PHYA5 ~ 20th June 2013 ~ A2 Physics

The inversion tube experiment is defined in the chapter on specific heat capacity, but it's really simple

you get a tube of known length, and a known mass of the substance whose specific heat capacity you want to test. mgl = the change in gravitational potential energy when the substance falls from one end of the tube to the other. You turn it over about 50 times, so 50mgl is the total change in gpe; then you assume that all of this was converted into thermal energy (when the substance hit the bottom of the tube), measure its temperature change, and hey presto, you can calculate specific heat capacity

Yeah I spoke too soon, I was looking for the experiments in those How Science Works boxes :redface:

I do actually know the inversion tube experiment, I just didn't realise that's what it was called. Thanks for your explanation though :smile:

feeling a bit less stressed now haha

Reply 1121

Serpentine111

12

Original post by Narwhaleninja

You can't retake it because it's the last exam, so no one ever does it in January so there's no papers.

Seems odd because they have papers for Chemistry, Maths etc in Jan as well... the grade boundaries for these papers also seem to be absurdly high for Physics :s-smilie:

Reply 1122

anuradha_d

1

Original post by Anythingoo1

Hey, it seems like you're really wised up about this, so I have a few questions to ask you if it's not too much trouble :smile:

Do we need to know what the Black-body radiation curves look like, as in do we need to be able to replicated them in some sort of way?

I don't really know what I'm doing regarding stellar spectroscopy, could you give me some pointers on those? Like the H Balmer series, I'm not really sure what they're for.

Evidence for the big bang?

Thanks!

Thought i'd chip in - you need to know the GENERAL shape of black body radiation curves, and understand why stars can be modelled as black bodies (continuous spectrum of light emitted). You also need to state that as the temperature of the star increases, its peak wavelength decreases (peak moves to the left) and most power radiated increases (leading to Wien's displacement law and Stefan's law)

Star spectroscopy - only need to know in detail about the balmer series due to excitation of H atoms in the n=2 state. This can only happen in hot stars (ie. classes O and B) and no star is hot enough to excite atoms in the n=1 state. In terms of explanation, its pretty much the same as photoelectric effect but in a new context :biggrin:

(edited 10 years ago)

Reply 1123

Jack93o

15

can you guys try this calculation on your calculator?

I got 390.65586

so shouldn't the answer be rounded to 391, not 390?

Reply 1124

igloo1

Original post by crc290

Nooo, don't say that. I know neither :s-smilie:

p152 of AQA book

Reply 1125

fuzzybear

12

june 2011, question 2b ii) in astrophysics

the answer in the mark scheme is given in 3sf, would I lose a mark for giving it in 2sf?

I know you're suppose to leave it in sf's that match the data you're given, but nowhere on that question was there a figure that was in 3sf :confused:

Reply 1126

Thr33

Original post by Jack93o

can you guys try this calculation on your calculator?

I got 390.65586

so shouldn't the answer be rounded to 391, not 390?

According to those numbers, yes.
Are those numbers taken from an earlier part though - so they themselves are rounded?
May just be a mark scheme error.

Reply 1127

aerf

Original post by bugsuper

This depends on which option module you're taking, though

Sorry, that was for turning points

Reply 1128

fayled

9

Original post by anuradha_d

Hey - six marker predictions? (Astro and Core) also im worried about the lack of explanation about Carbon Dating (or i may not have read it properly :P)

Not sure we need to be able to explain it, but if so, something like this I suppose.

Measure background count rate due to carbon-14 in plant matter. Measure count rate of a fresh sample of, for example, a piece of wood. Measure the count rate of the sample of wood being dated. Correct these count rates. Then use the equation x=x_oe^-lambdat with x as count rate of dated sample, x_o as count rate of fresh sample, and calculating lamda from the half-life of carbon-14, to calculate t which is the age of the sample.

Maybe consider some limitations to it, such as the fact that carbon-14 may have made of a different abundance of carbon at the time the dated sample was made, or the presence of contaminants that affect the count rate.

Reply 1129

BucketOfJam

In binary star systems, is the higher apparent magnitude when the brighter star passes the dimmer, or vice versa? I'm a little confused because the notes we got say (referring to a graph of this): "The deeper dips are caused by the dimmer star passing in front of the brighter star (magnitude is higher). The shallower dips occur when the dimmer star passes in front of the brighter." Doesn't really help that much. :frown:

Reply 1130

anuradha_d

1

Original post by fayled

Not sure we need to be able to explain it, but if so, something like this I suppose.

