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37

We tend to think that our modern electronic devices are very energy-efficient so mechanical mainsprings etc. must be enough but they're not. After all, the (Intel i7) microprocessors have over 1 billion transistors per chip and each transistor has to consume some nonzero (and not "totally" negligible) energy, after all, to do an operation and they do ...


34

An alternative way to generate random numbers, that truly is quantum, and also quite easy: put a small radioactive source near a Geiger counter. Radioactive decay is a truly random event in the quantum sense, and is basically not subject to thermal noise at all. For maximum visual impact, replace the Geiger counter with a cloud chamber. That way you can ...


17

Corresponding wavelength is 22.11 meters long, but we want also to emit our EM waves into the environment. This means if we get a nice half-wave dipole antenna we would need it about 11 meters in length, $\lambda/2$. Which is quite large for mobile device. Ok, lets reduce size by using quarter-wave antenna as in WiFi, based on ideas of quarter-wave ...


11

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 ...


11

If I understood correctly, what you are trying to build is a hardware based random number generator, where you want to use some quantum mechanics-based mechanism to supply the randomness. I'm no experimentalist, thus, take my comments with a grain of salt. Your suggestion is to use Schottky noise from a illuminated photodiode. I believe that it's a pretty ...


10

I found a general, qualitative answer in David Blackstock's book Physical Acoustics, on page 46: Impedance is often described as the ratio of a "push" variable $q_p$ (such as voltage or pressure) to a corresponding "flow" variable $q_f$ (such as current or particle velocity). I also received a nice answer to this question on another Q&A site ...


10

I'll give the answer to this question using an unusual method that showed up in the American Mathematical Monthly's problem section perhaps in the late 1970s. This is not necessarily the easy way to solve the problem, but it works out nicely from an algebraic point of view. The way most people solve most resistance problems is to use series and parallel ...


10

What makes it a good idea to use RMS rather than peak values The rms value, not the peak value, is the equivalent DC value that gives the same average power. Recall that power is the product of voltage and current: $p(t) = v(t) \cdot i(t)$ For a resistor, we have: $p(t) = R[i(t)]^2$ To find the average power, we must take the time average of both ...


10

To add to Nathaniel's Answer because (1) it is a good answer and (2) I get nervous recommending radioactive materials handling to anybody I don't know: I would really think about the cloud chamber idea, especially since you're a software guy with a math background. It would need to be inside a darkened container, but you could run a webcam to show what is ...


8

Use a star-delta transform to simplify part of the circuit. You may also use the principle of superposition.


7

I'll take it step by step here. First I'll write the answer for the first few cases with circuit analysis. Then I'll apply a reduction to show the pattern that the problem arrives at. N=1 $$Z = R+R=2R$$ N=2 $$Z = R+\frac{1}{\frac{1}{R}+\frac{1}{R + R}} = R \left( 1+\frac{1}{1+\frac{1}{1 + 1}} \right)=\frac{5}{3} R$$ N=3 $$Z = R+\frac{1}{\frac{1}{R}+\...


7

Dims is almost correct, in that you would only see resonant tunnelling effects at very low temperatures. In other words, at very low temperatures the electrons will sit at very well defined energy levels within the transistors. Under certain biases (voltages), the energy levels on either side of the thin barriers between devices will line up, and electrons ...


6

For any given $n$, you can work it out via the rules for series and parallel resistors, but to get a general formula, valid for all $n$, doesn't look easy to me. The best way I know of is to get a recursive relationship giving the resistance of an $n$-step ladder in terms of an $(n-1)$-step ladder. If I'm not mistaken, the $n$-step ladder can be thought of ...


6

A x----x-----[1]-----x-----[2]-----x----x B | | | [4] [3] [5] | | | |-------------x-------------| $\uparrow$ Fig.1. OP's original circuit. As suggested by Manishearth, one can perform a $Y$-$\Delta$ transform from $Y$-resistances $R_1$, $R_2$ and $R_3$, ...


