OCR PHYSICS B G495~ 18th June 2015 AM ~ A2 Physics

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Why is the answer A?

Reply 83

Original post by Mutleybm1996

I hear G484 was horrible this year
With even top students barely scraping 40

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Guess it's just physics in general! And I'm going to be doing it at uni, I enjoy learning about it but I hate the examinations on it...

Reply 84

Original post by Ghengis

Why is the answer A?

Flux is uniform in the whole iron core, it's flux density that is different due to the change in the number of coils on either side of the transformer. It is not flux linkage as there are different numbers of coils on either side of the transformer, hence flux linkage isn't the same.

Reply 85

Original post by RealTime

Flux is uniform in the whole iron core, it's flux density that is different due to the change in the number of coils on either side of the transformer. It is not flux linkage as there are different numbers of coils on either side of the transformer, hence flux linkage isn't the same.

thanks

Reply 86

Original post by urz13

How/why does the emission of secondary electrons from the sample limit the resolution of the electron microscope?
Also, is the resolution essentially determined by the size of the diffraction spots, which theoretically could be as small as the wavelength of the electrons? Can't quite get my head around these ideas which seem to be suggested by the questions on P1, so if someone could clear it up for me that'd be great :smile:

I'm unsure about the secondary electrons thing, I don't want to give you a potentially wrong answer.

However, resolution is the smallest resolvable thing, which in terms of visible light would be half the wavelength of the smallest wavelength of light ~250nm. Diffraction spots are like the maxima and minima in Young's double slit experiment where the separation of maxima and minima corresponds to a factor of the wavelength of light, so if you can work out the wavelength from this then the resolution would be half of this calculated wavelength,

Reply 87

STATER

Original post by urz13

How/why does the emission of secondary electrons from the sample limit the resolution of the electron microscope?

What makes you say that?
According to wikipedia, a secondary electron detector in fact maximises the SEM's resolution.

(edited 8 years ago)

Reply 88

What's the formal definition of flux linkage?

Reply 89

urz13

8

Original post by STATER

What makes you say that?
According to wikipedia, a secondary electron detector in fact maximises the SEM's resolution.

This question:
A de Broglie wavelength of 5 pm for 50 kVelectrons suggests that the resolution of an electron microscope isconsiderably better than the best optical microscopes. What factors may limitthe resolution of an electron microscope in practice?

And the answer:
-Electrons all have negative charge so repel one another, limiting minimum spot size
-Electrons may cause local heating of sample that leads to secondary electrons.

It has me a bit confused!

Reply 90

STATER

Original post by urz13

This question:
A de Broglie wavelength of 5 pm for 50 kVelectrons suggests that the resolution of an electron microscope isconsiderably better than the best optical microscopes. What factors may limitthe resolution of an electron microscope in practice?

And the answer:
-Electrons all have negative charge so repel one another, limiting minimum spot size
-Electrons may cause local heating of sample that leads to secondary electrons.

It has me a bit confused!

I would just mention relativistic effects tbh. You can say that the it becomes impractical to decrease the de Broglie wavelength of the electron beyond a certain point due to relativistic effects. Seems like much less of a headache.

Reply 91

what is the relationship between no of turns in a coil and the flux density?

Reply 92

urz13

8

Can anyone help with this? Cheers

Use the relationship Bqr= energy of particle/speed of light

to show that the unit of magnetic field strength, the Tesla, is equal to N A^-1 m^-1

(edited 8 years ago)

Reply 93

Rhetorical Hips

14

Don't know about you guys but I'm just winging this exam, haven't done any past papers yet :/

Reply 94

Mars256

8

Original post by Mutleybm1996

90% average across both
You could get 95% then 85% and get an A*

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..

just checked the spec, you're right, my bad

Reply 95

qivo

Original post by urz13

This question:
A de Broglie wavelength of 5 pm for 50 kVelectrons suggests that the resolution of an electron microscope isconsiderably better than the best optical microscopes. What factors may limitthe resolution of an electron microscope in practice?

And the answer:
-Electrons all have negative charge so repel one another, limiting minimum spot size
-Electrons may cause local heating of sample that leads to secondary electrons.

It has me a bit confused!

I think the problems with the secondary electrons in this case is that they are not being 'released' by the bombardment from the electron beam but by heating of the sample and this therefore interfers with the secondary electrons produced by the beam, which is what you are trying to measure??? this may be possibly the reason?

Reply 96

Original post by Rhetorical Hips

Don't know about you guys but I'm just winging this exam, haven't done any past papers yet :/

oooh

good luck! Hopefully it is a reasonable exam

Reply 97