Questions tagged [electromagnetism]

The kind of magic without which all of this wouldn't be possible. This tag should be used for questions about the **physics** of the fields that charged particles create and how these fields interact. It should not be used for all questions involving electromagnetism, that would be everything on the site.

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40
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8answers
8k views

Why isn't there a potential difference across a disconnected diode?

I know this question sounds silly, as if there was a potential difference a current would be created when the terminals are connected together and this would mean energy has come from somewhere. The ...
16
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3answers
54k views

How does the tester screwdriver work?

How does the tester screwdriver work? If I put the tester screwdriver inside the "hot wire" of an electrical socket, it lits up if I press my finger against the metal cap on top of the screwdriver. ...
30
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5answers
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Stealing energy from radio towers or power lines

From this answer: How much energy can be harvested from nearby radio towers or power lines? What circuits would you use? How much would you have to absorb for it to be noticeable by the producers or ...
25
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4answers
12k views

Why aren't solenoid/electromagnet windings insulated?

This has been bugging me for a long time. Take this video for example. I have always thought that electricity will take the shortest path. When the electromagnet's windings are uninsulated, it seems ...
38
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6answers
3k views

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 ...
3
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2answers
194 views

How current and potential propagate through electrical circuits?

I have something of a theoretical question. In any given electrical/electronic circuit there seems to be an electric field developed inside (and outside the conductors?) to push the electrons in the ...
5
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1answer
13k views

Open Source or free electric field mapping software

Looking for recommendations for 2D (or 3D) electric field mapping software. I want to be able to populate a series of electrodes in a plane and understand the equipotentials present when these ...
14
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3answers
2k views

Does Shielding “Electrically” shield “Magnetically” too?

I know it sounds like a newbie question but I can't wrap my mind around it. An electromagnetic field is electric + magnetic field. So this means that when shielding an equipment offensively, for ...
12
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4answers
10k views

Why is flyback air gap needed for energy storage?

Why do so many sources say something along the lines "since a flyback transformer stores energy, an air gap is needed"? I have seen this reasoning in textbooks and app notes. I thought air gaps ...
3
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4answers
898 views

Magnetic field strength and flux density from hysteresis curve

I am trying to measure the magnetic hysteresis curve of a toroidal nanocrystalline ferromagnetic transformer core made of Nanoperm (datasheet linked) using the following circuit. Each of the windings ...
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1answer
2k views

emi standards/best practices for PC cases

According to an EE friend, the FCC specifies EMI tolerances (max outputs) for PC components, including cases. However, here it says that there are standards for a number of components but not cases. ...
0
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1answer
1k views

Powerful electromagnet?

Is it possible to produce over 1000 lb of magnetic force over a distance of 4 inches(0.101 meters), using an electromagnet? How difficult would it be? Explaining the general spec's of such an ...
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7answers
7k views

Are E and B fields in phase in electromagnetic radiation?

I recently wrote this answer, in which I said: Radio waves are electromagnetic radiation. Electromagnetic radiation contains of two components, one electrical and one magnetic. These components ...
13
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4answers
3k views

Is it valid to consider back-EMF in a DC motor equivalent to increased inductance?

I know that back-EMF can be considered as a voltage source in series with the motor which is proportional to speed. This is the common understanding, and I totally get it. Before I understood this, I ...
2
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1answer
435 views

Good resources/books for electromagnetics and other advanced concepts

I'm looking for some good resource that teaches the fundamental concepts behind electronics. I'm not looking for a beginner book. I'm already in the final year of my Electronics and Telecommunication ...
0
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3answers
212 views

Electromagnetic charging

I'm building a circuit that uses a really low voltage and current. I thought that instead of using a standard battery it would be nice if I could use a rechargeable battery and charge it with ...
8
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6answers
1k views

Circuit yields a peculiar contradiction between KCL, KVL and Faraday's law

I don't know if this particular circuit/loop is covered in another question but I stumbled upon a video where a peculiar consequence occurs for the following circuit: For the above circuit loop ...
6
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2answers
1k views

Intuitive explanation of Skin Effect?

I'm trying to wrap my head round Skin Depth, we've derived the wave equation from Maxwell's equations, using the conditions that it's a "good conductor" ρ = 0, σ >> ωε and ...
4
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2answers
366 views

Why don't my coil work?

I disassembled my old Niko RC car today. Thought I could play around with the parts. But what I can't seem to understand is the coil. The powersource of the rc car is a 7.2v NiCd 620mAh battery. And ...
3
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1answer
379 views

Does electricity flow in open circuits?

Say I have an open circuit with a battery. I have a wire connected to the negative terminal and a wire connected to the positive terminal. The wires aren't connected. In this scenario, would their be ...
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6answers
302 views

Magnetron Working Principle

I was reading this simple description of magnetron working principle. Can you explain me why when electrons go near the cavities, they start to resonate at microwave frequency? I do not see the link ...
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2answers
712 views

Cable type and decreasing EMI/RFI

I am interested which cable/wire type is better to protect against electromagnetic and radio interference/leakage of signal. Is a twisted pair wire better than an untwisted, to protect against EMI/...
5
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1answer
3k views

AC solenoid valve with DC

I have a 24VAC/60Hz solenoid. I used 12VDC just to try, and it worked good (well needless to say it worked as expected with 24VAC/60Hz). What are the effects of a solenoid that's designed for AC ...
3
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1answer
1k views

Advantages of parallel over serial connections

As far as my understanding goes, parallel data connections are faster than serial at the same frequency. However, when you increase the frequency in parallel you start getting bits 'cross-talking' and ...
2
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2answers
303 views

If induced voltage (back-emf) is equal and opposite to applied voltage, what drives the current?

