the radio antennas receives millions of signals. how our radio selects the appropriate signal as it's input?  which characteristics of the signal are involved for this selection.
123568
I have some understanding of what frequency modulation entails - it entails "adding" a new frequency to the frequency spectrum of a signal. But how, exactly, is a modulated signal selected in the receiver? Say you're listening to your car radio and want the radio signal modulated at 80 Hz or something. How does the actual system select it? In general, not in car radios specifically.
38884
Despite being overall near-netural, I would expect the individual electrons and proton to radiate long EM waves as we accelerate around the Sun or rotate around our poles. Is the acceleration so small that the radiated energy is miniscule?  Or is it that somehow the macroscopic neutrality prevent radiation? If this is so, what mechanism causes single electrons or nuclei to fail to radiate?
57832
Why can I listen to my mobile phone's radio clearly inside the bus, inside the train, but not inside the elevator? Also, there is an interference of radio signal when the train is accelerating or decelerating. What is the reason behind this?
104718
My somewhat basic understanding of the concept comes from lectures I've attended about the Bohr-model, which explains the phenomenon as arising from the fact that certain configurations of an atom can only absorb certain wavelengths of light and other configurations can emit the same wavelength and change into the first configuration.  Now what I cannot understand is why these effects do not cancel out, and why in some instances absorption wins out and we observe absorption lines, and in other instances it is the other way around. Also, if I haven't misunderstood there are also some instances where we observe both absorption and emission at once.  So my question boils down to this: Why?
542
it once happened that when I touched the positive terminal of the Oscilloscope input probes letting its negative terminal open, I observed sinusoidal voltage signals with 0.5V peak to peak and of frequency nearly 50Hz. I don't understand the exact cause for these signals. When I touched the other, negative terminal output signal reached zero with a very little noise...(with the positive terminal connected to the body..) Is it because the power lines near the place I observed these are 50Hz and those induced a potential difference across me with respect to the supply ground which is also the ground of Oscilloscope... or something else.. When I touched a wire or a resistor with positive terminal of Oscilloscope.. there was no change in the Output.. Its because the equivalent model of our body includes capacitance and inductance with resistance.. If so then actually how we are being induced with such voltage signal..? Actually the discontinuity in signal is because of the difference between the frequency of actual signal and sampling frequency of my camera.
38507
Why can't you hear music well well over a telephone line?  I was asked this question in an interview for a university study placement and I unfortunately had no idea.  I was given the hint that the telephone sampling rate is 8000 samples per second.
67701
Why can energy invested in RF radiation not be harvested fully? What are the main factors that affect this? Is there any significant research nowadays that fully describes this process? I stand on the conclusion: "Energy moves from one form to another.", but in this case why can it not be harvested in that quantity to use it for powering low-power devices or store it in significant quantities in some batteries for later use? Why is it not profitable nowadays and why is it not used today like energy from water, wind and other energy sources?
28726
I hope many people might feel little awkward to see this question. But I feel this is important. Because in my place where I am living I can get a radio (Audio Receiver) very cheaply. I don't know till what extent they allow the audio frequency ranges. I guess they use  ICs for Frequency Demodulators, Frequency Clippers, etc. If the Frequency demodulator doesn't work properly and allows all the frequencies that comes through air then its going to be a big problem to the humans. Can somebody answer which are all the frequency ranges that are available and which causes human hazards?
37721
In the theory of electromagnetism (EM), why is the principle of superposition true? Can we read it off from Maxwell's equations directly?  Does it have any limit of applicability or is it a fundamental property of nature?
10270
Both light and radio waves are electromagnetic waves.that means they have almost similar properties. Both are EM waves ,E and H fields are there in both waves.....but My question is "radio waves can pass through walls ,but why can't a light pass through walls or opaque medium???
60107
First of all let me just state that I do not feel confident enough to tell anyone anything about how electric circuits work or anything about the physics behind them because I simply do not know or understand it all.  But I have many times read that there has to be a closed path for current to flow in a circuit, leading to a fact that if there isn't a closed conductive loop nothing can happen.  And I have taken that to be a definitive truth, but I wonder about something(and I might just as well be terribly far of the path of reason here).  If I was to design a circuit board which contains traces through which very high frequency signals(currents) will flow then I have to consider things like signal reflections, I don't know what reflections consists of in purely physical terms(but I have to imagine that a reflected signal is a certain amount of the current(s) that was originally sent through the trace) but apparently if I send a high-frequency signal down a trace(or wire) then under certain conditions the signal can travel down the trace(wire) only to bounce off of something and then travel all the way back to where it first came from. Where it might bounce off of something again and so it can bounce back and forth travelling the length of the trace over and over again getting smaller and smaller until it dies out.  This is just stuff from the top of my head, stuff that I have never acquired a fair understanding of in the first place. But if we restrict the scenario to this very high frequency situation, if a signal or current can be reflected back towards where it came then why would it even have to be relevant whether there are a closed loop or not.  Couldn't a broken loop present paths for such currents to bounce around in?  I know that I am at a relatively very low level of insight into these complex matters but I don't now why that wouldn't be possible. I would be very happy if anyone could enlighten me.  I have one single hypothesis without anything what so ever to support it, but perhaps the very high frequency scenario alters the way that a traces copper is utilized so that it in some respect is a closed loop in it self?
