OCR Physics A G485 - Frontiers of Physics - 18th June 2015

Original post by Zoze

Can someone please explain to me the evolution of the sun? The cambridge book explanation is confusing and I feel it's wrong? Every source I've looked at says that the fusion of protons in the hydrogen shell causes the expansion. :s for example http://www.astro.cornell.edu/academics/courses/astro201/evol_sun.htm and wikapedia and lots of other sources :frown:

Hydrogen fusion in the core decreases as the core mainly consists of fusion products such as helium nuclei. Hence the core will collapse due to the radiation pressure decreasing. The loss of GPE and gain in KE further increases temperature and the pressure of the core.The increase in pressure of the core causes the Sun to expand, becoming a red giant. At temeperatures of 10^8K, helium nuclei will fuse at a phenomenal rate and when this fusion comes to an end, the Sun will become unstable and will radiate most of its mass outwards in huge ion sprays. The rest of the mass and the core will shrink, forming a white dwarf.

Reply 644

Can someone please tell me what you would say for this...

Compare and contrast decay of radioactive nuclei and decay of charge on a capacitor a C-R circuit

It's straight out of the specification.

thanks.

Reply 645

Elcor

14

Apparently the spontaneous nature of radioactive decay means the decay can't be induced, but why doesn't fission count as induced? Is it because when a thermal neutron collides with an unstable nucleus you're actually making it a very unstable isotope for a split second, so that doesn't count as inducing the decay?

Reply 646

Elcor

14

Original post by sagar448

Can someone please tell me what you would say for this...

Compare and contrast decay of radioactive nuclei and decay of charge on a capacitor a C-R circuit

It's straight out of the specification.

thanks.

Both are exponential
Rate of charge decay depends on C and R
Rate of radioactive decay depends on the decay constant

Reply 647

Original post by Elcor

Both are exponential
Rate of charge decay depends on C and R
Rate of radioactive decay depends on the decay constant

Thanks

Reply 648

hol31

Can anyone explain electromagnetic induction for me? I wouldnt know where to begin explaining it. Especially with regard to emf induced in a coil.

Thanks

Reply 649

coder4

Original post by rachelc142

I feel like they're slightly better for 485 than for 484
some of the mech stuff was stupid so I avoided them
I'm just going over my notes atm

Which textbook do you use? theres 2 out for physics, the one i have this year is pretty good

Reply 650

boristeve

Original post by hol31

Can anyone explain electromagnetic induction for me? I wouldnt know where to begin explaining it. Especially with regard to emf induced in a coil.

Thanks

in what context? A transformer?

if so, in a transformer an AC is passed through the primary coil of the transformer. This changing emf in the PC produces a constantly changing magnetic flux. (since a magnetic flux is produced when there is a change in the emf, using DC would produce flux when it is switched on, as that is a change in emf, but then the flux would dissapaer since the emf is not changing. so transformers do not work with DC.)

The iron core of the transformer causes a flux linkage between the two coils, and that means a changing magnetic flux is produced in the secondary coil. Due to faradays law, this changing flux induces an emf in the secondary coil, and the emf is proportional to the number of turns on each coil.

(edited 8 years ago)

Reply 651

boristeve

Original post by Elcor

Apparently the spontaneous nature of radioactive decay means the decay can't be induced, but why doesn't fission count as induced? Is it because when a thermal neutron collides with an unstable nucleus you're actually making it a very unstable isotope for a split second, so that doesn't count as inducing the decay?

No, when the neutron is absorbed by a heavy nucleus, it excites it with it's kinetic energy, the nuclei splits into two daughter nuclei and emits more neutrons. The reason this isn't decay is because i think it is not exponential and has nothing to do with the decay constant. I also (though i could be wrong) don't think there is a weak force interaction involved with nuclear fission, whereas there is in other kinds of radioactive decay

Reply 652

Tiwa

7

Original post by rachelc142

any nice revision suggestions/sources? I think I've exhausted past papers

Some revision notes and some extra past papers in this link. http://www.st-ambrosecollege.org.uk/1273/academic/science---physics/physics-a2-level/

Reply 653

When a capacitor is charging or discharging through a resistor then the voltage across the capacitor relative to time should be a curve up or curve down since it's exponential.
The question in January 2013 1ii2 says it's a straight line through the origin? Please help?

Reply 654

L'Evil Fish

OP

18

Original post by sagar448

When a capacitor is charging or discharging through a resistor then the voltage across the capacitor relative to time should be a curve up or curve down since it's exponential.
The question in January 2013 1ii2 says it's a straight line through the origin? Please help?

I just did that one, lost a mark there... Not sure why

Reply 655

boristeve

Original post by sagar448

When a capacitor is charging or discharging through a resistor then the voltage across the capacitor relative to time should be a curve up or curve down since it's exponential.
The question in January 2013 1ii2 says it's a straight line through the origin? Please help?

the key is it says current is constant 40 microamp. since

Q = CV

It = CV

V = It/C

I and C are constants

V = kt

there fore a straight line through the origin :smile:

Reply 656

Oraeng

9

On Jan 13 6)c)i) there's a question about calculating intensity using I=Ioe^μx
Why don't you have to convert the units of x and μ from cm to m if the initial intensity is given in Wm^-2 and the final intensity is in Wm^-2?

(edited 8 years ago)

Reply 657

L'Evil Fish

OP

18

Original post by Oraeng

On Jan 13 6)c)i) there's a question about calculating intensity using the I=Ioe^μx
Why don't you have to convert the units of x and μ from cm to m if the initial intensity is given in Wm^-2 and the final intensity is in Wm^-2?

Because mu and x cancel out the units

Reply 658