# 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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### How does a gold leaf electroscope measure voltage?

It’s my understanding this device can be used to measure voltage. How is this accomplished? What electromagnetic equations are applied and how is it calibrated?
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I’m trying to understand the physical laws applying to Faraday’s induction experiment as pictured below. The Wikipedia site doesn’t go into detail. When the smaller diameter inductor is inserted into ...
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### Signaling on a medium with more than one frequency at once?

Does anyone know how it’s possible to have different carrier frequencies in a medium like copper cable and separate the original signals at the receiver? For instance, looking at image A below, this ...
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### Voltaic pile: question on components

I have some questions on voltaic pile components. The following show images of the voltaic pile. The vertical pile in I1 has four outside (labeled m in Fig 2) bars and one bar in the center ...
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### How does a voltmeter determine voltage?

For example, using a battery. Does the meter place a small resistance in line with the two terminals, then measure current and solve for V using V=IR? I’m looking for the general theory.
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### Inductance depends on the number of turns in a solenoid. Is this the case with reluctance as well?

The total flux ($\Phi$) through an solenoidal inductor of length $l$ and $N$ turns is proportional to the current through the inductor and the inductance $L$ of the inductor according to \...
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### Tesla AC motor: operational question with field direction

I have a question on the direction of the magnetic field for each of the four windings of the pictured AC motor. I ask because the wires are not clearly labeled as to direction of each winding, in ...
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1 vote
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### Tesla AC motor: question on component

The following is a picture of Tesla’s AC motor patent. Does anyone know what appears to be slip rings on the left end of shaft a’ coming from generator G are for? Also, what is the function of the ...
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### Tesla AC motor: what is this component?

Can anyone explain how the pictured component that looks like a long stem connecting to the generator G operates? Most specifically of the left side connecting to the terminals? My understanding is ...
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### Tesla’s AC motor: questions on operation

The Tesla Patent for the AC motor claims a circular disk can be used in place of the rectangular piece of steel which rotates in the motor. The disc is more efficient. I can see how the rectangular ...
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### How can one calculate at surface the electric or magnetic field strength for 50Hz 240VAC wire at 1 meter depth?

How can one approximately calculate at surface the electric or magnetic field strength for 50Hz 240VAC wire buried at 1 meter soil depth for a given current? Say in sandy clay soil with no mineral ...
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### Antenna port S11(dB) > 0

I'm simulating a microstrip patch antenna in microwave office at 35 GHz. When plotting the S11 in dB of the antenna port I get values greater than zero at frequencies below 28 GHz. How is this ...
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### Lavet type stepper motors design guidelines

For a client we need to provide a Lavet-type stepper motor - I'm looking for some design guidelines - there seems to be very little technical literature on this type of motor. Wikipedia seems to be it ...
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### GMR Sensor voltage output into magnetic field

I used a GMR sensor (Model AA003-02) inside a electricly adjustable homogenous magnetic field generated by a long coil. In other words the current is proportional to the magnetic field strength. ...
1 vote
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### Modelling a single trace PCB?

I am trying to model a PCB trace as a R, L, C circuit. Assumption: My PCB has a a single trace running through it. Just giving input from one side and taking output from other. There is not even a GND ...
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### Why is the bridge rectifier circuit widely used to make power supplies?

Why is the bridge rectifier circuit widely used to make power supplies? Which is better - a bridge rectifier with capacitor or without capacitor?
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### Frequency Response of Antennas

Electromagnetics and antennas are not my main area of expertise. However, I have a specific question about antennas that I want to get right in order to make sure my signal processing is right. What ...
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I am designing a receiver antenna for a wireless power transfer application. The transmitter is a 2m x 2m horn antenna array that transmits a power signal at a frequency of 35 GHz from 100 meters away ...
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### How does balancing currents within a cable or conduit cancel its magnetic fields?

I know the NEC requires all current carrying conductors of the same circuit to be included in the same cable or conduit [300.3(B)], but why? I think it is because the magnetic field generated by ...
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### DC Vacuum Contactor Coil - Failed Close

I am using a 350A DC, normally open vacuum contactor that experiences a failure while the contactor is closed (coil energized by 24VDC) for an industrial 1000VDC application. The manifests as the ...
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### Real life example of lossy medium

I’m learning about electromagnetic fields in class and a real life example of lossy medium would be helpful. I also don’t know how intrinsic impedance connect connects to lossy medium - probably ...
61 views

### Is this circuit measuring the electrical field?

I have found the following circuit on the web (https://electroclinica.org/2018/03/23/fabricacion-de-un-sencillo-medidor-de-campo-de-rf-diy/) which claims to be an "RF field" detector/meter/...
197 views

### Wierd noise on GND when I touch a metallic object to it

I am facing a problem. I have a PCB designed which fails ESD tests. The PCB is powered by an external power supply that provides 12V to the system (from 240V AC.) The power supply is CE certified and ...
50 views

### When the electric field EMF radiation hits tin foil why does it result in a magnetic field?

I have a cheap Erickhill EMF reader that measures “electric field” and “magnetic field”. I’ve been experiencing an odd phenomena that I can’t quite explain of a electrical sound emanating from a ...
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### Planar PCB inductor coil simulation

I am working on a planar PCB coil with a complicated shape (not the traditional circular/rectangular spirals.) It is essentially an inductor made out of PCB traces. I want to figure out the inductance ...
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### Difficulties in tuning magnetic levitation system on Arduino and MATLAB?

