The classical theory of electric and magnetic fields, both in the static and dynamic case. Also covers general questions about magnets, electric attraction/repulsion etc. Distinct from electrical-engineering.
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44 views
Why is the Maxwell Stress Tensor symmetric?
What is the physical meaning of the Maxwell Stress tensor symmetry?
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0answers
7 views
Magnetic moment in level circuit
Suppose we have a circuit in the xy-plane, with a random geometry (it just needs to be closed). I now wanna calculate the magnetic moment of this setup:
$\vec{m} = \frac{1}{2} \int_V d^3 x' \vec{x}' ...
2
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1answer
93 views
What is the meaning of a constant magnetic scalar potential?
Let a spherical shell of inner radius $a$ and outer radius $b$ have a uniform magnetization $\mathbf{M}=M\,\hat{\mathbf{z}}$,
$\hskip2in$
I've found that the magnetic scalar potential ...
-7
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0answers
35 views
EMP gun practical issue [closed]
I tried to build an EMP(electro magnetic pulse) device. I read about it on the internet and I found some possible circuits. For build the device I used a diode, an high voltage capacitor, a swich, a ...
-1
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0answers
19 views
Writing right and left polarized waves in terms of parallel field components
How can I write:
in terms of:
(or vice verse - I can't see how they relate).
I don't see how you can right and left polarized waves in terms of parallel E and B components?
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0answers
43 views
Calculation of magnetic force magnitude from a parmenant magnet
I was wondering how strong a magnet should be in order to be able to horizontally attract 0.2 Kg of pure iron at a distance of 0.3 M. Since I know nothing about magnets, I started looking for ...
1
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0answers
27 views
How to size an electromagnet
How can I size an electromagnet? For example if I would to attract a mass of $x$ kg what are the calculations that I must do to size the ferromagnetic core and of course the solenoid? And thus, how ...
28
votes
7answers
910 views
How can Magnets be used to pick up pieces of metal when the force from a magnetic field does no work?
I learned that the force from magnetic fields does no work. However I was wondering how magnets can be used to pick up pieces of metal like small paperclips and stuff. I also was wondering how magnets ...
2
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0answers
46 views
Current to cancel Earth's magnetic field
I'm making a lab report where I've used a Helmholtz coil and a magnet to find Earth's magnetic field intensity in the area by relating the frequency of oscillation of the magnet when a certain current ...
1
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1answer
30 views
Energy Change in a Capacitor by Inserting Dielectric
This is a practice Physics Subject GRE problem.
A parallel-plate capacitor has plate separation $d$. The space between the plates is empty. A battery supplying voltage $V_0$ is connected across ...
3
votes
1answer
38 views
Magnetic field insulators
I was wondering if there is any way to stop the magnetic field, without having the insulator turned into magnet. Let me present this as a simple case, there is a magnet to the left and a piece of iron ...
2
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0answers
27 views
Relationship between current, voltage, magnetic field in a torus
I'm trying to find the relation between current density $\boldsymbol{j}$, voltage $v$, and magnetic field $\boldsymbol{B}$ for the time-harmonic approximation in a cylindrically-symmetric coil (a ...
3
votes
1answer
51 views
Why do black lights show bacteria and proteins?
Why are blacklights used to show proteins and bacteria? Is it related to their ability to make white colors glow? Don't black lights just radiate in the ultraviolet/purple range?
3
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1answer
69 views
What is an anapolar moment?
I just read this: Anapolar Dark Matter
I'm not sure i've heard the term 'anapolar' before, so i google and i found this:
http://en.wikipedia.org/wiki/Toroidal_moment
This confuses me, my ...
3
votes
2answers
39 views
How to calculate resistance of bunches of wires
I figure that the resistance of multiple wires would work just like adding in parallel, a bunch of wires that is. But couldn't any wire just be viewed as a bunch of wires, albeit small ones? I know ...
11
votes
5answers
206 views
Why are wires in simple circuits approximated as equipotentials?
I just answered this question:
Voltage in a circuit
by asserting that any two points joined only by a wire, but no other circuit elements, are at the same value of the electric potential. It is, of ...
1
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2answers
52 views
What is wrong with this form of the Maxwell-Faraday equation?
What is wrong with this form of the Maxwell-Faraday equation?
Line integral of the electric field is equal to the double integral of (partial derivative of magnetic field with respect to time).
