# Questions tagged [physics]

Physics is the natural science that involves the study of matter and its motion through space and time.

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45 views

### Why galvo mirrors have different speeds? [closed]

I want to understand why galvo mirrors have different speeds for different axes for raster scanning in a traditional laser scanning confocal microscope.
• 11
92 views

### Will a circuit oscillate at all its resonance modes or only the most dominant one?

Will a circuit oscillate at all its resonance modes or only the most dominant one? Consider an amplifier with a feedback loop that meets the Barkhausen criteria at multiple frequencies. Will it ...
• 1,175
580 views

### If electromagnetic waves are indeed created by a changing electric and magnetic field, then why don't we see magnet based antennas?

I understand that exciting electrons, basically freely moving electrons cause electromagnetic waves. The faster they accelerate the higher the frequency of the waves. If this is indeed the case, then ...
2k views

### Why does this battery use 4.2 V when its nominal voltage is 3.7 V?

I changed my headphone power circuit and I noticed it was using 4.2 V to charge a 3.7 V battery. I am confused why it is using 4.2 V. I know to charge a battery you need more voltage than its rating, ...
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54 views

### What is the reactive power of a capacitive circuit?

I want to understand the reactive power in a purely capacitive load. Surprisingly, I couldn't find the formula for this anywhere, so I derived it myself, and would like to ask 1. Is this formula ...
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228 views

### Behaviour of uninitialised RAM in an ASIC

This is a question for anyone with experience designing or with a deep knowledge of volatile memory in an ASIC. E.g. chip designers or silicon process engineers. We are using the ET1200 EtherCAT ASIC (...
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105 views

### Why are current, voltage and resistance all low in this scenario?

My understanding here is that compared to the internal resistance of the source, the resistance of a human is very low so terminal pd is also low. Then if current chooses the path of least resistance ...
2k views

### Why does an inductor's inductance decrease with current?

I am struggling to understand what seems like a fundamental behavior of an inductor. I am looking at a chip mount LQM18PN2R2MFR inductor. One of the diagrams on the manufacturer's website shows a ...
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138 views

### Measuring current with a multimeter around a closed circuit [closed]

A question from physics lab: Referring to the circuit you built to measure currents, what would happen if we completed our circuit with a wire and measured the current with the multimeter at the same ...
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46 views

### Can a vacuum tube convert tube heat to power

In playing with the triode component in the Falstad simulator, I often observe behaviors that seem nonsensical, with a tube sometimes acting as a power source. If a triode just had three connections ...
• 47k
123 views

### Why do electrons move in a circuit? [closed]

I am trying to make sense of why electrons move in a circuit. I did my research and the first answer I got is because of the difference of electric potential between the two points of the battery that ...
1 vote
67 views

### Limit of applicability of "quasistatic"/capacitive picture of devices

When analyzing the transitories associated with switching in semiconductor devices, in textbook treatments one often sees the time dependence considered by including (potentially nonlinear) capacitors ...
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140 views

### LC Resonance with Inductor with Solid Metal core

Consider inductor with air gap and solid metal core made from material with relative magnetic permeability 1 regardless of temperature (such as copper or aluminium). There is Air gap between coil and ...
• 39
98 views

### RC response for any given voltage [closed]

Lets imagine a simple RC circuit in which the response voltage is defined by the following equation: $$V(t) = V_{inf} \cdot (1-\exp(-t/\tau)$$ This is only true when a constant voltage is applied (...
1 vote
640 views

### How does a boogie board/LCD writing tablet/ChLCD reset actually work?

there are those boogie boards, which essentially are just special LCD screens: ChLCD screens. which work by reflecting light on the places where it is activated. in a boogie board, the pressure causes ...
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1 vote
100 views

### Confusion about why pn junction discussion insensitive to doping variations faster than Debye length

Sze (Physics of Semiconductor Devices, 3e) gives the following discussion which relates to how equilibrium electric fields/equilibrium potential build-up at thermal equilibrium are related to doping ...
• 1,161
1 vote
133 views

