# Search results

1. ### Signal to noise ratio for thermal and shot noise

Homework Statement The output of a sensor which produces 10000 electrons is connected to parallel RC circuit with Resistance= 50 ohm, and the capacitance=10pF. Calculate the signal to noise ratio at temperature 300K. Homework Equations SNR= 10 log (power of signal/ power of noise) Or...
2. ### Quantum mechanics , total orbital angular momentum?

Homework Statement A hydrogen atom is identified as being in a state with n=4. What is the magnitude of the total orbital angular momentum for the largest permitted value of l? Homework Equations n>l, l is bigger or equal to m The Attempt at a Solution The allowed l= 3,2,1 The allowed m for...
3. ### How does FET amplifier amplify?

BJT ,the output voltage is amplified because there is dc voltage supply to keep BJT active ,but what about the FET? The voltage supplied only control the gate only, anything to do with the output? Thanks
4. ### Shannon entropy - use to calculate the bit needed to encode a symbol

To encode a symbol in binary form, I need 3 bits ,and I have 6 symbols. So I need 6*3=18 bits to encode "We are" into binary form. As shown in http://www.shannonentropy.netmark.pl/calculate My question: 3 bits to encode one then I have to use 16 bits, _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _. How to...
5. ### From Planck's law to derive the stefan Boltzman constant.

The following is the Planck's derivation for black body radiation $${\rho}({\lambda}) d{\lambda}=E({\lambda})*f({E(\lambda}))*D({\lambda})d{\lambda}------equation 1$$ $$\int_0^\infty{\rho}({\lambda})d{\lambda}$$ is the density of radiative energy. From...
6. ### Derivation for black body radiation. What is the 2 bout?

$${\rho}({\lambda}) d{\lambda}=E({\lambda})*f({E(\lambda}))*D({\lambda})d{\lambda}$$ $${\rho}({\lambda}) d{\lambda}$$ is density of radiative energy, $$E({\lambda})= k_BT$$ is the energy of an atom vibrate in 3D, $$f({E(\lambda}))=1$$ is the probability distribution. Equals to 1 because we...
7. ### Dulong Petit derivation.How do we get this formula?E=kT

Dulong Petit use energy,E=$k_B$T and the probability distribution as f(E)=1. Internal energy,u=3N$k_B$T $$C_v=∂u/∂T=3NkT$$ Three there because there is 3 modes in each atom. Then my question is why do we use E=kT? I understand 1 atom has 3 degree of freedom,and 1 freedom has kT/2. A...
8. ### Refractive index- will it travel back along the same way?

Refractive index----- will it travel back along the same way? From the picture http://en.wikipedia.org/wiki/File:RefractionReflextion.svg If I travel a light ray from n2 to n1 , I think the light ray will travel in the opposite direction shown in the first picture. But what about the...
9. ### Monostable multivibrator no input, the transistor1 in active mode?

If T1 is in active mode all the time, will there be any current flow in this circuit? ( I think it is impossible to flow) thanks
10. ### Bound state of a square well, no allowed bound state mean?

Homework Statement Show in the graph ,there will be no allowed bound states with odd-parity if the well depth is less than ${V_min}$ Find ${V_min}$ in terms of k and a.where a is the half of the well width. What does no allowed bound state mean? Homework Equations $cotz=-pa/z$ where p^2...
11. ### Bound state of finite square well, why do we make this statement?

Reading from http://quantummechanics.ucsd.edu/ph130a/130_notes/node150.html Again we have assumed a beam of definite momentum incident from the left and no wave incident from the right. Why is the above statement made? What does the reflected wave mean? There is now all why reflected...
12. ### Free particle wavefunction represent a fixed profile= wavepacket?

Why the Griffiths book says : any function of x and t that depends on these variables in the special combination (x±vt) represent a wave of fixed profile, traveling in the -+x direction at speed v... I don't really get the reason why from 2 terms of wavefunction can become only one term...
13. ### Divergence formula derivation ?

Homework Statement How to get equation 1 from the thumbnail? h1 h2 h3 doesn't have to be constant. The most I can try is equation 2 . Please guide thanks. Homework Equations The Attempt at a Solution
14. ### Re- equivalent for a common emitter

The questions is in the thumbnail, please tell me am I correct.thanks
15. ### What is the physical meaning for a particle in harmonic oscillator ?

