63

tl;dr- The maximum data rate you're looking for would be called the maximum entropy flux. Realistically speaking, we don't know nearly enough about physics yet to meaningfully predict such a thing. But since it's fun to talk about a data transfer cord that's basically a $1\mathrm{mm}$-tube containing a stream of black holes being fired near the speed of ...


34

The electrons themselves don't move all that fast. The wave energy is the part that moves quickly. Picture it this way. You have 500 meters of pipe, with a small hole at the other end. The pipe is full of water and you increase the pressure at your end. Water will flow out the other end immediately. This is the electrical energy (pressure) and the copper(...


33

The Shannon-Hartley theorem tells you what the maximum data rate of a communications channel is, given the bandwidth. $$ C = B \log_2\left(1+\frac{S}{N}\right) $$ Where $C$ is the data rate in bits per second, $S$ is the signal power and $N$ is the noise power. Pure thermal noise power in a given bandwidth at temperature $T$ is given by: $$ N = k_BTB $$ ...


31

The physical 'meaning' of the imaginary part of the impedance is that it represents the energy storage part of the circuit element. To see this, let the sinusoidal current $i = I\cos(\omega t)$ be the current through a series RL circuit. The voltage across the combination is $$v = Ri + L\frac{di}{dt} = RI\cos(\omega t) - \omega LI\sin(\omega t)$$ The ...


28

An capacitor has one intuitive property: Its voltage can't change instantly since its voltage is dependent on the charge it has stored, and charge doesn't move at infinite speeds (there is always resistance somewhere), therefore you can't instantly charge up a capacitor without infinite current. More capacitance means less voltage for the same amount of ...


19

Yes. That is the operating principle of this device, among many others:


14

In fact, electron's speed is not so fast that light bulb glows up immediately. It is the electromagnetic field which travels in the circuit at near the speed of light that is resposible for it. After turn on the light, electron only acquires a little speed in addition its thermal speed. The thermal speed of electron can be estimated by $mv^2/2\approx k_BT/...


13

You might find the Yahoo "home_transistor" group a useful resource. There's also a series of videos on YouTube by Jeri Ellsworth including some where she makes transistors. In one, in particular, she takes the crystal out of a germanium point-contact diode and turns the crystal into a point-contact transistor (much like the Bell Labs transistor.) There ...


13

Anyone who's ever set up a public address system knows that we do have this issue. It's generally called feedback, and tends to result in a high-pitched screaming sort of sound. It can be kept under control by careful use of EQ (graphic equaliser) and correct positioning of the microphone and speakers. (Pro audio people probably have lots of other tricks for ...


12

For a given circuit in a given technology, power increases at a rate proportional to $f^3$ or worse. You can see by looking at the graph in @Martin Thompson's answer that power is superlinear in frequency. $P=c V^2 f + P_S$ is correct, but only superficially so because $f$ and $P_S$ are functions of $V$ and $V_{th}$ (the threshold voltage.) In practice ...


10

Calling it a built-in voltage is something of a misnomer. People usually think of "voltage" as "what you measure with a voltmeter". So "voltage" is normally synonymous with "electrochemical potential of electrons" (in stat mech terminology) and with "difference in fermi level" (in semiconductor terminology). Under this definition, the built-in "voltage" is ...


10

What is the physical behaviour which allows a capacitor to act as a high or low pass filter? A capacitor alone cannot act as either. To create a filter you need a combination of resistance and capacitance or inductance and capacitance (or RL). You need two immittances, at least one of which is reactive. Let's take a practical example, an RC circuit. This ...


9

There is a physical meaning behind the imaginary component of the impedance. You can re-cast the complex impedance $Z = R + jX$ (using engineering's notation $j$ for the imaginary unit) in polar form to get $Z = |Z|\exp(j\phi)$. $|Z|$ is the magnitude of the impedance, and scales the amplitude of the current to get the amptlitude of the voltage. $\phi = \...


9

Imaginary components in physics often mean phase shifts. In this case the impedance is like a resistance, but it kicks in when the current is changing by messing with its phase.


9

Strictly speaking, a phase coherent electron device is an electronic device whose dimensions is smaller than the phase coherence length of the electrons. This definition is the one adopted in mesoscopic physics. So, what is a phase coherence length? To each electron, one associates a wave-function $\Psi=\Psi_{0}e^{i\varphi}$, with $\varphi$ the phase of the ...


