cyeokpeng

Assuming your floor is your potential step V0. Then solve your time-indepedent Schrodinder wave equation (Use time-indedent version since we are not interested in the effect of dynamical disturbance of the system). (-hbar2/2m)*d2Psi/dx2 + V(x)Psi(x) = E(x)Psi(x) Similar to the electron incident on the potential step. Solve it in the same way.

Yes Let's take for example glass. Glass is transparent to the visible light spectrum and shorter wavelength EM radiation like the UV light. However, glass is opaque (or block/totally reflect) infrared radiation and radio waves (Not sure about radio waves. Correct me if I m wrong) which are your EM spectrum at longer wavelengths. It also depends on the material. Most materials are opaque to a lot of frequencies of EM radiation, otherwise you can see through your tables, walls and most of your common materials.

I wonder in cellular communications, the base station obtained the desired signal from a mobile user from multipath effects, so that Doppler effect due to motion of user and reflection of radio waves by barriers causes a certain amount of interference since different radio rays are not exactly in the same phase as each other. How does the receiver end decode such signals at the demodulation stage? How can demodulator be so smart to decode such signals with so much interference?

True, there must be a net movement of electrons for the current to flow. Random thermal motion of the electrons cannot be considered current, since the probability of electrons traveling to the left is roughly the same as that travelling to the right. And the net movement of electrons must be due to any applied potential difference, so that there will be a net movement of the electrons in the direction opposite to the electric field. However, if there is a high temperature at one end, and a low temperature on the other end of a conductor, there will be a net current flow. The termal energy seems to be causing the current. Why is that? Anyway, this effect is what is known as the thermocouple effect.

Oh mistake. I forgot that Yes correct.

Oh I see. I fail to notice that. Thanks. But the effect is very minimal unless it is turned into a coil which will increase the inductance.

What do you actually mean by thermal equilibrium? From what I know, a system in thermal equilbibrium cannot do external work or else it will violate the 2nd law of thermodynamics. And why do we always use Boltzmann statistic to model and derive so many semiconductor equations?

Any idea what is the dielectric strength of the air in our atmosphere for such dielectric breakdown to occur in lightning?

Temperature does affect the resistance of a wire, since resistance at the microscopic level is related the movement of electrons and very low temperatures will overall increase the mean free path of the electron movement. And this means less collisions and hence lower resistance. But on the topic of frequency dependence, form what I know, only impedance of the capacitoror inductor is frequency-dependent and not the real resistance of a wire. So I do not think that any change in frequency of the current flowing through the wire will change its resistance.

What is the difference betwen air resistance and uptrust?

If this is a totally elastic collision, since you assume this is hypothetical, then applying the conservsation of momentum, also assuming that no friction is present, then the massive object should bounce back from the wall with exactly the same velocity, unless energy is lost by other means like heat of sound. Think in terms of billiard ball bouncing off the wall if it approach at right angles. Then applying F = d(mv)/dt and since mass is constant, average impact force = m*dv/dt = m*(vf-vi)/t Is this correct?

Mmm, what do you mean by virtual states? All the energy states should be acounted for in QM, in materials as complicated as dielectrics, metals and semiconductors, there are allowed bands (in which all your allowed energy states lie) and forbidden bands (in which all the electrons cannot possess that energy level). In dielectrics, the forbidden energy gap between the conduction band and the valence band is so much greater, about 5eV or more, but where do this virtual states lie in the energy band diagram?

From what I know, speed of light is c in vacuum. However, it seems to slow down as it pass through transparent material like glass or water. I have learned that light has actually not slowed down as it pass through glass, it only appears to slow down. Actually, the speed of light is still c as it pass through glass, but the electronic polarization effects of the material causes EM energy to be absorbed through dielectric loss (due to the imaginary dielectric constant) and this in effect make light appear to us as having slowed down in the glass medium. Assuming magnetic effects of the EM radiation to be negligible in glass, we have the relationship refractive index n = square root of dielectric constant assuming square root of magnetic permeability to be one. However, how does this EM energy being absorbed? What is the real mechanism behind this. Is it due to energy bands and bandgap?

Assuming if the tungsten filament has an ohmic property, Power dissipated = I^2*R where R= resistence of the filament. So as current increses, the intensity also increases by the power square law.

Just think it like this. Though at the peak of its path its intstantaneous velocity is zero, take an elemental time after that. Its velocity will start to increase but in the opposite direction, since it is under the influence of gravitational acceleration g. So in effect, it still is accelerating at the peak, or else it would stay stationary there and not fall down, which you wil never see it happening like this.

You should not apply it in the x-direction, since the acceleration is not g. Anyway, an easier method is to treat the ramp as the x-direction and use this coordinate system to solve the problem.