### How to derive the maximum power transfer theorem using the relation p=v²/R?

In most derivations of the Maximum power transfer theorem the derivation is done using the relation of power : P = i²R And on further derivation it comes out to be Pmax = Vth²/4Rth If I derive the ...
1 vote
86 views

### Lightning board 3 meters

In France, you will see some signs like the one below: Those signs are next to buildings which have lightning rods. They also have some conductors to make the current go from the rooftop to the earth ...
• 2,478
1 vote
82 views

### Why don't oppositely charged ends of separate batteries and capacitors produce current flow? [duplicate]

I have a question here regarding the flow of current from charges. I understand if you connect a simple circuit to a battery it creates surface charges which allow the current to flow. My questions ...
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1 vote
87 views

### Thermodynamics of a simple electrical conductor

I derived an expression for the time constant of the temperature of an electrical conductor and was looking for some constructive feedback. Initially there is no current flowing through the conductor ...
179 views

### Physics diagram to electrical schematic

After waiting a reasonable amount of time with no answer here, I have duplicated this question at Physics Stack Exchange. I am asking this question on behalf of user furious.neutrino. This question ...
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37 views

### Sizing MOSFET VDS to account for Stepper Back EMF

I'm having difficulty conceptualizing how the Back EMF of a motor, stepper in this case, can damage an H-Bridge MOSFET if the Vds aren't sized correctly. My main issue is my understanding of how the ...
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1 vote
70 views

### Seebeck Voltage [closed]

I understand that the voltage of two metal elements is V=(Sb-Sa)(T2-T1) regarding a thermocouple. But, what if mass was introduced to find the voltage? Can mass also be used to find voltage with ...
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1 vote
93 views

### Transient Charge Density in Conductor

Veritasium's (Derek Muller's) video How Electricity Actually Works makes the claim at 4:44 that: The truth is, if you average over a few atoms, you find the charge density everywhere inside a ...
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1 vote
576 views

### Understanding the formula of the saturation current in a PN-junction diode

According to Wikipedia, the saturation current in a diode is given by the following formula: I_S = qAn_i^2\left(\frac{1}{N_D}\sqrt{\frac{D_p}{\tau_p}} + \frac{1}{N_A}\sqrt{\frac{D_n}{\tau_n}}\right)\$...
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955 views

### What does it mean that an integrator has an infinite DC gain?

I mean, I get that, in the ideal case, $G\rightarrow \infty \text{ as } s \rightarrow 0$, but it doesn't make sense for a physical device to be able to amplify a signal to an arbitrarily large value,...
1 vote
133 views

### How does electricity flow if circuit is not complete? [closed]

https://youtu.be/oI_X2cMHNe0 What is he saying at 13:20? How can bulb glow if the circuit is not complete?
165 views

### What is happening at the end of this circuit? Why is the voltage zero at the bottom?

What is happening to the voltage at the bottom? Why is it zero? How does the voltage go back to the positive terminal?
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48 views

### Fundamental Question - how does magnetic permeability work for weak signals?

This is basically a physics question. If a sophisticated answer is needed, please go for it. The standard explanation of magnetic permeability is something like this: In a ferromagnetic material, ...
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70 views

### What goes on inside the transistor that amplifies the signal?

I understand that, for example, in BJT NPN transistor in common emitter configuration used as amplifier, feeding the small signal at the base will produce the larger signal at the collector. But what ...
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1 vote
60 views

### Igniting and maintaining plasma in low pressure chamber

I'm trying to create and maintain plasma inside a vacuum chamber (up to 50 Pa), I have a home-made feedthrough done with an HV BNC sealed with Viton gaskets on a KF50 flange (makes a ≈25 mm gap). I ...
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2k views

### Do MEMS or optical gyroscope record Earth's rotational and revolutional angular velocities?

I am using a 3 axis gyroscope (Android device) to record angular velocities using a sensor data logger application. When the device is placed stationary, the gyroscope recorded the following values: ...
1 vote
207 views

