# 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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### What limits CPU speed?

I've recently talked with a friend about LaTeX compilation. LaTeX can use only one core to compile. So for the speed of LaTeX compiliation, the clock speed of the CPU is most important (see Tips for ...
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### How do I prove to my physics teacher that adding a battery in parallel doesn't double the current?

simulate this circuit – Schematic created using CircuitLab My physics teacher said that the current through the resistor is 4A because each battery has a current of 2A if hooked up to the ...
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### MOSFET: Why the drain and source are different?

Why the drain the source terminal of the MOSFET function differently while their physical structure is similar/symmetrical ? This is a MOSFET: You can see that the drain and source are similar. So ...
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### How fast does electricity flow?

I get confused on the low-level physics of electricity from time to time. It came up in "Which way does electricity power a circuit," and I don't totally get it. How fast does electricity flow? Is ...
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### What is the difference between emitter and collector for BJTs?

the (surely simplistic) model of a bipolar junction transistor one is taught in foundational physics course appears to be symmetric. - So, what is the difference between the collector and the emitter ...
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### Why are electrons not pushed out from the conductor at an open circuit under the influence of a power source?

Frequently, electric current is compared with water flow. For example, if I make a hole in a water tank, water will flow till the tank pressure and the atmospheric not become equal or the tank becomes ...
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8k views

### Why is there no rolling shutter when using a mechanical shutter?

So I know it might not be the best place to ask this question, but maybe some of you are familiar with the mechanics of digital mirrorless cameras and the technology of CMOS sensors. I don't quite ...
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68k views

### How do BJT transistors work in a saturated state?

This is what I know about NPN BJTs (Bipolar Junction Transistors): The Base-Emitter current is amplified HFE times at Collector-Emitter, so that Ice = Ibe * HFE <...
14k views

### How does a nine volt battery make a spark?

With a nine volt battery, touching the two terminals together (or using a faulty terminal) will cause a spark roughly where I would want it to be. How is this possible? Is it ionizing only a very ...
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### Physics behind signal reflections and series termination

I have been looking for cause of signal reflections in transmission lines. Everywhere it is concluded that the reason is impedance mismatch. I can understand if the impedance changes in the path of ...
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### Why does the thickness of a wire affect resistance?

A teacher explained why by using a highway analogy. The more lanes you have, the faster the cars go through, where the number of lanes obviously represent the wire thickness and the cars represent ...
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### Why the prevelance of mechanical oscillators in electronic circuits?

The clock sources in modern electronics seem to come invariably from quartz and MEMS oscillators, both of which generate vibrations mechanically. The amplitude and frequency of the vibration are ...
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### Which everyday components involve flows of charge that are not electrons?

I like this explanation of why there's nothing wrong with conventional current being the opposite direction from electron current. It mentions batteries and fluorescent bulbs as two cases where the ...
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### Why does capacitance value changes with applied voltage?

I searched many forums and papers on google and haven't came up with anything. Even asked my teachs and they did not know. One said something about piezo effect but she was not sure about it. So here ...
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### Why doesn't higher frequency mean higher data rate?

My question is very primitive :) I've read multiple articles today and answers here, on Stack Exchange, but still don't understand one thing. Why doesn't frequency itself affect data rate in mobile ...
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### How did scientists deal with electronics' problems before Kirchhoff and Ohm's laws?

Both physicists developed really powerful laws which still nowadays rule the electronic behavior of circuits. These help us every day to solve problems, calculate circuit variables… but how did ...
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491 views

### Is it sensible to always use larger diameter conductors for carrying smaller signals?

This question as originally written sounds a little bit insane: it was originally asked to me by a colleague as a joke. I am an experimental NMR physicist. I frequently want to perform physical ...
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### Why do LEDs have a maximum current?

So I understand that LED's have a maximum current (like 20mA for instance), but scientifically why is this? Using the water analogy it seems like a high voltage would be the thing that would mess up ...
5k views

### Acceleration when device is on tilt

I'm currently working on device that uses always-on 3D accelerometer(using scale +-2g) and 3D gyroscope(using scale +-250g) -sensor. I can read every possible vector (X,Y,Z) and their acceleration (g'...
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### Saturated BJT Transistors. $V_{CE} < V_{BE}$ : AKA - Things that make you go "Hmmmm"

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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### 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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### Why aren't wires capacitors?

A parallel plate capacitor consists of two parallel conductors with opposite charges. In the above diagram, the wires are parallel and conductors so do they act as capacitor plates? If they do, if ...
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### Is charging a capacitor to a new DC voltage fundamentally lossy?

There has recently been a question about switching losses and Landauer's principle entered the discussion. I included it in my answer because I believe, a capacitor fundamentally cannot be charged ...
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### Constant current in a circuit?

simulate this circuit – Schematic created using CircuitLab A battery pumps electrons by creating an electric field and converting electric potential energy to kinetic. Near the positive ...
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### How does electricity produce heat, and where do the electrons go?