The second observer B will see the 1st observer A's time slow down also, since though the 2nd observer is moving at 0.9c wrt observer A, imagine B's frame of reference is fixed, A will appear to be moving at 0.9c in the opposite direction. Another analogy: If a train is approaching a station, observer on the platform will see passengers on the train moving say to the left. On the other hand, if you are the passenger on the train, you will see people on the platform to be moving at the same speed to the right. So B will still see A's time slow down due to this high speed 0.9c relative to B.

Unless somehow we can travel faster than the speed of light, then can we go back in time through space-time continuum. The light cone is 45 degrees everywhere, and you need to travel faster than the speed of light, i.e move at even greater angles more than 45 degrees before you can move to a point in space time that is history. But, as you know from special relativity, nothing can travel faster than the speed of light, or else its mass will be infinite. So I believe we cannot go back in time. Another strong reason stated by Stephen Hawking that time travel is not quite possible. If it is possible, then we would have experienced future people to have travel back to the present, but this does not happen. Again, if this is true, we would have experienced Grandfather Paradox, and history will be changed. Through the intimate relationship of causality, it is deemed impossible to time-travel.

What you say make sense. However, from what I know, work is done on the moon by the earth unless the moon is not moving in constant orbit round the Earth unless there is a change in gravitational potential, that is moving further away or nearer the Earth. But since it is at the same radius (i.e equipotential), no work is done on the moon by the Earth. Work = (Final Gravitational Potential - Initial gravitational potential) * Mass of moon

Hope I m still on time. The fault is not the equation of motion's fault. It is correct whichever equation of motion you use, you will still get the right answer. In the positive y-direction (vertical) -g=(v-u)/t v = u-gt Since the final velocity in the y-direction is zero, t = usin60/g This is correct! However, when you apply the displacement equation s = u*sin60*t-0.5*g*t^2 You should remember that this is the vertical y displacement, NOT THE DISPLACEMENT UP THE RAMP! since u*sin60 is your initial velocity in the vertical y-direction and your acceleration is your g. To get the correct distance up the ramp, use trigonometry to find the ramp distance for this vertical distance 137.8m. I have a feeling that your teacher formula is wrong, unless you give me slightly wrong question. COS sin60 = 137.8/ramp distance ramp distance = 137.8/sin60 = 159.1 m Check your question again!

It refers to only materials capable of producing magnetic field around them, such as paramagnetic materials like cobalt and nickel, correct me if I'm wrong cos I m too a little bit confused about the concept of paramagnetism, diamagnetism and ferromagnetism. Only a certain class of materials produces intrinsic magnetic field around them and they form your natural magnets. As for iron, they are ferromagnetic and this means than only if magnetic field from other sources is nearby, they will be able to exhibit orientation of tiny magnetic cells or regions such that they aid in the magnetic field. Also, they have the ability to retain the magnetism even if the external magnetic field magnetizing the iron is removed!!! An important concept of magnetic field is that it exist as dipoles, meaning that if there is a North pole, there must be a corresponding South pole in the field. This does not occur for other fields like electric field in which you can have one indepedent positive charge creating a field. That is why if you break up a piece of magnet into small magnets, they will be many individual North and South poles. So should answer your first question, only materials exhibiting paramagnetism and magnetized ferromagnetic materials can produce magnetic fields around it such that if like poles are put close to each other they repel and vice versa.

I reckon we have to use quantum mechanics to explain why glass can transmit visible light through but not objects like ceramics and metals. In addition, why is glass able to block off or reflect totally certain cutoff wavelength of radiation in the infrared range? We know that because this is the mechanism of Greenhouse Effect. In addition, I know that when light propagates and is incident on a material, 3 things can happen. Certain range of wavelength of light is absorbed and other ranges of light is reflected, which is the basis of the theory of colors. Lastly, light can also be transmitted right through the material if the material is transparent to this range of wavelength of light. How can we explain that quantum mechanically? Also, as I have learned, as EM waves of certain wavelength is incident on a cystal lattice, it can get Bragg reflected and there will be peaks at certain angles depending on whether Bragg condition is satisfied and also on the atomic structure factor (ASF directly affects the intensity of the peaks in addition to controlling certain disallowed diffraction planes). Can someone shed some light on this esp. on the atomic structue factor as I still do not understand the mechanism of how the atomic structure factor affects totally disallowed 1st diffraction planes and also on the intensity of the diffraction peaks observed at allowed diffraction planes.

It is very simple. Light as it pass through a optical tranparent material, gets absorbed by the material (exciting an electron to higher energy level) and emitted as the excited electron goes back to a lower energy state. This in effect, will seem to slow down the speed of light in the optical medium due to a series process of absorbtion and trasmission of visible light photons. The slowing of speed of light causes refraction of light. This means that the optical tranparent material must have energy band gap that is sensitive to optical frequencies, so that they can absorb light photons.

