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

Original post by ChloeA

I = nAve

For x
V= I/(2Ane) As X has twice the cross sectional area
V= 2x10^-5

For y
V = I/Ane
V= 4x10^-5

:smile:

great thanks :biggrin:

Reply 143

Original post by sweetascandy

Experiment to Determine the Planck Constant

• Connect an LED of known wavelength in an electrical circuit (which has a battery, voltmeter, LED and potential divider)
• Start off with no current flowing through the circuit, then adjust the variable resistor until a current just
begins to flow
• Record the voltage (V0) across the LED, and the wavelength of the LED
• Repeat this experiment with a number of LEDs that emit different optical wavelengths
• Plot a graph of the threshold voltage (V0) against 1/? (where ? is the wavelength of the light in metres)
• On the graph, you should get a straight line, of gradient 'hc/e' where h is the Planck constant, c is the speed
of light and e is the elementary charge
• From this, Plank constant (h) can be calculated

You do not need to know the wavelength of the LED, obviously it does help. You can use the n(lambda)=dsin(theta) to find the wavelength - LED through diffraction grating and bright/dark fringes.

This would take longer but my teacher does reckon this will come up as it involves quantum mechanics, waves and electricity - all three.

Reply 144

Pangol

15

Original post by lifesabeach

You do not need to know the wavelength of the LED, obviously it does help. You can use the n(lambda)=dsin(theta) to find the wavelength - LED through diffraction grating and bright/dark fringes.

Although this is very useful to know, I doubt that you would be expected to combine the two practicals into one. For one thing, it is very hard to see the bright spots from LED light through a diffraction grating; you would normally need something much more intense like a laser beam.

They may ask about either of these things, and it is great to know both in detail, but I would be surprised if they are combined.

Reply 145

sweetascandy

18

Original post by lifesabeach

You do not need to know the wavelength of the LED, obviously it does help. You can use the n(lambda)=dsin(theta) to find the wavelength - LED through diffraction grating and bright/dark fringes.

This would take longer but my teacher does reckon this will come up as it involves quantum mechanics, waves and electricity - all three.

Why would you go through all that when you can do it the simple way (my simple way)?! :-/

Reply 146

Original post by Pangol

Although this is very useful to know, I doubt that you would be expected to combine the two practicals into one. For one thing, it is very hard to see the bright spots from LED light through a diffraction grating; you would normally need something much more intense like a laser beam.

They may ask about either of these things, and it is great to know both in detail, but I would be surprised if they are combined.

You are right and if was the case, it would probably suggest the wavelength is unknown in the question, or it may be a 7 - 8 mark question with plenty of writting space. I would judge whether I would put it in as to how they worded the question and if it just said 'Describe an experiment to determine planck's constant' then I would probably leave it out, depending on whether I think I could make say, 5 clear cut points, for a 5 mark question.

Reply 147

Original post by sweetascandy

Why would you go through all that when you can do it the simple way (my simple way)?! :-/

Your not asked to carry out the experiment, only describe it. I was only making you aware, if you wern't already, that you can do it that way, as well.

Reply 148

wow, pretty complicated, but atleast theres two ways of doing it

Reply 149

wibletg

3

Original post by shyro1000

wow, pretty complicated, but atleast theres two ways of doing it

yeah, I think eV = hc/lambda will be my method :tongue:

Nice and simple.

Reply 150

Original post by wibletg

yeah, I think eV = hc/lambda will be my method :tongue:

Nice and simple.

i agree, thank god for the simple formula's rather then the real long awkward difficult ones :smile:

Reply 151

Do I revise for the exam tomorrow or go watch Elton John who is playing in my town tonight?

Reply 152

Original post by lifesabeach

Do I revise for the exam tomorrow or go watch Elton John who is playing in my town tonight?

revise half the day then elton john :biggrin:

compromise ftw

Reply 153

Original post by shyro1000

revise half the day then elton john :biggrin:

compromise ftw

Sounds a plan. What do you think is the hardest part of this module?

Reply 154

wibletg

3

Original post by lifesabeach

Sounds a plan. What do you think is the hardest part of this module?

The gay way they ask the questions. The content isn't particularly difficult, but the way they ask to see your knowledge is. :mad:

Reply 155

Joseppea

OP

2

Original post by wibletg

The gay way they ask the questions. The content isn't particularly difficult, but the way they ask to see your knowledge is. :mad:

Not as bad as one of the Chemistry past papers, it was something like
"Compare the percentage yield and the atom economy of this reaction"

And the first point on the mark scheme was "Ignore any comments regarding percentage yield." :eek:

Though I agree Physics has some ridiculously worded questions :frown:

Reply 156

kingboom4

4

anyone willing to share their revision notes? Will be greatly appreciated.

Reply 157