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Discussion in 'International A And AS Level' started by XPFMember, Sep 29, 2011.
thank you so much
yh shes right ive left these ques
Lots of thanks
Aishayasin , anastasia grey113 , Hamnah Zahoor
who else is doing A Levels???
can someone help me for part 1
i dont understand it, i tried comparing the question to the observation of photoelectric effect but still got no luck :c
for the second part:
it says that wavelength is reduced so i thought that frequency is increased now since the wavelength already exceeds the threshold frequency it means that all it will do is allow electrons to have a greater maximum kinetic energy. But the markscheme says photon energy is larger so hence larger to kenietic energy is also larger what does that even mean!!!!!
and more maximum photoelectric current it says fewer photons per unit time so maximum current is smaller????
Explain the function of the non-uniform magnetic field that is superimposed on a large uniform
magnetic field in diagnosis using nuclear magnetic resonance imaging (NMRI) (4marks) nov16/p42
I am having difficulty to write a model answer for this question.
When nuclei are placed in magnetic field, they precess at Larmor frequency and emit r.f. pulses. This frequency depends on strength of magnetic field as w = Bγ. Different rf pulse frequency emitted allow nuclei to be located. Changing field also allows position of detection to be changed.
For part 1, the photoelectric effect will not help because it supports the particulate nature of light.
Look into the wave nature instead which is very different.
According to that theory, there is SUFFICIENT TIME DELAY between electron emmision and light incidence.
This is because electrons GAIN ENERGY FROM MORE THAN ONE PHOTONS and when they 'collect' all this energy and it's equal to work function, they escape. This obvio takes some time.
This also means, that if suppose a low frequency light is shone, the electrons will still be emitted, only more time will be taken because as low energy photons are present it will take more time for electrons to gather up energy equal to work function. So electrons will be EMITTED AT ALL FREQUENCIES.
But if intensity is increased, more photons given out per unit time so electrons will gain more energy and more quickly so their max. kinetic energy will also increase.
Write the points I wrote in capital.
And remember: THIS IS JUST THE WAVE THEORY OF LIGHT. THE PARTICULATE THEORY IS VERY DIFFERENT FROM THIS.
1- When wavelength decreases, frequency increases.
2- When frequency increases, photon energy increases.
3- When photon energy increases, Max k.e. increases.
(ONLY FOR PARTICULATE THEORY)
And vice versa.
ohh achaaaa it makes sense now. thx
but what would happen to the photoelectric current tho
learn the formula I=nhf where I is the current, n is the no. of photons and f is the frequency.
You can also write it as I=nh3x108/λ so that means
I is inversely proportional to the wavelength.
So if n (intensity or no.) is constant and wavelength is reduced, the current will increase.
i understand how to do part one, its basically alpha=(Z2-Z1)^2/(Z2+Z1)^2 simple acoustic calculation
but part 2 I done understand how they did it,
i'm stuck what do to next?
this is what i did
alpha is 48 from the table
but why did the marksheme say that alpha x is (48 *2x) and then they multiplied 0.018 with alpha when Ln (ing) bothsides ik that the 0.018 is the reflection coefficient at the boundary between fat and muscle.
but the part 2 only asks about the thickness of fat so surely i can just say the that alpha is 48 since its the absorbtion coefficient for fat???
It's because the ray passes twice through the layer of fat before it's intensity is detected.
So the expression is more like:
0.018 x (e^-48 x x*10-2)2
If you solve it this way you will get the same answer
Can you please explain why are you multiplying 0.018 by e^(-48*10-2)2 i
I understand e^ part but why is 0.018 being multiplied to it.
shouldn't it just be this:
i just dont understand why you're using the coefficient alpha thats been calculated in the first part of the question
Because itn't the formula:
the Intensity cancel out, but what i dont understand is that why are we using the α and the µ at the same time in this question.
i'm so confused.
okay imagine the beam entering
There will be three phases.
1- It will move through the layer of fat. As it moves, it will attenuate and intensity will decrease. (e^48 x*10-2)
2- It will be reflected at the tissue boundary. But some of it will be transmitted thru the muscle tissue. (0.018 of the total is reflected).
3- It will move across the layer of fat and will be equally attenuated once more (e^48 x 2*102)
So you just have to multiply all these three phases in order to find the intensity detected.
The detected intensity is already given (0.012) so put it's value in the equation which is like.
Intensity detected = not attenuated x reflected x not attenuated
In short, some of the beam is also 'transmitted and NOT REFLECTED' at the tissue boundary. In this case only 0.018 of it is reflected. The rest is not so obvio it is not detected.
To get an accurate value, this should also be considered.
Can anyone help me in the following questions from oct/nov/2017/13 Q36 Q37
Can anybody explain last part of Q6?
can someone plz help in b) the ans is 0.85 and 0.51
One possible nuclear fission reaction is
235 1 U + 092 n 141 1
56 0 Ba + 92
36 Kr + 3 n + energy.
56 Ba) and krypton-92 (92
36 Kr) are both β-emitters.
Barium-141 has a half-life of 18 minutes and a decay constant of 6.4 × 10–4 s–1.
The half-life of krypton-92 is 3.0 seconds.
(b) A mass of 1.2 g of uranium-235 undergoes this nuclear reaction in a very short time
(a few nanoseconds).
(i) Calculate the number of barium-141 nuclei that are present immediately after the
reaction has been completed.
Ans is 3.1×10^21
(ii) Using your answer in (b)(i), calculate the total activity of the barium-141 and the
krypton-92 a time of 1.0 hours after the fission reaction has taken place.
Ms says krypton activity is negligible . Why?
I consider when the amplitude increases so does the resultant amplitude/displacement thus more or greater constructive interference occurs resulting in more brighter fringes produced. In other case A^2 is directly proportional to Intensity thus increase in A will ultimately inc. the Intensity of the bright fringes no effect on any other results.
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