### Construction of a Lagrangian function for a circuit

I was reading the paper "Introduction to quantum electromagnetic circuits", by U. Vool and M. Devoret and I am stuck in the construction of a Lagrangian function for the example LC circuit ...
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1 vote
512 views

### Equation for diode saturation current in photovoltaic cell model

I've been looking for mathematical models for photovoltaic cells. Most of the references I've found use the following circuit to the model the behavior: Particularly, the diode current is modeled ...
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270 views

### What is equilibrium carrier concentration in a semiconductor? Is it material specific? How to calculate it?

Is equilibrium carrier concentration dependent on doping density? If it is, then how to know how much the doping density should be for a particular semiconductor device?
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1 vote
388 views

### How does the electric field inside a conductor remain zero?

I'm learning about current in conductors, and came across the statement that "The electric field inside a perfect conductor is zero", this is frequently explained used Gauss´s Law by saying ...
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68 views

### Why do we use silicon in the crystal direction [1 0 0]?

In MOSFET, we use silicon in crystal direction [1 0 0] , while we use direction [1 1 1] in BJT. What is the reason? I expect the opposite, because we see more atoms on the surface in the direction [1 ...
375 views

### Why don't we consider drain-bulk voltage instead of source-bulk voltage in body effect?

Why don't we consider drain-bulk voltage instead of source-bulk voltage in body effect? For example, in the threshold voltage formula, we have source-bulk voltage, not drain-bulk voltage.
1 vote
582 views

### How does a electrical device draw its rated current?

I am learning electronics and this question has been bugging me quite a bit. If I have a device, let us say rated at 6 volts and 350 mA, this means the device will work when there is a potential ...
4k views

### How do I calculate threshold voltage from the graph?

I have a table of values given to me which contains drain current Id (in μA) and gate voltage (in volts) calculated in the saturation region for a thin film transistor. I need to find the threshold ...
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108 views

### Calculate complex impedance value from voltage in time domain waveform

I have problem. I was doing an electrochemical impedance spectroscopy (EIS) test on my Randles cell. simulate this circuit – Schematic created using CircuitLab The data from potensiostat's ...
1 vote
158 views

### Difference between arsenic and phosphorus in IC fabrication technology

What is the difference between arsenic and phosphorus in IC fabrication technology? When do we prefer one of them over the other?
195 views

### Is birefringence exploited or otherwise meaningfully observed anywhere in electronics?

Birefringence is a property of certain materials where the refractive index of the material is dependent upon the polarization and direction of propagation of light through it. I recently learned ...
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1 vote
501 views

### Calculating steady-state temperature for electrically heated wire

I am working on a program aimed at finding optimal parameters (voltages and feed rates) for a hot wire foam cutter. One starting problem that has to be solved before even attempting to evaluate heat ...
130 views

### Why is the gamma term missing in the first case of single transistor example?

This is the video. How is the gamma term present in one case and absent in another?
284 views

### Maximum weight for robot arm - MG996R [closed]

I want to build a robot arm (see here) to move a dumbbell of 2 kg (roughly 2.2 lbs). However, since I have no background in engineering, I have no idea how to calculate / find out if the servos used (...
942 views

### Why does current flow direction matter in welding?

In DC welding, there are two configurations - direct current electrode positive (DCEP) and direct current electrode negative (DCEN). When the electrode is negative, the current flows from electrode to ...
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554 views

### How can I calculate how long it will take equipment in an enclosure to overheat?

This is kind of a follow-up question about determining the heat dissipated from equipment based on power consumption. I have a situation where I have to install a router and UPS system in an enclosed ...
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51 views

### How to compute flux through a sphere when flux density is given in rectangular co-ordinates?

Compute the total flux of D leaving a sphere of radius 1m, and hence the total charge enclosed by that sphere. D(x,y,z) = xX̂ Attached is my current working and the worked solutions provided to us. ...
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