6

Attempts to find an average value of AC would directly provide you the answer zero... Hence, RMS values are used. They help to find the effective value of AC (voltage or current). This RMS is a mathematical quantity (used in many math fields) used to compare both alternating and direct currents (or voltage). In other words (as an example), the RMS value of ...


6

Capacitors, as used in electric circuits, do not store electric charge. When we say a capacitor is charged, we mean energy is stored in the capacitor and, in fact, energy storage is one application of capacitors. Now, for an ideal capacitor in a circuit context, the current through is proportional to the rate of change of the voltage across: $$i_C = C \...


5

It's surprisingly difficult to find a nice simple description of how a transistor works. This description is from my old physics book - I suspect this may be oversimplified and I'm sure a complete description would run to lots of equations! Anyhow, this is what an NPN transistor looks like: so as you say, the collector-base junction is reverse biased and ...


5

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 ...


5

While I agree with Alfred Centauri's answer, I am not sure it gives a direct answer to your specific question for the following reason. If there is a receiving antenna somewhere, there is always some reflection, however minute. If the antenna is connected to a circuit, the conditions of reflection will change, and the transmitter can "notice" that. In ...


5

Well, believe it or not, eels have been used to power Christmas trees (youtube link), so powering an electric motor isn't quite out of the question. However, Eels emit that 400 V at 1 A = 400 Watts (though the youtube video says that the eel was emitting 800 Watts). An electric car requires over ten thousand Watts of power to operate. So in theory, you ...


5

The way every electric car works is by converting electrical energy into kinetic energy. The externally released energy of a discharge of Electrophorus electricus has been studied and has been found to be around $17 \mu\mathrm J$ per discharge. Since kinetic energy is $E_k = \frac{1}{2}mv^2$, if we assume a combined driver+vehicle weight of 1000 kg, then ...


4

Your approach is not correct. Once you connect the two capacitors what happens is that the plates that are connected with a wire will have the same potential because they form a conductor. Suppose the initial charges are $Q_{1i}=Q$ and $Q_{2i}=0$. Because the potential difference across both capacitors is equal you get $$ \frac{Q_{1f}}{Q_{2f}}=\frac{C_1}{...


4

The $A/W$ units refer to the current (in Ampère) produced per Watt of light incident on the photodiode. This current-production happens when the diode operates in the so-called photoconductive mode. Since your question wasn't on the inner workings of a photodiode, I won't expand on this, but Wikipedia contains some more information if desired.


4

As an intermediate step, consider a sinusoidal source driving an infinite transmission line with some characteristic impedance $Z_0 = 50\Omega$. The source "sees" a real impedance of $50\Omega$ and so, power is delivered to the line and, since the TL is infinitely long, the power is transported down the line, via an electromagnetic wave, without reflection. ...


4

It's called a diode because the device has two terminals. Devices that have three terminals are called triodes, and those with five pentodes. Words of that type have fallen by the wayside except in the realm of vacuum tube electronics ... except for the word diode, which has hung on. Note that the Wikipedia article that you cite refers specifically to ...


4

Assuming that each sensor sees only the light from the other night light, and assuming that each night lights is bright enough to reliably trigger the other's sensor, then you have discovered a configuration that computer engineers call a "flip-flop". https://en.wikipedia.org/wiki/Flip-flop_%28electronics%29 It has another name, "bistable multivibrator." "...


3

To (hopefully) answer both your questions simultaneously, think of the concept this way: let's say I have an electrical circuit which consists of a battery (your EMF) connected by wires to some unknown electrical setup within a black box. Nothing appears to be melting or catching on fire within sight (which would imply the likely existence of a short ...


3

Here is how I would do it, following the method outlined by kleingordon in a comment. This method is less cool but more general than Carl Brannen's answer, because it will work even in the case where there are crossing wires and you can't rearrange it into a single sheet of resistive material. Let the electric potential at $A$ be $V_A$ and that at $B$ be $...


3

Resistors are generally used to dimension electrical devices to the ranges in voltage, current, time constants, what have you, that are needed. In this specific example the resistor is used to dimension the voltage drop in case one of the inputs has low voltage (lower than $V$), so that a current flows from $V$ to the input (it can only flow in this ...



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