Lets revise physicsA2 togather ultimate thread

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habosh
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#121
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(Original post by haz136)
Hey, doin Edexcel A2 physics and just discovered this wonderful thread. Did a few of the questions but I'm struggling on a lot of Unit 4 stuff. Just been brwosing through a few unit4 papers and got stuck on a few. Was wonderin if some ppl could help me out:

1) Basically there was a question on equipment to produce a stationary wave using a signal generator. Then it says:
How is this phenomenon used to describe the behaviour of the electron in a hydrogen atom?

2) A student observes LED from a distance of 0.20m. The pupil of her eye has a diameter of 6.0mm. Calculate the number of photons which enter her eye per second.
you need to see from the graph the intensity at .2m,then use IA where A is the area of the girl's eye,then use power/energy of the photons you found the previous part,don't worry I couldn't do that either I didn't think of getting the intensity from the graph
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haz136
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Sorry, also says the energy of a photon of this light is approximately 3 x 10^-19 J. My fault! STill can't do it tho
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sumitk87
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i done this question
ok this is how u do it............

raduis of eye is 0.003 area is therefore pi*0.003^2

from graph 0.2m gives Intensity of 0.03Wm-2

0.03wm-2 x pi*0.003^2 = 8.48x10^-7 W ( Js-1)

8.48x10^-7 W ( Js-1) / 3.21x10^-19 J = 2.64x10^12 s-1

so answer is 2.64x10^12 per sec (hence units was s-1 , per second)

Hope that helps
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sumitk87
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if you dont understnd why i have done somin let me kno and illl explain the stages to u if you want but i think u should understand it now since you have got the question infront of you
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sumitk87
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#125
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bump
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nas7232
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#126
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That's the best way to answer them type of questions at A2 standard as you will probably need to do something similiar in phy6.
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Goddess
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I am stuck in a circular motion question.

A car travels around a circular bend of radius 1km and at a constant speed of 108 kmh-1 . Calculate ite angular speed in rad s-1 .

I used v = rω but I don't know how to convert it to rad s-1 .

Can anyone please help?
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Bezza
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#128
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(Original post by Goddess)
I am stuck in a circular motion question.

A car travels around a circular bend of radius 1km and at a constant speed of 108 kmh-1 . Calculate ite angular speed in rad s-1 .

I used v = rω but I don't know how to convert it to rad s-1 .

Can anyone please help?
To get it in rad/s you just need to make sure you're using standard units so convert 108 km/h to 3 m/s then use ω = v/r making sure you use r = 1000m too
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CrazyChemist
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#129
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(Original post by Goddess)
I am stuck in a circular motion question.

A car travels around a circular bend of radius 1km and at a constant speed of 108 kmh-1 . Calculate ite angular speed in rad s-1 .

I used v = rω but I don't know how to convert it to rad s-1 .

Can anyone please help?
Like Bezza said: convert to SI units and do the calculation:

108 km.h-1 = (108 *1000)/(60*60) = 30ms-1
and 1 km = 1000 m

Since v = rω,
ω = v/r
= 30/1000
=0.03 rads.s-1

Edit: If you leave the units as km.h-1 and km then you would finish with an answer in rads.h-1 - you would have to divide this by (60*60) to get it in rads.s-1.
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Goddess
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(Original post by Bezza)
To get it in rad/s you just need to make sure you're using standard units so convert 108 km/h to 3 m/s then use ω = v/r making sure you use r = 1000m too
Like Bezza said: convert to SI units and do the calculation:

108 km.h-1 = (108 *1000)/(60*60) = 30ms-1
and 1 km = 1000 m

Since v = rω,
ω = v/r
= 30/1000
=0.03 rads.s-1

Edit: If you leave the units as km.h-1 and km then you would finish with an answer in rads.h-1 - you would have to divide this by (60*60) to get it in rads.s-1.
Thanks for all the help, guys !!!!!
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CrazyChemist
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Would somebody mind checking my understanding of transformers? I’d appreciate it to see if I’ve got it sussed (btw, I’m doing Edexcel, Salters):

Right – an ac current flows into the primary coil. The alternating current sets up a magnetic field, which changes with the current (collapses, re-forms, collapses, etc). The changing magnetic field induces a current in the secondary coil. I’m pretty sure about that lot – but is the current induced in the secondary coil dc? I seem to remember reading that it is, but I can’t find anything to confirm it in my textbook – it makes a kind of sense that it would be dc, but nothing I could explain in an exam.

