# Tag Info

Accepted

### How does this baby rattle work?

As you said, it's probably not magnetism if the balls are free to rotate; there is no reason they wouldn't just flip over and stick together, north to south. You can test this by buying some of those ...
• 103k

### Fourier transform of the Coulomb potential

I really appreciate the physical explanations made in other answers, but I want to add that Fourier transform of the Coulomb potential makes mathematical sense, too. This answer is meant to clarify ...
• 840
Accepted

### Why are atoms electrically neutral?

Being neutral does not exclude having an electric field. Any dipole is neutral but still has electric field. Your question is based on a wrong premise. "Neutral" simply means zero net charge....
• 8,153
Accepted

### Is it possible for a system to have negative potential energy?

Yes, potential energy can be negative: consider Newton’s law of gravitation $$V = -\frac{GMm}{r}$$ Where $G$ is Newton’s constant, $M$ and $m$ are masses, and $r$ is the distance between them. It can ...
• 3,265

### Can positrons attract electrons?

They attract each other and form a system completely analogous to a hydrogen atom, called positronium. This will then annihilate into at least two photons. The half-life time depends on whether the ...
• 25.6k
Accepted

### Basis for the Generalization of Physics to a Different Number of Dimensions

Great question. First of all, you're absolutely right that until we find a universe with a different number of dimensions in the lab, there's no single "right" way to generalize the laws of physics ...
• 48.5k
Accepted

### Can we add attraction forces?

You need to multiply by four not by two. To see why let's draw the situation: You are assuming the situation is as shown in the top diagram. So the two $M_1$s attract each other and the two $M_2$s ...
• 358k

### Why are atoms electrically neutral?

Atoms do have "complicated electric field", and occasionally may lose or gain a few electrons - in which case they are called ions. In fact, this happens all the time in chemical reactions. ...
• 60.5k

### Negative mass effect?

If $E = (m+M)c^2+k\frac{Q^2}{r}-G \frac{mM}{r} < 0$, then $(m+M)c^2-G \frac{mM}{r}$ must also be less than zero. Thus we must have $r < \frac{GmM}{(m+M)c^2}$. This is less than the ...
• 41.7k
Accepted

### How can electric field be defined as force per charge, if the charge makes its own, singular electric field?

It's true that a point particle with finite charge is problematic in electromagnetism because of the infinite field and associated energy near such a particle. However, we don't need that concept in ...
• 59.8k
Accepted

### Comparing the strength of gravity and electricity

Yes, they can. Both interactions can be modeled using perturbative quantum field theory, where their strength is parametrized by a dimensionless coupling constant. Electromagnetic repulsion between ...
• 35.6k
Accepted

• 1,485

### Why do we use cylindrical coordinates for infinite line charge?

You very well can. But the coordinates are used with the required symmetries in mind. In a line charge, the system is cylindrically symmetric about the axis of the line. This makes the vector ...
• 1,246
Accepted

### Can Newton’s law of gravitation be derived from Coulomb’s law?

those two laws look similar because they both describe the propagation of a long-range field through three-dimensional space which produces a force that acts (in the simplest example) between pairs of ...
• 96.2k
Accepted

### How can supercapacitors not implode?

This is a good question. It comes down to two factors: The 'plates' have dielectric material separating them, and the effective size of the plates is large, relatively speaking. The dielectric ...
• 3,697
Accepted

### Deriving Coulomb's Law from Gauss's Law

You are correct that Gauss's law alone cannot be used to derive Coulomb's law. Instead, you need to supplement it with the hypothesis that space is isotropic, but nothing more. The thing is really in ...
• 134k

### Why is the electrostatic force felt in straight lines?

Following R.W. Bird answer: Consider an isolated system of two particles: Since system is isolated, angular momentum (and linear) is conserved. I.e. $\vec\tau_{net}=\Sigma\space \vec r\times\vec F=0$ ...

### Can the electric force lift heavy bodies?

Unless I miscalculated, this does not seem possible for the following reason. Let us assess the electric field required to suspend a man with a mass of $m=70kg$. For simplicity, let us consider a ...
• 27.1k

### Basis for the Generalization of Physics to a Different Number of Dimensions

Here is one line of reasoning: E&M is supposed to be a fundamental theory. Having an action principle may facilitate developing a consistent quantum theory. The structure of the Maxwell Lagrangian ...
• 208k

### What will happen if we keep bringing two protons closer and closer to each other, starting from a large distance?

This really depends on the energy scale at which you bring them together. At CERN they do proton-proton collisions, where the protons move at speeds close to the speed of light. At those energy scales,...
• 291

### How is Gauss' Law (integral form) arrived at from Coulomb's Law, and how is the differential form arrived at from that?

@Qmechanic's already provided a nice answer. I would like to provide another one. Consider a charge $q$ be enclosed by any surface (not necessarily a sphere). Something like this - Now, you write ...
• 947

### Why do elements, as they have more protons, need a higher amount of neutrons to stabilise them?

The nuclear force only acts between nearest neighbors to hold them together- but the electrostatic force driving them apart has infinite range, so any one proton in the nucleus feels the repulsion of ...
• 96.2k

### Gravity vs. EM: action at a distance

Yet, no texts on electromagnetism make the same comment about the electrostatic potential $\nabla^2\phi_e = 4\pi\rho_e$. I disagree with this claim. In fact, most EM textbooks do make this statement. ...
• 104k
You're forgetting one thing: a particle cannot feel its own electric field, so a point charge that generates a $1/r^2$ field doesn't do anything unless acted upon by an external field. You also can't ...