70119
In general, to kill processes we generate signals like SIGKILL,SIGTSTP etc.  But how is it known who ordered that particular signal, who has sent it to a particular process, and in general how do signals perform their operations? How do signals internally work?
91851
What is the maximum/minimum possible sampling rate for the signal to be full recovered for a passband signal?
41527
My  objective is to classify sensor signals.  The concept of my solution so far is : i) Engineering features from raw signal ii) Selecting relevant features with ReliefF and a clustering approach iii) Apply N.N, Random Forest and SVM  However I am trapped in a dilemma. In ii) and iii), there are hyperparameters like k-Nearest Neigbours for ReliefF or the window length, for which the sensor signal is evaluated, or the number of hidden units in each layer of N.N.  There are 3 Problems I see here : 1) Tuning feature selection parameters will influence the classifier performance 2) Optimizing hyperparameters of classifier will influence the choice of features. 3) Evaluating each possible combination of configuration is intractable.  So my questions are : a) Can I make a simplifying assumption, s.t. tuning feature selection parameters can be decoupled from tuning classifier parameters ? b) Are there any other possible solutions ?
35138
I am an electronic engineer so I am thinking about this from an electronics outlook.  How does the addressing system work, As I see it, the nervous system is small parallel branches attached to larger "serial" lines (spinal cord).  The serial line is then attached to the brain, but how does it then figure out where the message came from and then get the message back to the place it came from.  If I were to build a robot I would have an addressing system with each message and maybe some kind of error checking to make sure the messages don't become corrupted, but as it's not an electronic system I don't see how the brain could possibly do this. unless it is a fully parallel system and there is a single wire going from every sensor back to the CPU..
114090
We know that any object above absolute zero emits electromagnetic radiation. So hotter the object shorter the wavelengths. In the electromagnetic radiation spectrum radio waves has the longest then microwave, then infrared, then visible light, then ultraviolet, then x-ray, then gamma rays. Why do we use microwaves in microwave oven when infrared and visible light are much hotter and how do microwaves cook food when they are cooler than visible light and others.
81783
In thinking how to ask this question (somewhat) succinctly, I keep coming back to a Microwave Oven.  A Microwave Oven has a grid of holes over the window specifically designed to be smaller in diameter than the wavelength of the microwaves it produces, yet larger than the wavelengths of the visible light spectrum - this is so you can watch your food being heated without getting an eye full of microwaves.  The "realness" of electromagnetic waves seems indisputable - both from the microwave example above, and also because if I want to broadcast a radio wave with a certain wavelength, then I need to make sure I have an antenna of corresponding length to produce the wave I'm looking for.  Furthermore, we discuss and treat these waves as real, measurable "objects" that exist and can be manipulated.     Now, if I want to describe the behavior of my Microwave Oven in the framework of QM (let's pretend my oven is going to only produce 1 photon of energy corresponding to a wavelength of the microwave spectrum for simplicity) then I'll describe the behavior of that photon as a wavefunction that evolves over time and gives a probability distribution within my microwave that similarly does not allow the photon to pass through the safety grid and exit the oven cavity giving me a retinal burn.  The difference is, the wavefunction is never treated as something "real" in this description.  When the safety grid is described as working to protect you because it has holes smaller than the wavelength of the classical waves it's blocking, this is a useful description that seems to describe "real" objects/be-ables.  While it is possible to describe why an individual photon has low probability of passing through the same grid but extended physical properties such as wavelength (in space) are treated as non-real because we're dealing with a point particle and with behavior described by something we also treat as non-real (the wavefunction); it seems unclear to me why we insist this wave function which predicts behavior of physical measurements so well is somehow "non-real."    Put another way, if we have no problem treating EM waves as "Real," then why do we insist on treating the wavefunction that describes the same behavior as "unreal?"  I understand there is recent research (Eric Cavalcanti and his group for one) trying to argue this point, but as every respectable physics professor, I've ever encountered, has treated the wavefunction as an indisputably non-real mathematical tool, I needed to ask this community for an answer.