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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Magnetic field in a wire with constant current

I assume I have a wire parellel to the $z$ axis and with radius $R$. A constant current $I$ flows through it in the $z$ direction. I want to know the magnetic field inside the wire at distance ...
0
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1answer
28 views

Explaining induced emf: directions of lorentz force and electrostatic field in two reference frames

First question, what is the direction of electrostatic field? Here is the problem: Consider a conductor rod CD moving at a speed v in a magnetic field. The three are in right angles to each other. In ...
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3answers
61 views

Magnetic Energy Density Equations

I have a plasma with a magnetic field through it and I want to know the magnetic energy within the plasma. I have the simple equation U=(B^2)/mu for the energy density. But my question is then, if ...
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1answer
26 views

Magnetic Field Along the Axis of the Current Ring - Alternative way to compute

This is Example 5.6 in Griffith's Introduction to Electrodynamics (4th Edition): Find the magnetic field a distance $z$ above the centre of a circular loop of radius $\ R$, which carries a steady ...
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1answer
26 views

Arcing through an inductor

When a current is passed through an inductor coil, potential difference across it is given as $$V=L\frac{dI}{dt} \, .$$ Now, if we suddenly disconnect the coil from the circuit, the current will ...
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0answers
23 views

Does a superconducting coil need a minimum amount of electric current to be a superconducting magnet?

I'm not about that for a tiny amount of Cooper electrons perhaps we can't measure the magnetic field. I suppose that it needs some influental electric current below which the coil isn't a superfluid ...
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1answer
43 views

Field strength is always closed?

In electromagnetism, we can write the homogenous Maxwell equations succinctly as $$dF=0$$ Where $F=E\wedge dt+B$ is the field-strength 2-form. However, it is a well-known fact that the field-strength ...
0
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1answer
28 views

Magnetic field due to sheet current [closed]

Suppose we have current flowing along very long thin sheet with current per unit length as $\lambda$ so what would would be magnetic field at distance $r$ from this sheet? Using Ampere Law ...
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1answer
8 views

Magnetic field sensor readings varying depending on polarity

I was playing around with one of these Hall effect magnetic field sensors when I noticed something I thought was strange. I held a cylindrical magnet several centimeters from the tip of the sensor and ...
0
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1answer
38 views

Potential Difference and Electric field due to changing magnetic fields

Suppose above Frame is kept in time dependent magnetic field $B=\frac{B_0t}{T}$ where $T,B_0$ are constant , then What will be potential difference between Point $O , A$ and $A , B$ . I know that ...
2
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1answer
76 views

What does it physically mean to take the curl of the curl of a field (wave equation derivation)?

What does it physically mean to take the curl of the curl of a field in the derivation of the electromagnetic wave equation from Maxwell's equations, as presented here, on Wikipedia? Why was it a ...
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0answers
26 views

How does a synchronous motor know to increase current when the mechanical load is increased?

Consider a very simple singe phase synchronous motor, such as the one shown in figure 1. This motor will not be self starting, but if an AC voltage is applied and the permanent magnet is given an ...
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0answers
20 views

Weizsäcker–Williams approximation

I'm having some trouble understanding the Weizsäcker-Williams approximation What I think it is is the following: I have a charged particle at high speed, close to the speed of light, at this speed ...
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0answers
35 views

Does photon polarization take place without scattering processes?

Behind two perpendicular crossed polarisation filters (for visible light) the intensity of light will be zero. After installing a third filter between the two others non parallel to each of them some ...
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1answer
34 views

Ampere's law, do I include the electric field causing the current?

Let's say I have a long, straight wire with a time varying current, $I$ through it. Now if I take a circular Amperian path around this loop wire (and concentric with it) there is both a current $I$ ...
4
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1answer
63 views

How is the conservation of momentum satisfied in long-range attraction such as electromagnetism and gravity?

I'm not a physicist, but my understanding is that electromagnetism (including attraction between opposite charges) is mediated by the photon, and gravity is probably (hypothetized to be?) mediated by ...
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0answers
17 views

How to calculate the capacity of an electroscope out of the geometry and why is it independent on the pointer deflection?