Measure background count rate due to carbon-14 in plant matter. Measure count rate of a fresh sample of, for example, a piece of wood. Measure the count rate of the sample of wood being dated. Correct these count rates. Then use the equation x=x_oe^-lambdat with x as count rate of dated sample, x_o as count rate of fresh sample, and calculating lamda from the half-life of carbon-14.

Maybe consider some limitations to it, such as the fact that carbon-14 may have made of a different abundance of carbon at the time the dated sample was made, or the presence of contaminants that affect the count rate.

Thank you very much! :smile:

I suppose with argon dating it'd be the same except the equation changes and it's used for rocks etc. Thanks!

If anyone is doing astro - no idea how to summarise stellar evolution into a six marker!

Reply 1131

igloo1

Original post by MatLH

Use 10 as the base unit- The formula doesn't use natural logarithms

so how would you do it?

Reply 1132

Jack93o

15

Original post by Thr33

According to those numbers, yes.
Are those numbers taken from an earlier part though - so they themselves are rounded?
May just be a mark scheme error.

yeah two of those numbers being multiplied were calculated from earlier parts of the question

but presumably they used those exact numbers stated, so its strange that they rounded up the number wrong

Reply 1133

Thr33

Original post by Jack93o

yeah two of those numbers being multiplied were calculated from earlier parts of the question

but presumably they used those exact numbers stated, so its strange that they rounded up the number wrong

I expect that they expected you to use your previous answers. Annoying on their part, though...

Reply 1134

Jack93o

15

Original post by BucketOfJam

In binary star systems, is the higher apparent magnitude when the brighter star passes the dimmer, or vice versa? I'm a little confused because the notes we got say (referring to a graph of this): "The deeper dips are caused by the dimmer star passing in front of the brighter star (magnitude is higher). The shallower dips occur when the dimmer star passes in front of the brighter." Doesn't really help that much. :frown:

if you think about it, if you have a bright star blocking a darker star, this would result in overall more light coming through (to you as an observer) than the situation of a darker star blocking a brighter star

so yeah, higher apparent magnitude would be seen when the brighter star is in front of the darker star

Reply 1135

BucketOfJam

Original post by Jack93o

if you think about it, if you have a bright star blocking a darker star, this would result in overall more light coming through (to you as an observer) than the situation of a darker star blocking a brighter star

so yeah, higher apparent magnitude would be seen when the brighter star is in front of the darker star

Ok, thanks very much. :biggrin:

Reply 1136

MatLH

Original post by igloo1

so how would you do it?

m-M= 5 log(d/10)

So d= 10*(10^((m-M)/5)))

Reply 1137

VV.KKGSR

10

Hi, sorry if this has already been asked but does anyone know what s and r stand for in the equation s=r(theta). ??

Reply 1138

Jack93o

15

Original post by anuradha_d

Thought i'd chip in - you need to know the GENERAL shape of black body radiation curves, and understand why stars can be modelled as black bodies (continuous spectrum of light emitted). You also need to state that as the temperature of the star increases, its peak wavelength decreases (peak moves to the right) and most power radiated increases (leading to Wien's displacement law and Stefan's law)

Star spectroscopy - only need to know in detail about the balmer series due to excitation of H atoms in the n=2 state. This can only happen in hot stars (ie. classes O and B) and no star is hot enough to excite atoms in the n=1 state. In terms of explanation, its pretty much the same as photoelectric effect but in a new context :biggrin:

I would add that if a star is too hot, then the balmer series won't occur (or it would be less noticeable)

Because if the star is too hot, the 'default' energy state of hydrogen electrons would be in higher (i.e. at n = 5, 6, 7), so it won't absorb as many of the photons in the balmer series as it would've. To allow for maximum absorption of visible photons (for hydrogen spectrum), the electrons in hydrogen must be in n = 2

to give an example, if the electron is already in the n = 4 state (because the star is so hot), then the photon of a certain colour corresponding to the energy gap between n = 2 and n = 3 (as well as between n = 3 and n= 4) won't be absorbed.