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# Edexcel Physics Unit 2 "Physics at work" June 2013 Watch

• View Poll Results: The last question - Does resistance increase or decrease?
It increases ( using V=IR or some other method)
70.73%
It decreases using the 'lattice vibrations' theory
29.27%

1. (Original post by Branny101)
Just to clarify, does refraction only happen from a less dense to more dense medium?
And TIR only occurs in the more dense medium?

Thanks

EDIT: What is it you put on the top and bottom of the Smell's law equation, like when do you know what 'velocity' example to put on the numerator or denominator ?

Posted from TSR Mobile
Refraction is the change in the direction of propagation of wave as it moves from one medium to another in which it has a different speed. It can happen when a wave moves from a more dense medium to a less dense medium of vice versa.
TIR has two conditions:
1. The wave should be trying to move out of a more dense medium to a less dense medium
2. The angle of incidence should be greater than the critical angle.

It doesn't matter what velocity you put in the numerator or denominator as long as the ratio matches
It should velocity in this medium over velocity of that medium equal to angle in this medium over angle of that medium.

An easier method to remember, my physics teacher said, is to remember
Mu of medium A * wavelength or velocity or angle in this medium A= mu of medium B * wavelenght or velocity or angle in this medium

If you want to be so sure then you should substitute the medium that the wave is exited as 1 and the medium it is entering as 2 in the equation given in the exam.

Hope you get it!!!
2. (Original post by Branny101)
Just to clarify, does refraction only happen from a less dense to more dense medium?
And TIR only occurs in the more dense medium?

Thanks

EDIT: What is it you put on the top and bottom of the Snell's law equation, like when do you know what 'velocity' example to put on the numerator or denominator ?

Posted from TSR Mobile
Actually refraction occurs from a more dense to a less dense aswell the only difference being it will refract away from the normal and speed up, and this the very reason why TIR occurs in the more dense, as the angle of the incident ray in the more dense increases the angle of the refracted ray in the less dense medium will bend away from the normal, as you increase the angle of incidence the refracted angle will eventually reach 90 degrees from normal, at this point the angle of the incidence wave is called the critical angle and beyond this angle total internal reflection occurs, TIR does not occur in less dense to more dense because when the angle of incidence increases, the refracted ray bends TOWARds the normal, decreasing the angle of refraction

Posted from TSR Mobile
3. http://www.edexcel.com/migrationdocu...e_20100609.pdf
Question 5, multiple choice question
I reaaaaaaaally don't get why the answer is D...
4. http://www.edexcel.com/migrationdocu...e_20100609.pdf
And question 10
5. (Original post by Branny101)
Just to clarify, does refraction only happen from a less dense to more dense medium?
And TIR only occurs in the more dense medium?

Thanks

EDIT: What is it you put on the top and bottom of the Snell's law equation, like when do you know what, 'velocity', for example to put on the numerator or denominator ?

Posted from TSR Mobile
The refractive index cant be less than one, so the faster velocity on the top, is the way i remember it, or the larger value for sin theta

Posted from TSR Mobile
6. (Original post by pureandmodest)
http://www.edexcel.com/migrationdocu...e_20100609.pdf
Question 5, multiple choice question
I reaaaaaaaally don't get why the answer is D...
Well you should know two things to tackle questions like these
1. The longer the transition the more energy is involved for the photon. Photon energy=h*frequency , so longer the transition as it appears on the diagram the higher the frequency. Velocity is constant so higher frequency means less wavelength.

So longer the transition higher the frequency smaller the wavelength and vice versa

2. Photons can be absorbed or released during a transition. When a photon is released the electron moves DOWNWARDS and when a photon is absorbed it moves upwards. Photon=energy. Its like gpe. If you fall you lost gpe which is transferred to other form of energy.

So now you must get it as D.
7. (Original post by krisshP)
Yes, TIR occurs in the denser medium.

Not sure what you mean by refraction. Refraction can occur when going from a more dense to less dense medium OR from less dense medium to more dense medium.

U=velocity1/velocity2

Or

U=sin I/sin r

Both equations should give the same refractive index.

If they give you velocities, use U=velocity 1/velocity 2. If they give angles of incidence and refraction, use U=sin I/sinr
So if light is going from a less dense to more dense medium, the ray would be shown as bending away from the normal?

And I dunno I just heard something about the refractive index having to be more than one , thus, surely that would affect the appearance of the equation?