So I am dealing with a project that revolves around magnetically levitating an object below an electromagnet. I created a transfer function of it after making a mathematical model of it, and got this ...
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### What are "dummy terminals"

This picture is in datasheet of one relay: From this picture I assume that if there's current flow between 1 and 8 pins, the relay will connect 3 and 4, right? What are "dummy terminals"? ...
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### Does coil winding impact current direction? (Lenz Law)

When a magnet is moving toward a wire loop north-side first, the current goes counterclockwise according to the magnet's perspective, as shown in the image below from Wikipedia. Now, consider a ...
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### Strange result in modelling magnetic levitation system?? Negative Transfer function?

I'm working on this magnetic levitation system project where I basically levitate an object under an electromagnet using an Arduino. Currently, I'm stuck at the graphical simulation of my transfer ...
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### Difficulties in modelling magnetic levitation system?

I am currently working on and writing a project (not homework) surrounding magnetic levitation with an electromagnet having to levitate an object underneath itself. I have taken a look at relevant ...
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### How can I read an RFID tag at the back of a phone FROM THE FRONT?

I'm discovering RFID applications and I would like to be able to read an RFID card I strapped to the back of my phone (the most convenient location for me) by just putting the phone face down on the ...
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### How does electromagnetic shielding work?

How does electromagnetic shielding work in cables? I learned from school that foils are used as shields in coaxial cables to "shield" them from external electric and magnetic fields. It also ...
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### How do I mathematically model a magnetic levitation system?

I am working on a magnetic levitation system project using PID control, and I am getting confused as to how I am supposed to mathematically model this system for my paper. Where am I supposed to start?...
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### Does acceleration come from a change in current?

I have a current connected to a wire and let's say when we measure the current at a particular time it reads differently than it does after some time. Does this imply the current is AC? I'm asking ...
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2k views

### How do I build an electromagnet?

I want to make an electromagnet that can lift a paper clip off of the ground from a short distance (probably 7 to 8 inches). I naively thought of making an electromagnet as a simple task (because of ...
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1 vote
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### Computation of DC motor parameters [closed]

How does one compute DC motor parameters instead of measure? Example: inductance, backemf,torque? Would one use a EM simulator or is there another way?
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### How do clamp multimeter sense presences of AC voltage?

If a clamp meter is taken near a AC supply line, it's AC voltage sense LED starts to glow. The same coil that is used for current measurement is used to sense the presence of voltage. As far as I know ...
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### Electric field from charge density

This Maxwell law tells us that the rate of change of the electric field created by a charge density: depends on the charge density and the permittivity of the material outside the charge density. Is ...
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### Multiple questions about magnetic levitation including modelling and how to achieve it

I have several questions regarding magnetic levitation. If I have missed something in one of these questions then please do let me know. I am working on a magnetic levitation project using an Arduino ...
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### Can a pulsating signal in which the direction of the current may not reverse radiate from an antenna?

Normally an antenna uses a sinusoidal wave whose positive half-period corresponds with positive direction of the current and vice versa. A pulsating signal/current is a signal in which the direction ...
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### Induction on the center of solenoid

Why is the magnetic induction at the middle of the solenoid greater than at the end of the solenoid? I know it's bigger, but the question is why is that?
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### Solenoid magnetic induction graph

I need help solving this problem, if anyone had a similar problem it would help me a lot. A table is given with the current flowing through the solenoid,also values ​​for the magnetic induction vector ...
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### Determining the type of magnet used in a hand-crank torchlight

I'm trying to design a DIY dynamo that recharges a 5V battery used to power some sensors. However, I am having some trouble with deciding on the arrangement of coils and magnets. Currently, I am ...
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### How to Limit Current Using L293D?

I’m designing a circuit to control 4 electromagnetic coils and subsequent orientation of a sphere (pic attached below). I’m new to EE but thanks to this forum I’ve learned what and how to utilize PWM ...
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### Force on an Aluminium plate in a 1T field?

I am thinking of using a magnetic field to damp mechanical oscillations/noise in an Aluminium swing arm. The problem is that I do not even know how to calculate ballpark numbers for the effects I am ...
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### Relative Permeability of Rotor

I am working on an example question from a electric machinery textbook. The question provides a simplified version of rotor and stator as shown in the picture below (please excuse me for the poor ...
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### Listening to electromagnetic fields

In Nicolas Collins' book Handmade Electronic Music there is section on picking up electromagnetic fields and listen to them simply by connecting the wires to a RadioShack Mini Audio Amplifier. It says:...
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### Calculating number of turns an electromagnet has

The title, but can one calculate the amount of turns on a coil or find that out somehow? Because I purchased a 120m copper coil (originally intended on using it on another coil former before giving up ...
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### What is the EMF induced between two points in a closed wire loop?

Farday's law says if I change the magnetic flux through a closed loop an emf would appear. Between what two points is the EMF calculated? Let's say that my calculations give me an E=−dΦdt=10V. What ...
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