For infinite square well, ψ(x) square is the probability to find a particle inside the square well. For hamornic oscillator, is that meant the particle behave like a spring? Why do we put the potential as 1/2 k(wx)^2 ? Thanks
16. ### Ehrenfest theorem, is there any condition for the operator Q?

For commutator, HQ-QH = 0 . But for this case as shown below, complex ψQHψ - HcomplexψQψ= 0? If the operator Q is in term of (∂/∂t) and (∂/∂x) ,then the HQ-QH may not be zero. Is there any restriction for Q operator?
17. ### Sum of 1/(n^2) as n goes to infinity

Homework Statement Prove Ʃ1/(n^2) as n goes to infinity = (∏^2)/8 Homework Equations The Attempt at a Solution No idea how to start. Pls guide. Thanks
18. ### How do we get the expectation value formula?

<x>= ∫ complex ψ x ψ dx How do we get this formula? And why must the complex ψ must be placed in front? Please guide or any link to help,not really understand this makes me difficult to start in quantum mechanics. Your help is really appreciated. Pls
19. ### The moment of inertia of a rectangular plate about its diagonal

Homework Statement Find the moment of inertia of a rectangular plate of mass m, sides 2h and 2k , rotates about its diagonal. Homework Equations I= Ʃmr^2 The Attempt at a Solution I=Ʃ m(x^2 +y^2) , let 2h parallel to x-axis, 2k parallel to y-axis. I= m(x cosθ)^2 + m (y cos ∅)^2 where...
20. ### Dot and cross product?

Homework Statement A=(x,3,1) ,B=(x,-x,2) Determine the value of x if the vector perpendicular to A and B is given by C=(10,-4,-4) Homework Equations The Attempt at a Solution Find A cross B , let A cross B be D . Then D cross C = zero (since they are perpendicular to both A and...
21. ### Why magnet can attract liquid oxygen but not aluminium?both paramagnet

A magnet only attract ferromagnetic material because they have magnetic domains which will align to form magnetic dipole when external magnetic field applied. Paramagnet dont have domains. Due to the presence of some unpaired electron, they will cause the realignment of the electron path when...
22. ### Find v^2 in terms of x ?

Homework Statement A gun is shoot straight up. Assuming that the air drag on the bullet varies quadratically with speed, and the equation of motion for the bullet is (m/2)(dv^2/dx)= -mg-(cv^2). Find for the downward motion , v^2 in terms of x. Taking that upward direction is positive...
23. ### Mechanics - How to predict the motion after two objects collide in 2D?

I think no matter what are the mass of two objects, there must be a fix angle between the angle of two direction. Is that using the center mass reference to calculate? I say that because we can prove that when the two objects with same mass collide, then the angle between the direction of the...
24. ### How to integral ? How can they ignore that variable?

Homework Statement I don't understand case 1. In case 2 that is normal integration , isn't? Then why case 1 can simply ignore the r ? I don't think r is constant. Homework Equations The Attempt at a Solution Please guide. Thanks
25. ### How to do cross product if I have got only two coordinate?

a =(x,y), b =(h,k) a cross b =? I have idea what to type on google. Is that doing like matrices , a cross b = xk-hy? thanks.
26. ### What should i do after using Taylor series?

Homework Statement The first equation on the uploaded paper converts to the last equation. Homework Equations when i substitute ln (1-u)=-u-(1/2)(u^2) into the first equation, i can get the first term in (3rd equation). but the second term of the 3rd equation ? The Attempt at a Solution I...
27. ### Why must be that for curl vector in spherical coordinate?

The correct one is 2nd, but why not first? Please guide , or tell me any link that relate to this derivation. Thanks
28. ### How do scientists know the direction of Magnetic dipole moment?

How they know, how to see the spin of electron>?? Without external field applied, there is random dipole in the paramagnet. for diamagnet, without external field , there will be no dipole exist. Thanks.
29. ### Mechanics~polar coordinate & radial and transverse component

http://www.answers.com/topic/radial-and-transverse-components From the above link, 1) e θ is a unit vector perpendicular to r in the direction of increasing θ. Where is the direction of increasing θ? Is that a circle? θ Increase from 0 to 2∏.then eθ moves in a circle? direction always...
30. ### Uncertainty-what is the rate with 0.59% precision?

Uncertainty--what is the rate with 0.59% precision?? Homework Statement Assume that negligible uncertainty in timing of a clock, calculate the number of counts required to measure a rate with 0.50% precision. Homework Equations The least accuracy is 0.50%? The Attempt at a Solution...