Thanks

Posted from TSR Mobile
8. (Original post by Branny101)
So if light is going from a less dense to more dense medium, the ray would be shown as bending away from the normal?

And I dunno I just heard something about the refractive index having to be more than one , thus, surely that would affect the appearance of the equation?

Thanks

Posted from TSR Mobile

I don't remember the bend towards normal stuff or whatever it is since you are more vulnerable to forgetting it in an exam. See his ^ analogy. It's far more logical, so you more likely to remember it.
9. (Original post by Branny101)
So if light is going from a less dense to more dense medium, the ray would be shown as bending away from the normal?

And I dunno I just heard something about the refractive index having to be more than one , thus, surely that would affect the appearance of the equation?

Thanks

Posted from TSR Mobile

It moves towards the normal with its velocity decreased when moving into a more dense medium
And vice versa for a less dense medium.
If you include the refractive index then you must follow the equation strictly if not its not needed as long as the ratio matches. Just remember 1 as the medium it exits and 2 as the medium it enters.
10. Just remember for refraction his analogy in Khan academy
11. (Original post by StUdEnTIGCSE)
Refraction is the change in the direction of propagation of wave as it moves from one medium to another in which it has a different speed. It can happen when a wave moves from a more dense medium to a less dense medium of vice versa.
TIR has two conditions:
1. The wave should be trying to move out of a more dense medium to a less dense medium
2. The angle of incidence should be greater than the critical angle.

It doesn't matter what velocity you put in the numerator or denominator as long as the ratio matches
It should velocity in this medium over velocity of that medium equal to angle in this medium over angle of that medium.

An easier method to remember, my physics teacher said, is to remember
Mu of medium A * wavelength or velocity or angle in this medium A= mu of medium B * wavelenght or velocity or angle in this medium

If you want to be so sure then you should substitute the medium that the wave is exited as 1 and the medium it is entering as 2 in the equation given in the exam.

Hope you get it!!!
Thaaaanks, I think that explanation at the end made sense of it

Posted from TSR Mobile
12. (Original post by StUdEnTIGCSE)
Well you should know two things to tackle questions like these
1. The longer the transition the more energy is involved for the photon. Photon energy=h*frequency , so longer the transition as it appears on the diagram the higher the frequency. Velocity is constant so higher frequency means less wavelength.

So longer the transition higher the frequency smaller the wavelength and vice versa

2. Photons can be absorbed or released during a transition. When a photon is released the electron moves DOWNWARDS and when a photon is absorbed it moves upwards. Photon=energy. Its like gpe. If you fall you lost gpe which is transferred to other form of energy.

So now you must get it as D.
ohh! that makes more sense, thankya
14. June 2010 Q 13 a ii)

I got the right answer but the method they used on the mark scheme looks peculiar; why is it they divided the resistances?

EDIT : same paper as above.

Posted from TSR Mobile
15. http://www.edexcel.com/migrationdocu...e_20110117.pdf
question 2) and 5)
i'm so bad at anything to do with circuits so help would be much appreciated!
16. (Original post by pureandmodest)
Okay so potential difference is split between components, so you need to find where voltage across resistor + voltage across bulb = 9 v and also the current has to be the same for these voltages, so by inspection looking on the graph, when the voltage across the resistor is 6 and the voltage across the bulb is 3, and thr current is 0.3amps for both, and 6 + 3=9
17. (Original post by Branny101)
June 2010 Q 13 a ii)

I got the right answer but the method they used on the mark scheme looks peculiar; why is it the divided the resistances?

EDIT : same paper as above.

Posted from TSR Mobile
What do you mean divided by resistances?

Posted from TSR Mobile
18. Bit confused by what actually is emf and potential difference and the differences between them
19. (Original post by pureandmodest)
http://www.edexcel.com/migrationdocu...e_20110117.pdf
question 2) and 5)
i'm so bad at anything to do with circuits so help would be much appreciated!
2). Find the resistance of the paralell combination, : 1/200+ 1/200= 1/R gives R is 100, so the first resistor has 200 and the combination of the next 2 is 100, voltage get split between components and the ratio of this voltage split will be the same as the ratio of the resistors which in this case is 2:1, since the terminal p.d is 12v, you have to split this in the ratio 2:1, so 8:4, leaving 4 volts as the p.d across the paralell combination

Posted from TSR Mobile
20. also guys what does this syllabus statement mean
"investigate and explain how the potential along a uniform current-carrying wire
varies with the distance along it and how
this variation can be made use of in a
potential divider"

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