How can one calculate the capacity of an elektroscope like the following from the geometry of the setup? How does this formula implies that the capacity does not (approximatly) depend on the ...
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6answers
540 views

why is the magnetic field circular

According to relativity, If magnetic field is just an electric field viewed from a different frame of reference, why is the magnetic field around the wire is circular?
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1answer
41 views

Magnetic Susceptibility

maybe very silly question :( In Magnetism: \begin{equation} \ B=H+4\pi M (SI) \end{equation} \begin{equation} \ B=\mu(H+M) (CGS) \end{equation} in these formulas M is Volume Magnetisation ...
2
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1answer
33 views

What's the strength of a $B$-field inside a magnet?

I understand that the strength of a magnetic field decreases as you get further away from the source of the field. However, whats the strength of the field within a magnet? Is there a field in a ...
2
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1answer
31 views

Question regarding Van De Graff generator Belt

I have made a VDG generator with a rubber band as the belt and a glass roller. It doesnt seem to work because I think the rubber band may be conductive. I was thinking of using other materials for ...
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0answers
46 views

Aether/Ether in Electrodynamics?

From reading Maxwell's original papers on the formulation on the dynamics of electromagnetism, I saw that Maxwell kept mentioning aether in his paper. I know today in modern physics the Michelson ...
2
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1answer
45 views

Electric field in a hollow object

I am currently visiting a course about electrodynamics. In my last lecture it was said that if a hollow sphere is inside of a bigger sphere, but only in the bigger sphere there are charges, the ...
0
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1answer
23 views

Monochromatic wave between two conducting planes

I'm trying to do a question which reads: Perfectly conducting planes are positioned at $y=0$ and $y=a$. Show that a monochromatic wave may propagate between the plates in the direction $z$ if the ...
0
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1answer
62 views

The role of sine function in the Biot-Savart Law

$$ d\vec{B}~=~\frac{\mu_0}{4\pi}\frac{I d\vec{\ell}\times \vec{r}}{r^3}. $$ If we change the $ d\vec{\ell}\times \vec{r} $ with the $ d\vec{\ell}\times$r . sin(Ө) = $ d\vec{\ell}\times$R in order ...
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2answers
66 views

Classical Hall effect when current has neutral charge

Suppose I have a current of both negative and positive charges(I know that there is also current from only negative and only positive charges,I'm not confused) along an infinite wire of square ...
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1answer
33 views

Why does a current loop generate the same magnetic field as a flat permanent magnet?

Imagine a current loop perpendicular to the screen pointed up and down with the current flowing out of the top end. Next to it, imagine a flat permanent magnet lying in the plane the screen with the ...
2
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1answer
25 views

Explanation for electrostatic energy expression

My question is about the electrostatic energy $We$ expression for $n$ point charges. I just can't figure out where the factor "$\frac{1}{2}$" came from: $We=\frac{1}{2} \sum_{i=1}^{n} q_iV_i$
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3answers
59 views

Why use different arguments to plot a sine wave?

In my Electronics class was given some examples of sine wave graphs that represent voltage in respect to time, $v(t) = Asin(wt)$ and other graphs whose $x$ axis is $wt$ instead of $t$, like in the ...
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1answer
36 views

Signs in derivation of capacitor discharge differential equation

In deriving the discharge current for a capacitor I have seen two different approaches: By Kirchhoff's law we have: $$ \begin{align} 0 &= I R + \frac{Q}{C}\\ \implies 0 &= \dot I R + ...
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0answers
24 views

Effective Area of Isotropic Antenna: Explanation?

I'm reading some introduction to antenna theory and I've often puzzled on the equation: $$ A_{eff} = \frac{\lambda^2}{4\pi} $$ which relates the effective area by which an antenna captures radiation ...
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6answers
675 views

Is electromagnetic vector field a sum of E and B?

I have a hard time to fully understand (classical) electromagnetic field theory with respect to Helmholtz's decomposition. Let me start from Helmholtz's theorem: Any vector field of class ...
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0answers
32 views

Faraday's law in free space explaining away the constant vector?

