diffract through one slit, and then diffracts through another 2 slits (coherent) before reaching the screen?

+rep going your way (all of you!) :smile:

EDIT: "You have reached the limit of how many posts you can rate today!"..... Sorry :colondollar:

(edited 12 years ago)

Reply 195

Dale12

Original post by sweetascandy

Thanks so much guys. But btw, is the 2source interference pattern for light waves EXACTLY the double slit experiment? What I mean is that does it literally initially diffract through one slit, and then diffracts through another 2 slits (coherent) before reaching the screen?

+rep going your way (all of you!) :smile:

you use a laser so the light is coherent and monochromatic (only one wavelngth present) then its shone trhough two slits to create the diffraction, then it hits the screen to show the interference, if that helps :smile:

Reply 196

Toshiya

1

Anyone have the Jan '11 paper and mark scheme? Can't find it, and the lack of past papers because it's a new syllabus is really annoying. I want to get one more in that I haven't done before the exam tomorrow!

Reply 197

Dale12

is it just me or does anyone else get confused on the power section about calculating e.m.f and determining p.d through resistors in parallel etc :colondollar:

?

Reply 198

Dale12

Original post by goerigi

yeah for me personally its my weakest area :/

same andi keep doing question after question but still not sinking in abhout how to go about doing them :/ just gotta hope they dont come up tomorrow much :/