# Tag Info

## New answers tagged electricity

1

Indeed, an infinitely long and thin wire with a current $I$ has a magnetic field given by (in SI units): $$B = \frac{\mu_0 I}{ 2\pi r}$$ Now suppose your wire has a finite radius. Ampère's law shows that as long as cylindrical symmetry is mantained, the field depends only on the current and not on the detailed properties such as the wire's radius or the ...

2

You have a box of pebbles. There are some pebbles that do not move at constant velocity when placed closed to one another. We repeat the experiment many times with these pebbles and characterize their motion. We make a plot of their acceleration as a function of their position and we notice that the acceleration is always proportional to the inverse distance ...

5

It's not actually circular, speaking purely logically. One says "An X is a field produced by things having Y." The other says "Y is a property of matter that causes it to respond to an X." Putting these two things together reveals simply that some matter has Y, which produces an X, which affects other bits of matter which have Y's. In other words we are ...

2

It is not acceptable to change a question to invalidate two existing good answers to your question, you should have asked a new question. There are two types of particles in the universe, the fermions and the bosons. The fermions take up space in the sense that you can't have too many in the same region of (phase) space. And the bosons basically mediate ...

6

Recall that science is descriptive of nature. These words are defined so that they describe the effects of experiments (generally the descriptions that stick are the simplest ones that cover the range of behaviors actually seen). As a result, though they are circular in structure, they absolutely do not rely on circular logic for their validity: they rely ...

11

Definitions in physics are always somewhat circular, because they are not really definitions. Instead, they're descriptions of the world. The way to make sense of "circular definitions" is usually to think about the experiments that led to those definitions. Suppose you're Charles-Augustin de Coulomb. One day you discover that by rubbing objects on each ...

0

Voltage is potential energy per charge. It's a function of space, so if we move one electron from a place where the potential is 2V (Location A) to where it is 1V (Location B), that takes 1 eV of work (note the sign because electrons are negative). The reason one place is 2V and the other is 1V is because there is electric field in between, which pushes our ...

0

Voltage is, at heart, simply a way to quantify the fact that separate electric charges produce a force on each other. Similar charges repel, opposite charges attract. It is seen as a measure of the intensity of an electric field. An electric charge in an electric field will experience a force which tries to move it (an electromotive force), and it is this ...

1

Think of the amount of (molecules of) water in a hose. Does it change when we start pumping it around?

0

Martin's answer is right. I would like to add some points here though. In the old TV's we used to get electric shocks(huge ones) when we used to touch the circuit of TV even when the socket was off. Some people think this is because the TV set is using the electric current, even when the tv is off. But, something worth mentioning here is, the shock used ...

1

It depends on the TV. An old TV with a real physical switch wouldn't consume any power, there is a break in the circuit and whether that break is at a switch on the socket or a switch in the TV makes no difference. Even a slightly more modern TV with an electronic power switch would consume "almost" no power - the electronic switch would leak a tiny amount ...

9

What the picture shows is a corona discharge (see also Wikipedia). It isn't a circuit in the usual sense of the word. It happens because the voltage is so high that it raises the electron energy to above the work function and the electrons just leak off. In effect the coil is charging the air around it. The charge will end up on the furniture, walls, floor, ...

0

{1} Here is a link to a paper by Patrick C. Crane on the transmission line disturbances to RF: http://www.faculty.ece.vt.edu/swe/lwa/memo/lwa0168.pdf. I didn't read the paper, but it seems to be an extensive analysis, and mentions at least 3 other references on the subject. On page 3, you'll find a summary of effects on three types of RF: "The phenomena ...

0

The closest existing solution to what you're looking for is probably Peltier cooling. Of course Peltier coolers don't break the Second Law of Thermodynamics either: they are heat pumps, with a cold sink and a hot sink.

0

As long as you're not trying to violate the second law of thermodynamics, yes. There are thermo-electric effects where a voltage can be generated from a temperature difference. This mechanism isn't really considered efficient though. Thermodynamically, the situation isn't too different from a regular power plant anyways, so it's never used for large scale ...

0

If you are considering a fixed charge on the capacitor, it won't matter at all. The charges have plenty of time to distribute themselves evenly over the plates, so there is no current anywhere, so no voltage drop. If you are considering conditions during the charge cycle, the charges in the low resistance plate will spread out more quickly. If the leads ...

2

When considering whether two surfaces will have a high friction or a low friction when rubbed together, more important than whether they are individually smooth or rough is what the barrier to their passing actually is. Consider first two perfectly flat plates. Even if the two plates are made out of wood, which is rough, they slide relatively easily past ...