Suppose we have a circuit with a voltage source, a switch open and an inductor all in series. If we close the switch, the potential difference of the voltage source is instantaneously applied to the ...
0
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1answer
94 views

Inductance of a practical relay

I would like to design an SMPS to feed an electronic circuit that includes a relay. To study the stability of my SMSP I need to model the relay, but when I go through many relays datasheets, they do ...
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2answers
320 views

How to determine the correct graph for induced current

To be straight , this is a home - work question . This is the question. Okay, now let me give you the my attempt and what I believe to be correct. Here E.M.F induced across OA is constant, ...
16
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3answers
10k views

How does a Theremin “antenna” work?

I always assumed that the shape of the antennas was important; a vertical monopole for pitch and a horizontal loop for volume, thinking that this minimized their interference with each other. But it ...
31
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5answers
13k views

What is the speed of “electricity”?

In a distribution grid, what is the speed of energy signal Is it strictly the same? And how is synchronisation achieved? Is an electric grid essentially an orchestra where everyone is the chief ...
9
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3answers
22k views

How does a “coilgun” work?

I was wondering how that so-called "coilgun" works, I know what it does but I was wondering how it works under-the-hood and eventually how to make a basic one ? Also, how much energy would be required ...
24
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14answers
3k views

How is it that two electric currents can travel in opposite directions on the same wire, at the same time, without interfering with each other?

An Introduction to Information Theory: Symbols, Signals and Noise, by John R. Pierce, says the following: While linearity is a truly astonishing property of nature, it is by no means a rare one. All ...
12
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4answers
4k views

What is the difference between electromagnetism and electromagnetic radiation?

In a previous question, I asked if having 2 coils of wire with electricity flowing from one to the other is RF, and they replied that it was not Radio Frequency. I was puzzled because I thought this ...
11
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1answer
13k views

Controlling an electromagnet strengh with arduino

This previous question Controlling an electromagnet with Arduino deals with binary control (ON or OFF) only. On my side I need to choose the strength of the magnetic field. It is a home made ...
14
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2answers
5k views

How does this iPhone app measure the power flowing through an AC appliance cord?

My electric utility released an iPhone app called "DTE Insight" which (among other things) measures the amount of power flowing through an appliance's power cord by asking you to hold the cord against ...
5
votes
4answers
2k views

How does a transformer transmit power from the primary to the secondary?

I'm trying to get my head wrapped around transformer operation and in the process regretting the times I snoozed in my Electromagnetics class as a EE student back when I was a lad :) I'm looking for ...
3
votes
3answers
875 views

Transmission Line Effects

My question has to do with the rule of thumb we generally use when dealing with transmission lines. We say that if the length of the line is 10% or less of the wavelength, we could neglect the ...
3
votes
3answers
502 views

Transformer magnetizing inductance

I am learning about transformers and in the equivalent circuit the magnetizing inductance of the excitation branch is supposed to represent the current needed to "set up" the flux in the core due to ...
1
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0answers
223 views

Mistake in published paper about magnetic levitation?

As I was reading through this paper: Nonlinear Model & Controller Design for Magnetic Levitation System, I stumbled cross section 3.3 which I could not follow. Summarizing section 3.3 Non Linear ...
8
votes
5answers
678 views

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 ...
6
votes
4answers
7k views

Putting far more mechanical energy into a generator than its electrical load

Funny how I was pondering about this question for a long part of the day yesterday, and today I was at stackoverflow and in the box with the featured questions from other SE sites this very related ...
6
votes
2answers
1k views

Homebuilt magnetic tape head

I have seen a lot of projects that use a tape head, but there is very less information about how to build one yourself. Is this possible? I know that basically it is a metal core, with a coil around ...
6
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4answers
1k 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 ...
4
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3answers
1k views

Rotating a wireless power mechanism

I got this simple wireless power mechanism: http://dx.com/p/diy-wireless-charging-transmitter-receiver-solution-module-green-golden-dc-5-12v-194469 If I'll have the receiver end rotating at about 30 ...
4
votes
2answers
3k views

Why does a permanent, non-moving magnet, stuck to a long piece of metal generate a voltage?

I was just messing around with a very small permanent magnet (round) stuck to a piece of metal. The magnet is round, ~6 mm in diameter. I guess 1 mm thick. (I lack precision tools where I ...
3
votes
4answers
15k views

how much power is radiated by cell towers?

I want to know how much power is radiated by cell towers of GSM (1.8 Ghz), 3G (2.1 Ghz), 4G (2.6 Ghz) ? I want links to references if exist.
3
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1answer
2k views

Modes in waveguides

What is the meaning of Modes in waveguide? With what physical property of the Electromagnetic wave does it deal with? I have just started with parallel plane waveguide and in that case the constant m- ...
2
votes
2answers
642 views

Turns for saturating a toroid at a particular frequency

I need to know the winding turns required to saturate a toroid for a particular drive frequency. The toroid I have is having the following dimensions OD = 19 mm Thickness = 8 mm Effective Area (Ae) =...
2
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2answers
2k views

PWM controller to meet EMI/EMC regulations

I'm designing an application in which the current running into a load is controlled by PWM. A microcontroller generates a PWM cycle at 20kHz and this signal is used to control a MOSFET which is ...
1
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2answers
308 views

How to choose the correct graph

This is a question based on electromagnetism. Screenshot of the question and all the necessary stuffs are given below, This is the figure , And the choices that are given, First choice Second ...
1
vote
1answer
2k views

How to increase an electromagnet's magnetic field size?

I made this electromagnet out of a microwave oven transformer by taking out the secondary coil. It very strong but it can only attract the metals if I bring it really close. How do increase the ...