# Questions tagged [electric-fields]

For questions that utilize the concept of electric fields (commonly denoted by the letter E), or for questions whose answers likely involve electric fields. More specific than the [electricity] tag, as questions about the phenomenon & theory of electricity do not necessarily involve the discussion of fields.

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### Change electric potential along uniform electric field

I have a doubt. I do know that the potential at a point decreases as we move in the direction of electric field because $$V=\frac{KQ}{r}.$$ This was taught to me with a point change as an example. I ...
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### Charge in crossed $E$ and $B$ fields

This post describes the motion of a charge in crossed electric and magnetic fields as: $$x = r \cos ωt + \dfrac EB t\\ y = r \sin ωt\\$$ Can you please explain how these equations are derived. Why ...
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### How to find the electric field in a inductor? [closed]

There's a problem that I didn't understand when I was at school, and it's still unclear. Suppose a simple circuit, an AC constant current source $I=I_{0}\exp(j\omega t)$ the current is, and an ...
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### Potential of a capacitor

I'm learning about capacitors, and i cannot visualise what potential difference of the capacitor is. I do know that potential of a charge is work per unit charge which is (kq)/r , where q is the ...
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### Integral for $E$ field of spherical shell [closed]

Per Griffiths Electrodynamics problem 2.7, finding the $E$-field of a spherical shell of surface charge dist (sigma); and radius $R$. I don't understand my result or how it jibes with Gauss' law. ...
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### Disagreements in derivations of electrostatics

In the derivation of the potential energy of electrostatic systems we get (I am working in cgs) $$W=\frac12\int\phi\,dq=\frac12\iiint\phi\rho\,d V \tag{1}$$ But $$\vec\nabla\cdot\vec E=4\pi\rho$$ Thus ...
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### Why is the field of an electric dipole important in imaging? [closed]

Is it perhaps because the atoms and molecules in our scatterer/object become oscillating electric dipoles due to the electromagnetic field(light) that we shine on it? So can we actually model our ...
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### Classic Image problem

The classic image problem: Here, to calculate the potential above an infinite grounded conducting plate and a point charge q held at distance d above the plate, the image method is used. Here, I want ...
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### Does a perfect conductor has unlimited supply of charges?

In Griffiths, it has been written that A perfect conductor would be a material containing an unlimited supply of completely free charges. My question is, how is the unlimited supply of charges ...
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### Consistency of existence of point charges and energy in fields

In Feynman lectures, Volume 2 chapter 8 (https://www.feynmanlectures.caltech.edu/II_08.html#Ch1-audio) at the very end Feynman remarks We must conclude that the idea of locating the energy in the ...
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### How does the electromagnetic field behave in a complex circuit when first energized? Such as in a computer CPU booting up

From what I understand in the case of a simple circuit, the electrons in the circuit wiring are steadily excited, i.e. energized, in a fairly complex way by the external energy input, as the energy ...
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### Closed loop integral of Electric Field dotted with dL

I am struggling to wrap my head around Faraday's law: $$\oint\mathbf{E}\cdot d\mathbf{l} =-\frac{d\Phi}{dt} = -\frac{d}{dt}\int_S\mathbf{B}\cdot d\mathbf{S}$$ Integral of an Electric field dotted ...
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### How do we know that the electric field exists when there is no other charge nearby?

We have defined the electric field to easily calculate the force exerted by a system of charges on another charge. What happens to the Electic field if the test charge is removed? Does it still exist ...
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### Why is the total electrostatic energy bigger than the sum of separate energies?

I don't understand why, if we add the two energies $\,\boxed{1}\,$ and $\,\boxed{2}\,$ from two electric fields, the component $\,\boxed{3}\,$ is added and so it's different from their sum \begin{...
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### Magnitude of Electromagnetic fields are radially symmetric but not direction

Suppose we have a plane and a wire with current passing perpendicular to it. If we consider magnitude field of any point at a perpendicular distance $r$ from the wire, we have the following expression ...
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### Can we light a light bulb with earth's electric field? [duplicate]

This is a dumb question. I recently found out that earth has an electric field around 150V/m near it's surface. So,I was wondering if we held the two terminals of the bulb few centimetres apart, ...
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### Complex permittivity and density flux time response expansion

I've been studying complex permittivity and come across to expansion 3.4.2 on website https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Book%3A_Electromagnetics_II_(Ellingson)/03%...
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### Numerical Computation of Electrostatic Field Lines

After solving for the potential $V(x,y)$ from a Laplace Equation $\nabla^2V=0$ in a closed 2D domain, I can readily compute the isopotential lines $V(x,y)=V_0$ with $V_0=0$ to $1$, and the Electric ...
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### Macroscopic vs microscopic electric fields

What exactly is the difference between macroscopic and microscopic electric fields? Is the macroscopic one just the average of the micro over some not-to-small volume?
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### Why can we add the two terms in the Lorentz force?

The term "Lorentz force" in the title refers to (1). my textbook states: The Coulomb force is mediated by the electric field and acts on the charge $q$, that is, $\mathbf{F}_e = q\mathbf{E}$...
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### Parallel and perpendicular component of electric field

We know electromagnetic wave is moving along propagation direction and electric field, magnetic field & propagation direction are perpendicular to each other. Then please tell me what is the ...
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### Electric potential inside a polarised conductor

Say a conductor with an initial electric potential of zero is subject to an arbitrary charge. I understand that because if this outside charge, there would be charge distribution inside the conductor, ...
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### The definition of Electric field?

In Raymond Serway's physics textbook, the definition of the electric field vector, was that it's force vector acts on a positive test charge, given as force divided by the test charge: \...
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### Having positive charge density, yet a particle experiences an attractive force [closed]

Before flagging this as a homework question, note that I'm not asking to solve something. I have the answer but cannot interpret it correctly. I have a conceptual problem that appeared in a specific ...
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### Question about electric field [closed]

Here the cavity must affect the electric field lines because there were electric charges in the cavity then removed , so the lines must change and be no more radially out ward due to asymmetry of ...
1 vote
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### Question regarding Boundary Conditions for Electrostatics [closed]

When considering the boundary conditions for $E$ $fields$ of a infinite long sheet(in $xy$ plane) ,the E field is given by $\dfrac{σ}{2ε_0}\hat{z}$ (if the test charge is above the sheet) ,it ...
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### If changing electric field induces magnetic field, how can steady current (therefore steady electric field) create magnetic field around it?

In the search for understanding electromagnetism on deeper level, one of the earliest observations was that magnetic field is created around a current carrying wire. Then it was noticed that changing ...
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### Static electricity problem

Take the following diagram and explanation: I have two questions: Why is the table tennis ball with metal paint attracted to the positively charged rod. Although the negative charges move closer to ...
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### Representation of the electric field lines of a positive and a negative charge, where does this line end?

Below is an image taken from the Electric Fields entry of Encyclopedia Brittanica. The general rule for drawing field lines supposes that the lines are drawn starting at the positive charge and ...
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### Gauss law at the surface of a conductor

I came up with a conclusion where I find half of the result on the picture. Here is how can the half of the result: I thought the same gaussian surface. But since we know that, charges outside the ...
How do I prove that Gauss's Law works and is valid for a sphere with a circular disk on top? I'm given the radius of the sphere R$(\theta_0,\pi)$, radius of the disc $(R \ sin \ \theta_0)$, the center ...