Questions tagged [electrical-engineering]

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4answers
11k views

Why should a voltmeter have a higher resistance than of any circuit element across which the voltmeter is connected?

According to my textbook, it is said that, for an ammeter: It is essential that the resistance $R_A$ of the ammeter be very much smaller than other resistances in the circuit. Otherwise, the very ...
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1answer
110 views

Saturated BJT Transistors. V CE < V BE : AKA - Things that make you go “Hmmmm”

At the risk of being blasted for cross-posting, I am posting this question again here in the Physics department. It is a duplicate of this post in the EE forum. However, I would like to get some ...
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0answers
71 views

Saturated BJT Transistors. $V_{CE} < V_{BE}$ : AKA - Things that make you go “Hmmmm” [duplicate]

We use them every day and those in the know fully understand the functional characteristics of BJT transistors. There are documents and links galore explaining the operational math. There are even ...
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1answer
37 views

Unknown $c$ in torque equations [closed]

In a book I have come across a couple of unrelated equations with an unknown $c$, in equations of the form $$T_{en} = \frac{P_{en}}{2\pi n_n}c,$$ where $T_{en}$ is torque, $P_{en}$ is rated power and $...
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2answers
72 views

Short Circuit Current [duplicate]

Theoretically, shouldn't there be no current flowing through a short circuit? Across the short circuit, the voltage is 0, and the resistance is ideally 0 as well. So how does it make sense for any ...
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0answers
48 views

How to determine the best fit battery for a battery powered application? [closed]

This is an attempt to get some scientific information about how to best use batteries (I'm going to refer to both single cells or assemblies of cells as batteries for the sake of this question) in ...
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1answer
56 views

Resistance of a Large (relative to lead contact) Conductor

How do I calculate the effective resistance of a large piece of metal between two small electrical contacts? The thermal image (from an interesting but unrelated article called An Exception to Ohm's ...
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0answers
51 views

What is the optimum altitude for tapping atmospheric charge?

The atmosphere appears to have a large potential difference (voltage) in relation to ground. The obvious example is the thunder where this voltage becomes large enough to break the insulator - air. Of ...
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1answer
383 views

How does a gate voltage determine the state of a transistor?

It is known that in a transistor, like the FET below, it has an 'on' state when current flows from source to drain, and an 'off' state when there is no current flowing between junctions. This current ...
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1answer
654 views

Minority charge carrier concentration in pn-junction during forward bias

my question: During forward biased, is it true that the minority concentration (e.g. hole in n-type) could be higher than the majority concentration of the same type (e.g. hole in p-type, which is ...
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2answers
3k views

How is it I can see the infrared Face ID on my iPhone X? [closed]

I’ve tried using Face ID in the dark on my new iPhone X and I can very clearly see the infrared flood Illuminator light up as well as the infrared dot projector. Is it possible for humans to see ...
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1answer
471 views

Why did Shockley's Copper Oxide Field Effect Transistor fail and has anybody succeed in creating a Copper as appose to Silicon based FET?

Reading Tomas Lee's very interesting historic account of Semiconductor's used in the Electronic industry: https://web.stanford.edu/class/archive/ee/ee214/ee214.1032/Handouts/HO2.pdf I wondered, why ...
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1answer
58 views

What happen to organic solar cell when applied electric field? [closed]

For example, I have a PTB7:PCBM organic solar cell. I applied electric field in forward basis. Will it contribute to increase charge separation? Then, if I do in reverse basis, what will happen to ...
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2answers
255 views

Why don't radio waves ruin electronics?

Out of curiosity, I estimated the voltage amplitude for typical visible light. Wikipedia says that sunlight hits the earth at about $1000 W / m^2$. The intensity of an electric field is given by $$I =...
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1answer
438 views

Power consumption of a qubit?