Also, is there anything I’ve missed?
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SinghFello
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(Original post by CrazyChemist)
Would somebody mind checking my understanding of transformers? I’d appreciate it to see if I’ve got it sussed (btw, I’m doing Edexcel, Salters):

Right – an ac current flows into the primary coil. The alternating current sets up a magnetic field, which changes with the current (collapses, re-forms, collapses, etc). The changing magnetic field induces a current in the secondary coil. I’m pretty sure about that lot – but is the current induced in the secondary coil dc? I seem to remember reading that it is, but I can’t find anything to confirm it in my textbook – it makes a kind of sense that it would be dc, but nothing I could explain in an exam.

Also, is there anything I’ve missed?
First part is correct.
An emf is induced onto the secondary coil, due to the changing flux density across the coils (because field is perpendicular to the coils wires). The induced emf is AC. When the induced emf is displaced on a CRO, it it seen as a sinosidal curve.
Why would it be DC? Remember Lenz's law. The emf induced is in a direction as to appose the change in magenetic flux. So the emf produced would be in the opposite direction as the direction of the current in the primary coil, with the a different frequancy.
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CrazyChemist
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(Original post by SinghFello)
First part is correct.
An emf is induced onto the secondary coil, due to the changing flux density across the coils (because field is perpendicular to the coils wires). The induced emf is AC. When the induced emf is displaced on a CRO, it it seen as a sinosidal curve.
Why would it be DC? Remember Lenz's law. The emf induced is in a direction as to appose the change in magenetic flux. So the emf produced would be in the opposite direction as the direction of the current in the primary coil, with the a different frequancy.
Got it. Thanks
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Mr Tom
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(Original post by sumitk87)
So u took phy4 in january :eek: , lucky our school dont do january exams .
what did u get in the exam if u dont mind me askin and was it a hard paper?
I dont kno what to expect from the paper
any topics that came up alot in the jan paper? (Does anyone have that paper ?)
Sorry so late to reply, been "working" (watchin big bro live)
I got the lowest B available, and the general consensus in our college was that is was a very hard paper (only 2 of us ARENT retaking) and was harder than all the past papers. so..... this june should be easy to balance out
in general though the paper cant throw too many surprises at you, so worry more about phy6

and you say lucky, but every jan module i have taken i have had to retake... who revises over xmas?
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Mr Tom
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(Original post by Mr Tom)
Sorry so late to reply, been "working" (watchin big bro live)
I got the lowest B available, and the general consensus in our college was that is was a very hard paper (only 2 of us ARENT retaking) and was harder than all the past papers. so..... this june should be easy to balance out
in general though the paper cant throw too many surprises at you, so worry more about phy6

and you say lucky, but every jan module i have taken i have had to retake... who revises over xmas?
having now read full thread seen singhfellow say it was easy, looking back on it myself, and laughing, i too have seen the error of my ways.

At the time i found q7 on energy levels impossible, but it is a relatively straight forward question :mad:

how do u know what u got for each question singhfellow?
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mackin boi
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#136
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I h8d jan ph4 it was a bit dodgy im resittin...got a B...sum1 in ma skool got full marks...on questioning his reply was 'u have to feel the fysix'...i promptly slapped him....jkin OBVIOUSLY....
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SinghFello
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I got a copy of the exam paper from Edexcel. It cost £15, but it was worth it. At least you got a B! I only just scrapped a C, 54/90!!

Has anyone done the June 2003 PH4 paper? Now thats a HARD paper! I tried hard in exam conditions, and I scrapped a C! The mark scheme was incredibly strict. I then did the June 2004 paper and got near 90%. Strange :/
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sumitk87
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(Original post by SinghFello)
I got a copy of the exam paper from Edexcel. It cost £15, but it was worth it. At least you got a B! I only just scrapped a C, 54/90!!

Has anyone done the June 2003 PH4 paper? Now thats a HARD paper! I tried hard in exam conditions, and I scrapped a C! The mark scheme was incredibly strict. I then did the June 2004 paper and got near 90%. Strange :/
hey singhfello since u have done june04 can u help me with one question http://www.thestudentroom.co.uk/t119391.html
thanks

anyone else who can help me would be appreciated , 2morows the exam :eek:
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