1

The lightnings primarily depend on strong electrostatic fields. They're up to 100 volts per meter in the summer and 500 volts per meter in the winter. These fields are fluctuating. When a certain critical threshold not far from those values is reached, a lightning strikes. Does the electrostatic field move a metal? There is no direct electrostatic force ...

2

The exposition you give is fine in classical physics. Note though that in classical physics a particle cannot be a point particle, because something has to carry the charge in classical physics formulations. So the fact that one finds infinity at r=0 just hits on this constraint. One could use the argument as a proof by reductio ad absurdum that particles ...

1

When you rub your fingertips along a blanket you are exchanging charged particles like electrons and in some cases molecules missing an electron. This is due to friction. If you notice your fingers will actually become very hot if you do this over and over. These electrons are negatively charged and create what we call an electric field around them which ...

1

Your description is not very complete, but I guess what happens is exactly what you expected: to get significant repulsion (to counteract the atmospheric pressure), you need very high charge, which will be necessarily limited due to air discharge (maybe that is why you observed sparks). I don't see how replacing an aluminum shell with a plastic bag ...

0

The source and sink phenomenological description of charge - how realistic is it? It isn't realistic at all. To be perfectly honest it's totally misleading. I've heard over and over an electron described as a source of the electric field, but that is a misleading term. Yes it is, because like Timaeus said, it's the electromagnetic field. See ...

2

There is a grand tradition in electromagnetism to talk about the electric fields using the same terminology as we use for velocity fields. For instance we talk about the flux which rightly is a flow per area (and sometimes we multiply by the area and still call it a flux, which is even more confusing to call two things a flux) but it isn't a flow because it ...

0

As you have figured out, $V_{ab} \equiv V_a - V_b = - \int^a_b \frac{\lambda}{2 \pi \epsilon_0r} = \frac{\lambda}{2 \pi \epsilon_0} ln(\frac{a}{b})$. But we don't know what $\lambda$ is yet! In fact, we must use this equation to derive its value in terms of $V_{ab}$, which is assumed known. Then, to get the potential at any radius $r$, simply do your ...

0

Technically, you can maintain a constant (or near-constant) temperature of the conductor in spite of Joule heating by cooling it with temperature regulation. Practically, resistance typically does not change dramatically under moderate Joule heating.

1

Static electricity is not like regular electricity in that it does not involve closing a complete circuit; it just needs a large difference in voltage potential between one object and another. When you shuffle your feed on a nylon carpet and touch your finger to a doorknob, you are not closing a circuit. Instead, you are building up a large negative charge ...

1

I think an insulator does not completely stop charge transfer, if viewing it to act in the same way as maybe ie. a thermos cup, which does significantly increase the cooling time of a hot coffee inside, but does not fully prevent heat from escaping, hence allowing the hot coffee/material inside to cool.

1

So my question is where I am wrong or more appropriately what wrong steps I have made in my calculation. When you lower mathematically the resistance while fixing the current, this means electric field decreases, but magnetic does not. In the limit $\rho\rightarrow 0$ all the initial energy is in the magnetic field and can be expressed through ...

1

The problem is in your assumptions. If you have an E-field in the loop then the potential (voltage) would be increasing as you go around it. If you have a current in the wire, there must be a magnetic flux through the loop. The magnetic field has energy, which you are not accounting for. The magnetic field cannot penetrate a superconductor, so it is ...

0

A wide rubber band should work fine. Both the belt and the roller must be extremely clean. EXTREMELY extremely clean. A single fingerprint can be enough to mess things up, since the contamination distributes itself all over as the motor runs. If your belt and roller aren't clean, then oil film is touching oil film, and no rubber actually touches any ...

0

There must be some asymmetry at the start. It can go either way, positive or negative. It's like balancing a razor blade on edge, and "falls" one way or the other. But, under humid conditions, the Kelvin generator gives zero output. Build it, turn on the water, and nothing happens. This occurs because a many-megohms resistance appears across all the ...

1

Like all steady state circuit analysis things in parallel share the same voltage, and things in series share the same current. The question then really boils down to what the current vs. voltage curve looks like for a solar panel. A quick search turned up this: from https://www.folsomlabs.com/modeling/module/module_model So in this case it looks like you ...

2

How quickly discharge will occur in the situation you sketch depends entirely on the surface properties of the negative electrode. For current to flow, electrons need to be released from the negative electrode; once they are free, they will accelerate unimpeded to the positive electrode. They will arrive there with 1.5 eV of energy, causing a small amount of ...

