PHYA5 ~ 20th June 2013 ~ A2 Physics

I usually say that a black hole is very dense and has a very large gravitational field. It has an escape velocity greater than the speed of light, c. I think that would suffice for a definition. :smile:

Thank you

Do you have any predictions on the 6 markers for astro?

Reply 481

amish123

Original post by JoshL123

Thank you

Do you have any predictions on the 6 markers for astro?

I reckon it could be comparing the adv/disadv of refracting/reflecting telescopes. But I haven't properly started physics revision as I've been concentrating on my other maths exams. Just need to get S2 out the way on Thursday then I'll hit physics!

Reply 482

UnvesedSplash

hi peeps does anyone have any good resources for explaining like the differences between the electron and alpha scattering the textbook isn't that good in explaining it :/ i dont get why it more useful for estimating the diam of a nucleus

Reply 483

ehtisham_1

Don't really think that's going to come up this year as its already come up.

Reply 484

amish123

Original post by UnvesedSplash

hi peeps does anyone have any good resources for explaining like the differences between the electron and alpha scattering the textbook isn't that good in explaining it :/ i dont get why it more useful for estimating the diam of a nucleus

Alpha scattering:
In simple terms, you're firing alpha particles at a thin piece of gold foil; it has to be thin in order to prevent multiple scattering which can be hard to interpret. Now, we know a nucleus is positivley charged and also a alpha particle has an overall positive charge, so this experiment relies on electrostatic repulsion. What this means is, some alpha particles (1/2000) will just be deflected off the nucleus. Around 1/10000 will be deflected by an angle greater than 90 degrees. Some alpha particles will be on course to having a head on collision with the atoms in the foil, however due to electrostatic repulsion, the alpha particle will not actually touch the nucleus of the atoms in the foil. Instead, you can measure the least distance of approach which provides an upper limit to the radii of the atoms in the foil. But remember that the strong nuclear force also acts here, so this will recoil the nucleus and push it away from the atoms in the foil, giving a less accurate result for the closest point of approach. To sum up, using alpha scattering, you are not measuring the radius of a nucleus, you are simply measuring the least distance of approach.

Electron diffraction:
What happens here is that you are firing a monoenergetic (all electrons have the same energy) beam of high energy (we'll explain why they need such high energies later) electrons at a thin solid sample of metal. This experiment relies on electron diffraction and treats the electrons as behaving like a wave with a certain de Brogile wavelength. What actually happens is we can fire this beam at a certain angle to the perpendicular normal from the sample. As this angle is slowly increased and more and more data is gathered, you'll eventually end up with a electron diffraction pattern, similiar to that from the single slit experiment:

Using this data, and the equation Rsintheta=0.61lambda (where R is the radius of the nucleus and lamba is the de Brogile wavelength), we can work out an estimate for R. We can look at the first minimum from the diffraction pattern to work out a value for theta. Now, the de Brogile wavelength of the electrons needs to comparable to that of the nuclear diameter (~10-¹⁵m). In order to have such a short wavelength, we need to make the electrons travel very fast, hence they need to have high kinetic energies (in accordance with the formula lambda=h/mv, the bigger the value for v, the smaller the value for lamba). Electron diffraction gives a more accurate estimate to the value of R as it isn't affected by the strong nuclear force, it measures the actual value of R and not the least distance of approach.

That should be all you need to know, hope it helps :smile:

(edited 10 years ago)

Reply 485

JoshL123

2

Original post by amish123

I reckon it could be comparing the adv/disadv of refracting/reflecting telescopes. But I haven't properly started physics revision as I've been concentrating on my other maths exams. Just need to get S2 out the way on Thursday then I'll hit physics!

Yeah I;ve been focusing on C4 quite a bit after a disastrous mechanics one exam :/. You definitely seem prepared :smile:

! Are you going for the A* in physics?

Reply 486

sports_crazy

I made a list of possible 6 marker on astrophysics.

1) Stellar Evolution: Life cycle of small and large stars.
2) Advantages and Disadvantages of mirrors and lenses in telescopes.
3) Evidence of Big Bang or universe expanding.
4) Different types of telescopes: Radio, X Ray and optical.
5) Charged Couple Device
6) Line Spectra and absorption spectra. Can mix this with Doppler effect.

Telescopes came last year, it may not come this year. Revise everything to be on the safe side. :smile:

Reply 487

JoshL123

2

Original post by sports_crazy

I made a list of possible 6 marker on astrophysics.

