ALL CIE PHYSICS DOUBTS HERE !!!

[ashwinkandel]

SkyPilotage Virtual earth approximation is only true for inverting op-amp. For an ideal op-amp the gain must be infinite and gain = Vout/Vin. For gain to be infinite the Vin must be zero. So the difference between V+ and V- must be about zero(which only happens if both are same). In case of inverting amplifier one is earthed(P.D=0) so another must also have zero p.d so it is called Virtual Earth. I don't know the answer to second question(which i am also seeking)

[SkyPilotage]

I believe you are talking about the open-loop gain, but in non-inv amplifiers, there is a closed loop gain determined by Vout/Vin and it is not infinite. Therefore Vin cant be zero. I have tried looking it up and I found that only the Differential Input Voltage must be zero.
There tends to be confusion between Vin and the Diff. Input volt, where Vin is the voltage coming before the Rin Resistor. While, Diff input voltage is V+-V- which has to be equal to zero. I am not sure why it has to but I think it has -like you said- to with the open-loop gain which has to be infinite, but ill try makin sure.
Thanks for your help mate! 

[ashwinkandel]

SkyPilotage once check this out(thing that i have attached)

[Rvel Zahid]

Rvel Zahid I have attached solutions to your questions:

Bombastic Ashwin! Thank you soooooo much, makes things very clear.

one more questionhttp://www.xtremepapers.com/CIE/International%20A%20And%20AS%20Level/9702%20-%20Physics/9702_w09_qp_11.pdf
mcq 14 how it is A?

[ashwinkandel]

I was a bit unclear last time for q.n 14. Here i have attached another version

[Rvel Zahid]

I was a bit unclear last time for q.n 14. Here i have attached another version

yeah this mcq was hard to grasp...Thanks

one more
i can't understand how to calculate phase difference between two waves...

[SkyPilotage]

Do you guys agree that the signal to noise ratio is equal to 1 when both the signal and noise have equal power? because I think it should be zero because 10log1 = 0 ! What do you think?

[SkyPilotage]

yeah this mcq was hard to grasp...Thanks

one more
i can't understand how to calculate phase difference between two waves...

Its like you find the ratio of the wave lag to the wavelength. Here it is 1/2 lamda so thats the phase diff. and destructive interference will occur where they cancel each other out.

[Rvel Zahid]

Its like you find the ratio of the wave lag to the wavelength. Here it is 1/2 lamda so thats the phase diff. and destructive interference will occur where they cancel each other out.

ok thanks a bunch.
some more. attached the files below. take ur time not urgent. Thankyou.

[ashwinkandel]

Rvel Zahid  Unless someone else answers your questions , I will answer it tomorrow.

[Rvel Zahid]

Rvel Zahid  Unless someone else answers your questions , I will answer it tomorrow.

Ok brother dats so nice of you. i don't need urgent answers, so its ok no problem. Take care

[Rvel Zahid]

http://www.xtremepapers.com/papers/CIE/Cambridge%20International%20A%20and%20AS%20Level/Physics%20(9702)/9702_w08_qp_1.pdf

need help in 11 and 32

[haris94]

http://www.xtremepapers.com/papers/CIE/Cambridge%20International%20A%20and%20AS%20Level/Physics%20(9702)/9702_w08_qp_1.pdf

need help in 11 and 32

q11:
F=ma
For the 2kg box 20-T=2a
20 is the weight of the box ; T is the tension in the string ; 2 is the mass of the box ; a is the acceleration

For the 8kg box T-6=2a
6 is the frictional force

Solve both simultaneously and you get a=1.4

Answer is A

[haris94]

http://www.xtremepapers.com/papers/CIE/Cambridge%20International%20A%20and%20AS%20Level/Physics%20(9702)/9702_w08_qp_1.pdf

need help in 11 and 32

q32:
i need someone to please confirm this.
here's my theory - the steel cable will not be responsible for the resistance of the power cable since it is enclosed within the copper cables.
Now all the 6 copper cables are in a circle so i think it should be taken as if they are all parallel to each other.

So 1/R = (1/10)*6
R=1.67

Answer is B

[SkyPilotage]

I think the resistance of the steel cable is included for 2 reasons:-
1- Answer will be 1.64 i.e closer to B
2- They wouldnt have given you the value, never seen CIE do sth like that from year 2000 Examinations
What do you think?