1

Electrons (and other charge carriers, e.g., ions) in vacuum travel without resistance. However, as pointed out, correctly, in the other answers, there are no charge carriers in vacuum. Nevertheless, electrons can escape from the terminals if they have a kinetic energy which is bigger than the potential barrier of the terminal surface, i.e., the work ...

4

If the electric charge from lightning is captured and harnessed through circuits, eventually it will reach the ground, but once there it will join the general discharge process mentioned by Feynman. It won't lead to any build-up of charge. Incidentally, it would be very difficult to harness lighting with sufficient regularity to make a difference to the ...

1

Lines of electrostatic force exist between the positive and negative poles of the battery, even though they're separated by a vacuum. Vacuum permittivity is ε0 = 8.854 * 10^-12 farads per meter. By convention, this is called the dielectric constant of 1, a baseline against which the dielectric permittivities of other materials are compared. ...

1

In vacuum there are no charge carriers like ions or electrons. With nothing to carry charge, i.e. current, such a battery would discharge much, much slower than when the battery poles are connected by something that can carry charge like a conductor or an imperfect insulator.

0

The area element of the surface of a sphere is $\sin(\theta) r^2 d\theta d\varphi$. You can verify this yourself: $$\int_0^{2\pi}\int_0^\pi \sin(\theta)r^2 d\theta d\varphi=(2\pi)(-\cos(\pi)-(-\cos(0)))=4\pi r^2$$. So, if you integrate over only the $\varphi$ direction, you get the area of an infinitesimal loop: ...

0

The Peltier and Seebeck effects are each others opposites so to say. The Seebeck effect is described by the electromotive force or voltage $V$ generated at a temperature difference $\Delta T$ across the ends of a material: $$V=S\Delta T$$ $S$ is the Seebech coefficient and is a material constant that depends on charge carriers, material density and much ...

0

First of all, there is no need to worry over this. The reason we can get away with using conventional flow even though electron flow is what is really happening is that the two are indistinguishable except for in some experiments that are really hard to do. On to your questions. The GND is not an "electron reservoir". First of all, in most circuits there ...

0

The main issue with running a fluorescent light bulb backwards is that there are some irreversibility in the way that it convert electricity to light. The Mercury vapour in the tube emits UV rays. Theses rays are absorbed by the phosphor coating on the tube which emits white light. However, if we illuminate the phosphor with white light (reversing the ...

0

The easiest way is probably a material that changes color with heat and have the electric current heat the ink. This is how the meters on the side of batteries work. This is the stuff used to make novelty coffee mugs or t-shirts.

0

Without more detailed data, further experimentation, I cannot provide a definite answer. But with regards to your supply transformer that feeds the primary - could it possibly be a GFI protected transformer? Many of the later date neon light transformers (Franceformer for example) built GFI protectors into the transformers for safety. Using these ...

0

The teacher was right about the closed loop fact. The current can only pass through a closed lopp connecting two different levels of electrical potential. However in the case of thunder the potential differnce could be millions of volts, which ionize the gases between the cloud and the ground. The ionized gased behave as conductor providing the passage for ...

2

Is the working principle of light bulb reversible? No. Electric potential energy is converted into thermal energy in the light bulb filament. At a certain temperature range the filament will light up; that is, it will radiate with a wavelenght in the range of visible light. That this process is non-reversible might be clear if you consider some more ...

2

You are taking a shortcut when you say, "The voltage is zero." Voltage is always measured between two points. In electrical engineering, when we say the voltage at point X is V, we actually are measuring the voltage between point X and an implicit other point called "ground". In the electric power grid, "neutral" is ground, by definition. So the voltage ...

0

The key thing to remember here is that we are talking about an ideal wire. This means that there is (effectively) no resistance in the wire, and therefore no voltage drop along the wire. Now if there is a voltage difference between the two ends of the wire, that induces a constant electric field along the wire, causing current to flow.

1

If I understand your question correctly, the answer is "yes". For most energy conversion processes, the "inverse" process exists. Typically though, as you go from one to the other, and back again, you will lose some efficiency - think of it as the universe entropy increasing at every step of the way. Specifically, with regard to your two examples: The ...

0

The electrical resistance of a sphere is calculated in https://www.academia.edu/1841457/The_Notion_of_Electrical_Resistance

0

But according to the given answer if the electric field $E$ is changing then the flux should also change since $ds$ is constant. As you say, $E$ does change. However, it changes both where it enters $S$ and where it exits $S$. In the integral, the sign of $E \cdot ds$ depends on whether the field is pointing into or out of the surface. To make this more ...

Top 50 recent answers are included