Let's say that I have a plane electromagnetic wave travailing in free space, and I know the electric field part to be $\vec E$. If I am using Faraday's law to get the magnetic field part I will get ...
2
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0answers
21 views

Why does this 'Quantum Pinned' superconductor allow easy repositioning

I'm confused by videos such as this (popular demonstration of 'Quantum Levitation'): https://www.youtube.com/watch?v=Ws6AAhTw7RA So my current understanding of superconductors is that when in the ...
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2answers
51 views

Auto spinning turbine generator

This might be dumb question but I'm so curious to know if this actually works or is impossible. I was researching how you could generate electricity from magnets and copper wire and also have read how ...
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0answers
71 views

Does an accelerating electron also produce a constant magnetic field in addition to an electromagnetic wave?

I've been reading about the basics of electromagnetism, from a layman's point of view, how static charges are the source of the electric field and how constant moving charges are a source of a magnet ...
0
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1answer
34 views

Computing energy loss for a hysteresis curve

Let a hysteresis loop be as shown in the figure, with $B_0$ the applied external field due to a current $I$ in a toroid, and $B_M$ the additional field due to the ferromagnetic material. Hence we ...
2
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1answer
47 views

Is the Lorentz force expression valid for magnetic field created by a magnetic monopole?

Will the Lorentz force expression be valid for a magnetic field created by a magnetic monopole? I haven't seen any derivation of Lorentz force expression yet and I don't know whether it was derived ...
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1answer
44 views

Electro-magnetic radiation

What happens when a free electron and an electromagnetic wave interacts? Does it vibrate or move along the direction of the electromagnetic wave?
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0answers
55 views

Electromagnetic Induction in two coils placed in vicinity of each other

First read the question (No I'm not saying you to solve it): Now what I think: As we know $V\propto -dI/dt$, so $I\propto -\int Vdt$ or negative of area bound. First area is increasing first fastly ...
0
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1answer
88 views

How is the current equation calculated from Ginzburg-Landau (GL) free energy?

In the Ginzburg-Landau theory, we can get the current expression from GL free energy: $$F = \int dV \left \{\alpha |\psi|^2 + \frac{\beta}{2}|\psi|^4 + \frac{1}{2m^*} \mid (\frac{\hbar}{i}\nabla - ...
0
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1answer
17 views

How can DC-magnetic fields be produced with the aid of currents?

How can DC-magnetic fields be produced with the aid of currents? What's a possible theoretical explanation for this, and how does one achieve this in the real world? What are the maximum fields that ...
0
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1answer
24 views

Energy Loss during Sharing of Charge between two Capacitors

It is fairly easy to show that there is always a loss energy when two capacitors share their charge to attain the same common potential, but is it with the same ease that one can explain why it ...
0
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1answer
24 views

How to approximate the force on a magnet below a coil

o x o x Coil with 4 turns o x o x _ | | | | Neodymium magnet |_| I need to know the ...
4
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2answers
294 views

Integral vs differential forms of Maxwell's equations

As stated in this post, the integral and differential Maxwell equations should be identical. However, in a text I was reading it states that The integral forms of Maxwell’s equations describe the ...
0
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3answers
87 views

Why does the electric field vanish at infinity?

When $r \rightarrow \infty$, $E \rightarrow 0$ for a point charge or set of charges or a finite charge distribution. While this seems obvious, I cannot find a reason why this is true when inspecting ...
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5answers
1k views

Why does a ring falling through a magnetic field experience an upward force?

The Problem states: A metallic ring of Mass $M$ and radius $r$ falls freely under the influence of gravity in the direction along the negative Z-axis. A magnetic field $B_z = B_0(1-z\lambda)$ ...
3
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1answer
176 views

What happens in a gas of magnets?

This SMBC comic asks what happens if you make a gas of magnetic particles: I was wondering whether anyone has run into actual examples of this or something like it. A classical example similar to ...
3
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1answer
81 views

Biot-Savart law from relativity

Few days ago I came by a derivation of STR from Bio-Savart law. Since then I have been trying to derive Bio-Savart law from STR. The derivation mentioned previously used two parallel current-carrying ...
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1answer
38 views

Does the direction of the magnetic field inside a solenoid depends on the direction which it is turned?

Today trying to explain some magnetic stuff, just came across with a simple (at least seems) question. My question is brief: does the direction of the magnetic field inside a solenoid depends on the ...