I keep hearing “A quantum computer will consume so much power that it'll need a dedicated power plant.” I'm trying to find literature to back this up. I would love to get an estimate for the ...
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1answer
307 views

Patch antenna excitation

How can we excite the patch antenna in different modes like TM10 and TM30? If a microstrip patch antenna resonates at TM10 mode can it resonate in other modes also like TM30?
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1answer
190 views

Equivalent resistance of a circuit [duplicate]

Can anyone help me with this one please?! I want to find the total resistance but the circuit seems neither parallel nor serie. What should I do?!
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1answer
113 views

What's a reasonable difference in resistance between batteries in parallel?

I know that the differences in resistance between batteries in a circuit will create different drop, and as a result draw unequally. Unfortunately, I couldn't get my hands on a parallel D-cell ...
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3answers
3k views

Use of slip rings and its working in a AC generator

What is the use of slip rings in an AC generator and its working? What would happen if there was no slip rings?
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1answer
425 views

Explaining the phsyics behind a coil gun

I made a coil gun for my extended essay, but I'm having some trouble explaining exactly how it works. The research question is how does varying the initial position of the projectile affect the ...
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1answer
117 views

Speed in telegraph equation

From the telegraph equation, it is found that electrical signals travel at fraction of light speed over transmission lines. For copper cables, the speed is about 0.5 c. Is such speed experimentally ...
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3answers
4k views

What are crystals used for in radio?

What are crystals used for in radio? I think I sort of know the answer to this but I would like a more detailed explanation. To my current understanding, piezoelectric crystals are used to modulate ...
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1answer
39 views

Can the telephone signals be tapped by an intruder? [closed]

I have a doubt. There are lots of telecommunication signals moving across the world, at the same time. For example, take the case of mobile phone calls. Millions of speech signals are propagating ...
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0answers
210 views

TV screen goes black momentarily when a fan turns off - why?

I am a physics major teaching in a classroom when a students turns off a regular pedestal fan - at the same instant, the TV screen at the front of the classroom goes black for a couple of seconds. I - ...
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1answer
953 views

Larsen effect and pitch of sound

Why Larsen effect produces always high pitched sounds, never low pitched?
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2answers
2k views

What are the wavelengths of the red, green, and blue lights used for making LED pixels?

LED screens in televisions and smartphones use pixels that use a combination of Red, Green, and Blue diodes that can produce any color when used in the correct proportion of intensity. My question is,...
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1answer
541 views

About The foxhole radio: coil rotation times, whether capacitor's needed

I recently tried to make a DIY foxhole radio. Althogh I made it according to the manual (140times coil rotated, ground(220V), blued razor blade, pencil core and antenna (just a steel stick about 1 ...
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1answer
66 views

Impedance $Z$ for AC source [duplicate]

In AC, there are $R$, $C$, & $L$ or $R$, $X_C$, & $X_L$. $$ X_C = \frac{1}{\omega C} $$ where $\omega=2\pi f$. But sometime we're using the $X_C = \frac{1}{j\omega C}$. I understand the $j$ ...
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2answers
178 views

Example of transfer function and driving signal equation

In a text I am using it states that for linear dynamical systems, there is a very simple relationship between the power spectrum of the driving signal and that of any one of the system variables. If ...
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0answers
48 views

Resonance curve of oscillating curcuit

I took a resonance curve (see below) for different (series) oscillating curcuits plotting I:f and found that the amplitude differs for different values of C and L. Why exactly is that? Blue specs: $L=...
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1answer
48 views

Charging current of capacitor

In one of my books there is a figure where G is a neon lamp. Basically the capacitor gets charged once the switch is closed up to a certain spark-current $U_Z$ where the neon lamp gets switched on so ...
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0answers
53 views

Anti-theft through high frequency oscillating curcuits

As far as I know the anti-theft chips used in malls (see here) are basically high frequency (8.2 MHz) oscillating curcuits that resonate when stimulated by an external field (the two pillars at the ...
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1answer
90 views

Fixed voltage source & maximized power

The goal of a voltage source like a battery for example is that it will provide a fixed voltage regardless of the load resistance. The battery has internal resistance Ri If R_L≫R_i, the voltage ...
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1answer
40 views

Electrical induction limit

How much current can a metal object "hold" using electrical induction? For example a 1 kg ball of iron. Lets say i start to charge the item negatively. I suppose its up to the mass and the kind of ...
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3answers
3k views

Current being 0 in potentiometer

Why does the current become 0 in potentiometer when the potential difference is equal to the emf of the battery? I know how a potentiometer works, this is the only part troubling me.
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1answer
289 views

Why is the emitter current of a common emitter slightly greater than the collector current?