When electrons flow through a conductor it is subject to resistive loss, which has the unfortunate capability of producing heat from this current, leaving us with less current in the other end of the ...
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### 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: ...
11k views

### Is there a formula to determine the size of decoupling capacitors? [duplicate]

I have used many decoupling capacitors in circuits and have seen them in many textbooks, but they never give an explanation about why they picked a certain size of capacitor. Everyone seems to have a ...
2k views

### Wavelengths blocked by a certain metal mesh

Introduction: I've heard about a theory, according to which you can check if there is any leakage in your microwave oven by putting your mobile phone inside and call it. However, I gave it a try and ...
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### MOSFET channel length modulation - Glossing over physics

I am having trouble understanding an aspect of the channel length modulation/pinch off of MOSFETs. My current understanding (For NMOS) is that when a positive voltage is applied to the gate, the P-...
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### 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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### What mechanism happens inside a resistor for potential drop and constant current?

What process happens inside the resistor just after being connected to a battery? I am trying to get a picture of it. On applying a potential difference, the electric field it generates pushes the ...
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### How does a dual gate MOSFET reduce Miller effect?

How does a dual gate MOSFET reduce Miller effect? I am trying to gain a global understaning of the physics behind them, not necessarily exact formula's that come with it other than maybe some for ...
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### Why are(n't) rechargeable batteries damaged by partial charging?

Over the years, I've come across websites and people with different opinions on the "correct" way to charge rechargeable batteries (I'm more concerned with laptop, tablet and phone batteries than with ...
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### 5G and Radar Altimeter Interference - Am I Taking Crazy Pills?

I can't understand where the math supports the concept of 5G frequencies interfering with aircraft altimeters. Can anyone point out the holes in my logic or educate me? Knowns: Aircraft altimeter ...
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### Photoelectric effect vacuum tube

It occurred to me that it ought to be possible to make a vacuum tube which, instead of a heater, uses an intense light source, such as a laser, to kick electrons off of the cathode through the ...
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### How strong is the electrical field inside a microwave oven?

I'm interested in figuring out the approximate peak electrical field strength inside a microwave oven. The question is not so much about the exact number, but rather what reasoning to use to work it ...
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### Does "a signal is buried in noise" mean that the noise amplitude is still smaller than the signal amplitude? (Special case: Lock-in amplification)

I heard that Lock-in amplifiers (LIAs) especially play to their strengths when the signals are weak compared to the noise level. But then I talked with someone about it, who understands the principles ...
2k views

### How exactly do capacitors work? [duplicate]

So when a capacitor is being charged, it is connected to a voltage source and a current flows through it (for a time). Now, high-school physics says that when a capacitor made of 2 large parallel ...
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### How to understand this circuit?

I'm studying about Ohm's Law, and the following problem perplexed me. The answer is $I_{t} = 11$mA and $U_{r} = 41$V. What I need to find is $U_{t}$ and $I_{t}$. The only thing I've found so ...
7k views

### Is there any relationship between frequency of signal and distance it travels?

I'm currently researching 5G technology. Many videos/articles on this topic claim that 5G is designed for communicating on lower distances (than 4G), because of higher (than 4G) frequency. So to build ...
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### What ultimately determines the speed of electrical media

I have heard several myths about this. Some have said "thicker wires mean faster speed" which I don't believe. Others have said different voltages and others have said higher frequencies. Take Coax ...
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1k views

### How do electrons carry thermal energy in Peltier coolers?

I've read that when electrons enter from metal to semiconductor type N, they gain thermal energy and make that side cooler and the reverse happens when they leave semiconductor to the conductor. Why ...
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### Designing an analog circuit that Fourier Transforms or Laplace Transforms an input signal

This is currently my pet project but I am a bit stuck on some problems. First I'll go over my idea for the Fourier Transform: The Fourier Transform is defined as \\$F(\omega)=\int^{\infty}_{-\infty}...
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### Green LED: Forward Voltage

Yesterday in lab, we were asked why all of our green LED had large forward voltage than expected. We expected for the forward voltage to be at around 2.1V as according to hf=Egap=eV. Yet, we ...
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### Antenna Design for specific directions

I am designing an antenna for a device and was hoping to gain some insight in the correct design direction for the antenna. The Idea: A receiver & antenna that fits in a 6" diameter by 3" deep ...
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### Depletion Region Capacitance in MOSFET

Hi, I would like to ask about the behavior of a MOSFET as a capacitor, particularly at the onset of inversion/weak inversion - see Fig 2.6(c) At this stage in Fig 2.6(c), there is positive charge at ...
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### Color LEDs and voltage: why the discontinuity on blue?

Something is bothering me when I consider the forward voltage of tiny monochrome LEDs. Consider the following data from datasheets of tiny SMT monochrome LEDs of different colors (sources red yellow ...
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### Why does isolation transformer protect from shock?

Perhaps I can best explain the question with two examples: (1) there is PC powered directly from 230V mains (unprotected by RCD or similar), and there is fault in PC that causes line voltage to ...
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