1) Stellar Evolution: Life cycle of small and large stars.
2) Advantages and Disadvantages of mirrors and lenses in telescopes.
3) Evidence of Big Bang or universe expanding.
4) Different types of telescopes: Radio, X Ray and optical.
5) Charged Couple Device
6) Line Spectra and absorption spectra. Can mix this with Doppler effect.

Telescopes came last year, it may not come this year. Revise everything to be on the safe side. :smile:

Nice mate! They've done the telescopes one :smile:

Reply 488

amish123

Original post by JoshL123

Yeah I;ve been focusing on C4 quite a bit after a disastrous mechanics one exam :/. You definitely seem prepared :smile:

! Are you going for the A* in physics?

Ahh I see mate, hope it pans out for you! :smile:

Thing is, I know most of the wordy kind of questions in my head, I just find it hard putting it in writing! And given the time constraints of the exam, I sometimes get muddled up with my explanations, especially the dreaded 6 markers. Yeah I am, I did quite well in Unit 4 so my physics teacher has predicted me an A*, I'll just give it my best shot and hope for the best :smile:

Reply 489

amish123

Original post by sports_crazy

I made a list of possible 6 marker on astrophysics.

1) Stellar Evolution: Life cycle of small and large stars.
2) Advantages and Disadvantages of mirrors and lenses in telescopes.
3) Evidence of Big Bang or universe expanding.
4) Different types of telescopes: Radio, X Ray and optical.
5) Charged Couple Device
6) Line Spectra and absorption spectra. Can mix this with Doppler effect.

Telescopes came last year, it may not come this year. Revise everything to be on the safe side. :smile:

Great list, I reckon either 1 or 4 will come up. But by judging how my AQA exams have gone so far (quite different to the past papers), anything could come up, so I guess all round knowledge is required!

Reply 490

sports_crazy

Original post by amish123

Great list, I reckon either 1 or 4 will come up. But by judging how my AQA exams have gone so far (quite different to the past papers), anything could come up, so I guess all round knowledge is required!

Number 4 came twice before including last year. So I doubt it will come up.
Number 1 got a good chance. They haven't asked us about doppler shift.

Can you please sum up the key points for number 4 like you did for diffraction and alpha scattering.

Reply 491

amish123

Original post by sports_crazy

Number 4 came twice before including last year. So I doubt it will come up.
Number 1 got a good chance. They haven't asked us about doppler shift.

Can you please sum up the key points for number 4 like you did for diffraction and alpha scattering.

Good points there, I will sum no.4 when I get a chance to.

Reply 492

bugsuper

8

There are lots of different questions that could come up on the astrophysics module, but to my knowledge they've never done one on the lifecycle of a star. Whether that means it's not the sort of questions they'd ask as a six-marker, or whether that boosts the probability, I don't know.

Reply 493

sports_crazy

Original post by bugsuper

There are lots of different questions that could come up on the astrophysics module, but to my knowledge they've never done one on the lifecycle of a star. Whether that means it's not the sort of questions they'd ask as a six-marker, or whether that boosts the probability, I don't know.

True there are lots of possibilities. Can you please answer my significant figures question that I posted before. I am still confused on it.

Reply 494

josephtsui

Anyone doing turning points? There just seems to be so much to memorise, it's almost like history or biology than physics...

Reply 495

bugsuper

8

Original post by sports_crazy

So we will not loose marks if the answer is 795 and I get 796 as long as my working out is right. Unless they mention correct sf in the question.
One more example is that I multiply 25 x 971= 24275. So do I write it as 24000(2sf) or 24300(3sf).

I did not know that you use the exact worked out answer when you multiply it by something else. Never use the rounded answer if you are going to use it for more working out. :smile:

I thought I'd answered this question to the best of my ability.

The thing is, they'll never give you "25" and "971" as numbers. They'll normally be expressed in standard form, like 6.3 * 10^2 (2sf) or 6.30 * 10 ^2 (3sf)

In the case you specify, I'd probably go with 24300, because 25 hasn't been rounded to 2sf, it's an exact value

But, to be honest:

1) It really doesn't matter on questions where they don't specify that sig figs are important. When they do, you'll be able to follow the "no-more-than-the-data-in-the-question" rule, because they wouldn't write a question ambiguously like that.
2) If you're concerned, write your answer in the working to a greater number of sf, and then show that you've rounded it to a slightly different value. That way, they can't dispute that you've done the working correctly