I'm trying understand where electricians came to agreement that Ie = Ib + Ic. I know that the base of a NPN transistor is very thin, allowing electrons to flow through to the collector which explains ...
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2answers
447 views

How do radio-frequency transmission work?

Are sound frequencies (20-20,000Hz) included in radio waves? If not, how does sound get to our end (radio/receivers)? Is there some type of encoding going on inside those radio-frequency waves or ...
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1answer
101 views

What causes hum in a coil with an air core?

I am designing a controller to cut down heat production in electrically actuated solenoids (12VDC, 10A). This is not for proportional control. I am using a 50% duty cycle and have been experimenting ...
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1answer
283 views

Reflecting radio waves?

I have a laptop that I primarily use for music. I don’t want to have to bring my laptop to the basement every time I want to listen to music. I have bluetooth headphones and I was wondering if there ...
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0answers
151 views

Data Transfer & Physics

I am doing a large end of high school project on some physics, which has to involve an experiment. In lieu of this, I bought myself a micro oscilloscope - RSO138 - which I would use to listen to data ...
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1answer
453 views

Using RC circuits to solve differential equations

As I was thinking about RC circuits it dawned upon me that under the correct configurations one could very efficiently solve differential equations by programming them into an RC circuit (the ...
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3answers
59 views

Why is this a circuit in parallel if it is topologically different than a parallel circuit

I'm struggling to understand why the left circuit is equivalent to a parallel circuit. I'm trying every deformation in my mind I can think of but I can't seem to get out the second circuit. Secondly,...
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1answer
413 views

What is the physical quantity which makes a Reed switch open or close?

I would like to generate the activation map of a Reed switch, similar to the following: Using a magnetic finite element modeling software such as FEMM. Assuming the Reed switch closes for X Ampere-...
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1answer
293 views

Semiconductors ->Transistors npn and pnp

I have had a tough time with semiconductors, I cannot understand the working of a transistor in saturation, as a switch, and also the cases as to when which part is forward biased, I also tried google ...
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1answer
255 views

Question about the principles behind AC generators

I've learnt that when the normal drawn from the surface of a coil (the area vector) is in the same direction as the magnetic field, i.e. when the field is perpendicular to the coil, then there are a ...
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1answer
103 views

Mutual Inductance with non-homogeneous magnetic reluctance medium

Given two inductive circuit elements, it can be shown that the inductance coefficient between the first one and the second one is the same as the inductance coefficient between the second one and the ...
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1answer
163 views

How do Cogs and belts work? / Cogs and belts in an ICAS exam [closed]

Does the left red wheel turn in the same direction as the leftmost orange wheel?If so P and S go up? If not, Q and S go up? My reasoning: If so Leftmost orange cc Left red cc (by assumption) ...
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1answer
85 views

If you would replace ITO, would it ideally have a lower or higher relative permittivity?

ITO is used as the conductive layer in a touchscreen. If you would replace ITO in a capacitive touchscreen, would it ideally have a lower or high relative permittivity? The relative permittivity (...
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2answers
14k views

Surface charge density of parallel plate capacitor

If empty (filled with vacuum) parallel plate capacitor has two plates set to be $ d=0.0012m $ apart and connected to $ 1500 V $ voltage source, then surface charge density should be: $$ \sigma = \...
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1answer
464 views

Memristors: how are memristors modeled in terms of impedance?

How are memristors modeled in terms of impedance? I have been searching the net for memristors, but could not get much of an idea. I'm not interested in how memristors are made (by combining ...

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