9

Imagine electricity as water in a pipe. The current can flow in either direction (direct current, DC) or one way then the other way (alternating current, AC). Now put a rubber membrane in the pipe. This is the capacitor. Now it will slow and then stop DC, but AC can still keep wobbling back and forth. In this way, capacitors block DC but enable AC. ...


8

In this case, the magnitude is telling you how to scale your input signal, and the argument is telling you how to phase shift it. Complex numbers usually represent 'amplification' and 'twist'. So, say, 1 means 'leave it the same', 2 means 'double it', 0.5 means 'halve it', i means 'one quarter turn', -1 means 'one half turn', -3i means 'triple it and give ...


8

Well, it does. The Earth's magnetic field is about half a gauss, or $0.5\times10^{-4}\rm\,T$. So if you have a meter of wire carrying one ampere of current from east to west, it'll feel a magnetic force of $0.5\times10^{-4}\rm\,N$ in some mixture of upwards and the north-south direction that depends on the tilt of Earth's field at your location. (I'm in ...


8

A matter of thresholds The reality of spread-spectrum is complicated but let's imagine that the WiFi router and cell phone tower have both allocated 1 Watt to transmit to your phone, and it in turn can transmit 1 Watt back in both cases. If the WiFi router is 10 meters away and the cell phone tower is 1 km away, then it's possible to imagine a four order ...


7

This is very easy to understand why centeraltap transformer is needed in a full wave rectifier. Let us assume that we have a simple transformer, and there are two diodes and the central wire coming out from the transformer is not present there which is obvious since we are not using centeraltap transformer. So now see the figure In first case let A be at ...


7

Sometimes for physical intuition, it's nice to think about the extreme cases. For instance, a zero frequency signal is just a DC voltage. If we send it through the RC high pass filter, the capacitor is just like a break in the circuit, and prevents any current from flowing. Slightly more quantitatively, the capacitor equation $Q = CV$ implies that if we ...


6

In experimental physics it is required to use electronics as instruments. You must know how they work(amplifiers, ADC's, MCA's etc) in order to fully understand and design an experiment. Usually, you don't need too much electronics(filters, amplifiers, transistors, digital electronics-boolean algebra) is more often than not, more than enough. You need ...


6

The key difference between a Zener diode and a normal diode is that the Zener diode has a low breakdown voltage - typically in the few volts range. The breakdown voltage is low because the heavy doping means the depletion layer is very thin, and even at a low voltage the field strength over this thin depletion layer is very high. With a conventional diode ...


6

But how can there be current without electron potential (voltage)? In the case where there is no resistance, current (once flowing) does not require any voltage to continue flowing. If you start a current flowing in a superconductor, then even with no applied voltage, it continues to flow. It doesn't take any force to keep a ball rolling if there is no ...


6

To take your question at face value: what if we somehow connected the two ends of a charged capacitor with an ideal wire, that somehow avoids having self-inductance or radiation losses or anything? Well, we can write a differential equation using Kirchoff's voltage law: $$ \frac{Q}{C}=0 $$ Whoops! That's nonsense- our initial conditions are not $C=\infty $ ...


6

The short answer is that you need a complete circuit for a battery to work. However, I find a longer answer to be very useful to help with understanding. If you have a circuit, you are solving a problem in what we call "electrodynamics." Electrodynamics is studying what happens when electrons are moving. In a circuit, electrons are constantly circling ...


6

here is why you can kill yourself with the knife-and-toaster trick. the AC power line into which the toaster is plugged consists of a hot line (black wire) that has 120VAC on it and a ground return line (white wire) which is at zero VAC or very close to it. The toaster has a switch inside which feeds power to the bare resistance wire inside the slots when ...


6

It can, and does. However, the absorption of a photon with wavevector $\mathbf k$ causes an electron transition from $(\mathbf{k}_i,E_i)$ to $(\mathbf{k}_f,E_f)$ subject to the constraints that $E_f-E_i = \hbar c|\mathbf k|$ and $\mathbf{k}_f-\mathbf{k}_i = \mathbf k$. Consider a transition corresponding to an energy difference of $2$ eV. This would ...


6

Using imaginary numbers for current in reactive components just happens to make the maths a lot simpler. In AC circuits there is typically some phase difference between the voltage and the current. Manipulating these quantities without the use of complex numbers, but instead just keeping track of the phase differences (such as the power factor), is a right ...


Only top voted, non community-wiki answers of a minimum length are eligible