So ...
1
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2answers
28 views
Question regarding inverse relation of resistance with area of cross section
It is said that resistance is inversely proportional to area of cross section. But greater area will have greater electric flux, and greater electric flux will have greater magnetic flux, and greater ...
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0answers
25 views
Electrical properties of molten gallium arsenide
I'm looking for the resistivity and magnetic permeability of molten gallium arsenide, but can only seem to find the values for the solid material at room temperature (e.g., Wikpedia). Not even ...
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0answers
45 views
Energy harvesting from power lines [closed]
I want to harvest energy from an overhead power line of 50kV 50Hz and this shall be legal, since I am doing it for the company owning the overheads (A step-down transformer is concidered too ...
7
votes
1answer
123 views
How do I derive the Lorenz gauge from the continuity equation?
I was reading my old electromagnetics book (Elements of Electromagnetics, by Sadiku, 3rd edition) and after the author explained what the Lorenz gauge is mathematically and why it is useful in ...
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0answers
18 views
Electric Field of Electret
I've read that a method to measure Radon, is to use an Electret Ion Chamber. In particular here can be seen that, a voltage drop can be $2V$ in a single day.
I believe this very much, yet I don't ...
5
votes
1answer
90 views
Aharonov-Bohm Effect in Torus
I had a very brief introduction to the Aharonov-Bohm effect in class. The lecturer introduced the notion that $H(\Phi=\Phi_0)$ and $H(\Phi=0)$ gives identical energy spectrum and that the Hamiltonians ...
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1answer
82 views
Can a light be bent by a magnetic field?
I'm struck with two competing ideas on the question in the title.
Listing #1: http://van.physics.illinois.edu/qa/listing.php?id=2009
Q: "How far can a magnetic field bend light?"
A: "Unfortunately, ...
7
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2answers
145 views
What could magnetic monopoles do that electrically charged particles can't?
I understand the significance to physics, but what can a magnetic monopole be used for assuming we could free them from spin ice and put them to work? What would be a magnetic version of electricity?
...
6
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2answers
169 views
Is Biot-Savart law obtained empirically or can it be derived?
Well, there's already a question like this here so that my question could be considered duplicate, but I'll try to make my point clear. Is there a way to derive Biot-Savart law from the Lorentz' Force ...
3
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1answer
105 views
Is there any potential associated with magnetism
Can anybody please tell me if magnetism is a conservative force or if there is a field associated with it? How to reason? One thing I know is that the work done by a magnetic force is $0$.
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0answers
73 views
Velocity, Wave Equation, Differential Equations [closed]
Suppose you have a differential equation of the form:
$$
\frac{\partial^2 u}{\partial z^2} = C \frac{\partial^2 u}{\partial t^2} + D \frac{\partial u}{\partial t}$$
Is it possible to find the ...
4
votes
1answer
55 views
Physics of donut magnets levitating vertically on a pencil?
When you put identical donut magnets on a pencil horizontally, the magnets all repel from each other in a way that creates equal spacing between each magnet. However, spacing is not equal when the ...
1
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0answers
32 views
Exchange operator and magnetism
in my atomic physics lecture notes it is said that depending on the values of the exchange operator $K_{ij}$ there can be different kinds of magnetism, say, ferromagnetism and antiferromagnetism, how ...
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1answer
42 views
Chapter 35-Electricity and Magnetism [closed]
A light bulb connected to a 3.0-V battery draws 1.2 A of current. Calculate the resistance of the bulb.
0
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1answer
69 views
From where do the permanent magnets get energy from? [duplicate]
I have a doubt about permanent magnets. If a magnet is permanent it can attract some materials permanently.
Attracting something involves energy. If a permanent magnet can do this forever, from where ...
1
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1answer
45 views
Induced voltage of a conductor in a magnetic field
A book which I referenced for Electrical Machinery states that the voltage induced in a conductor inside a magnetic field is given by
$$ \mathcal{E}=(\mathbf{v} \times \mathbf{B})\cdot \mathbf{l}$$
...
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1answer
47 views
Electromagnetic Tensor in Cylindrical Coordinates
I understand that the Electromagnetic Tensor is given by
$$F^{\mu\nu}\mapsto\begin{pmatrix}0 & -E_{x} & -E_{y} & -E_{z}\\
E_{x} & 0 & -B_{z} & B_{y}\\
E_{y} & B_{z} & ...