Photons are not charged particles, they are electrically neutral in net effect. Photons are just electromangnetic waves and it can range from short wavelengths such as gamma and UV, to long wavelengths such as radio waves. Rememeber that EM waves are sinusoidal-varying electric field orthogonal to a sinusoidal magnetic field orthogonal to the direction of propagation. They are produced when a charge experience acceleration such as an electron vibrating about their equilibrium position due to thermal energy (causing blackbody radaition) or electron jumping from higher energy level to lower energy level (causing your emission line spectra). As to your question when two photons collide, we have to employ the wave-paricle duality in QM. Since we cannot observe individual photons and the photons do not interact with matter, the photons behave as a wave (the wavefunction do not collapse) when they collide heead on or at certain angles, and they superpose in a wave manner. For example, if two photons collide in phase, they will have contructive interference, and if they collide out of phase, they will have destructive interference. Since we do not know where the photons are positioned anywhere in space, according to Heinsenberg uncertainty principle, they is no way we can deduce that in the process of collision, they form a new photon with more energy (counterexample, what about destructive interference case??? they should have less energy) We therefore have to resort to the wave theory of light to explain the effect. Photons of different wavelengths do not just pass through each other. They superpose according to the superposition theorem of waves and produce a completely new wave with different frequency components according to the Fourier theorem. For example, two waves, one cos(2*pi*f1*x) another cos(2*pi*f2*x) superpose to give cos(2*pi*f1*x)+cos(2*pi*f2*x) = 0.5exp(j*2*pi*f1*x)+0.5exp(-j*2*pi*f1*x) +0.5exp(j*2*pi*f2*x)+0.5exp(j*2*pi*f2*x) Transform to frequency domain using Fourier transform, you have 0.5 del(f-f1) + 0.5 del(f+f1) + 0.5 del(f-f2) + 0.5 del(f+f2) In the frequency domain, the resultant superposed wave has two frequency components f1 and f2, and depending on the value of f1 and f2, we may have a fundamental frequency equal to f1 or even smaller. eg. f1 = 2 * fundamental frequency f2 = 3 * fundamental frequency Question: If I'm understanding things, photons don't have to be light we see, this one's confusing me.. Radiation given of by uranium or something like that is just photons of a kind we can't see, correct?? Does that mean that a radio transmitter is emitting photons we can't see but a radio receiver can "see" them?? Yes, photons do not have to be the light that we see depending on the wavelength of the photons. Our eye is only sensitive to the visible region of the EM radiation (400nm to aroung 650nm). Don't ask me why this is it. Maybe due to evolution on earth and the nearest star to Earth Sun is emitting EM blackbody radiation most intense in the visible light region. And so most living things on earth evolve to have eyes to be sensitive to the visible light range. As we know, we can see things by 3 ways, reflection of light from a body, absorption of light and blackbody radiation emission. Actually, most objects on earth are emitting infrared radiation at its peak intensity, and if somehow our eye can detect it, then we can do infrared imaging using our eyes, thereby no such thing as night-blindness anymore. Radiation from uranium if I'm not wrong is in the gamma wave region which has the highest frequency and shortest wavelength. Our eyes are not sensitive to gamma rays and so such radiation emitted from uranium during radioactive decay is invisible to our eyes. Yes radio transmitter transmits another type of EM waves with longer wavelength than visible light and lower frequency than visible light. These frequency is are not powerful enough to activate the neuro-sensor in our eyes, and hence radio waves too are invisible to our eyes. However, a radio transmitter is designed to detect radio waves by engineering the antenna to be of the same order of the wavelength of the modulated radio signals, so that the radio receiver can "see" these radio signals. Note that the radio receiver uses electrons in the antenna to detect radio signals, and the antenna material and length is designed carefully to be "sensitive" to the frequency of the modulated radio signal, in a way that the bandgap energy between the conduction band and the valence band is smaller than the energy of the photon, so that the electron can be excited into the conduction band to take part in electrical conduction in the antenna. This conduction will produce I-V characteristics that varies according to your modulated signal received at the receiver side.

OK, The electron diffraction is a strong example that electron can behave as a wave. We also know that if we use a light detector to observe which slit the electron goes into, we collapse the wavefunction and there will be no interference pattern anymore. However, there is something in the electron diffraction experiment, we can use delayed choice in 'tricking' the electron, by switching on the light detector only after the electron has passed through the slits. This is weird. The moment you switch on the light detector, the electron wave function collapses but it has already passed through the slits as waves!!! As we know from the result of the experiment, you will not see the interference pattern the moment you turn on the detector. But the electrons have already passed through the slits as waves, so why not the interference pattern being observed on the screen??? Does it mean that the quantum system can somehow predict the future that you are going to switch on the detector, and so it will collapse the wavefunction passing through the slit even before you turn on the detector???