1
vote
2answers
50 views
relation between permittivity and conductivity
I am able to measure relative permittivity $\varepsilon_r$ of a fluid, and I want to calculate conductivity of same fluid. Can anyone suggest me how to do this?
I found one formula to calculate ...
3
votes
1answer
60 views
What is the direction of an accelerated particle due to a variation of magnetic field?
Let an ion of charge $q$ and mass $m$ traverse a circular orbit of radius $R$ under the influence of a uniform magnetic field $\mathbf{B}=B(r)\,\hat{\boldsymbol{z}}$, where $r$ is the distance from ...
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votes
3answers
63 views
Right hand rule to measure direction of electric field [closed]
The diagram: a positive plate is on the left of the page and a negative plate is on the right. The velocity of the particle is going down the page. So, with a right hand rule, I would point my thumb ...
2
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1answer
43 views
Circuit Loop Law Doubt
In a circuit with a solenoid/inductor and a resistor and a battery .
Books say that $\Sigma \Delta V=0$ around a closed loop . That means work done by electrostatic field per unit charge is $0$ ...
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0answers
17 views
Electron Kinematics for Moving Field Region
I am working out a rough calculation for an experiment I am working on this summer: Suppose an electron is moving in the positive y direction with 300 MeV. There is a circular electric field region of ...
3
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0answers
44 views
Finding the terminal velocity of a magnet dropped in a solenoid
We have to find proportionality of the terminal velocity with the factors of the system:
Plot: a small dipole(mass $m$) with dipole moment $\mu$ is dropped in a long solenoid (radius $r$, ...
1
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1answer
74 views
How to add a potential term to the Dirac Equation?
I've read that if you have a Hamiltonian for the Dirac Equation, you can add a potential term to it simply by adjusting the momentum operator so that $p^\mu \rightarrow p^\mu-A^\mu$, where $A^\mu$ is ...
1
vote
2answers
54 views
Gauss's Law understanding
In the case of a point charge $q$ at the origin, the flux of $\vec{E}$ through a sphere of radius r is,
\begin{equation}
\oint \vec{E}\cdot d\vec{a} = \int \frac{1}{4 \pi \epsilon_0 ...
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votes
1answer
29 views
Gauss's Law - Electric Field outside a Shell?
I'm somewhat confused as to why the electric field outside a spherical shell is $\frac{Q}{4\pi r^{2}\epsilon_{0}}$
Going through the work:
$$ Q = 4 \pi r^{2} \sigma ; \frac{4\pi ...
1
vote
1answer
45 views
Superconducting diamagnetic sphere in uniform magnetic field
What is the size of the magnetic dipole moment $\vec m$ of a superconducting diamagnetic sphere $radius=R$ in a uniform magnetic field $\vec B_0$? Since there is no free current, we can solve for ...
3
votes
1answer
58 views
Electromagnetic black hole?
So I was thinking about something for the past while
Consider a large spherical foam-ball with homogeneous density. Where a foam ball is defined as an object that can absorb matter with 0 friction ...
0
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0answers
22 views
How to maximizing inductance of solenoid using Optimization method? [closed]
My teacher told me to maximizing inductance of solenoid using Optimization method, and I don't know how to find it if you know please answer me quickly as possible.
Hypothesis: length$=1m$, ...
1
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1answer
45 views
Magnetic force on a magnetic dipole
Can someone please help me understand and answer the question of "why are the forces on a magnetic dipole different in a uniform magnetic field and non-uniform magnetic field?" I know in a uniform ...
1
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1answer
37 views
Behavior of the electric field on boundary surfaces
Consider this picture.
Integrating over this infinitesimal box gives the following equivalencies:
$$\int_{\Delta V} d^3r~{\rm div} \vec{E}(\vec{r}) = \int_{S(\Delta V)} d\vec{f} \cdot ...
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0answers
39 views
How to calculate back emf? [closed]
I'm trying to answer the question:
The back emf in a motor is 85 V when operating at 1800 rpm. What would be the back emf at 2500 rpm if the magnetic field is unchanged?
So I don't know what ...
-1
votes
1answer
59 views
Magnetic field of an electromagnet with an iron core [closed]
I'd like to know how to calculate the magnetic field between two poles of a bent electromagnet ("U" or "C" shaped) and with an iron core.
