Questions tagged [electricity]

The study of the presence and flow of electric charge. Charges, currents, fields, potentials.

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2answers
2k views

Surface charge in a wire?

The image above shows the distribution of the surface charge in a current carrying wire. The surface charges distributes themselves to make sure the field inside the wire is always perpendicular to ...
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Why do lightning rods have a sharp point at the top?

We know that a lightning rod or lightning conductor is a metal rod or metallic object mounted on top of an elevated structure and, if we look closely, most of them have a sharp point at the top. What ...
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How did physicists know that only negative charges move?

I have phrased similarly another question about how physicists knew that two charges exist, positive and negative. The purpose of the question is not necessarily to educate me historically. It's just ...
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Why are materials that are better at conducting electricity also proportionately better at conducting heat?

It seems like among the electrical conductors there's a relationship between the ability to conduct heat as well as electricity. Eg: Copper is better than aluminum at conducting both electricity and ...
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What happens to capacitor’s charge when the plates are moved further apart?

In my physics textbook there is an example of using capacitor switches in computer keyboard: Pressing the key pushes two capacitor plates closer together, increasing their capacitance. A larger ...
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Why does it spark when I push a plug in the electrical socket?

When I slowly push a plug into the electrical socket I can often see sparks. Can anybody explain why? Can this be possibly harmful for the devices I plug in?
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computer vs heater coil comparison

If I have a computer and a heater coil that consume exactly the same amount of energy, which would be more efficient at heating my room? This is assuming that they both have the same fans and heat ...
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How is Kirchhoff's voltage law understood in the water flow analogy?

I met the Kirchhoff circuit laws in the past, but now I'm trying to associate them with a practical representation to be sure to understand them. Let's start with the Kirchhoff current law: If I say ...
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What happens if you connect a hot resistor to a cold resistor?

Kind of an extension to this question: If you heat up an object, and put it in contact with a colder object, in an ideal insulated box, the heat from one will transfer to the other through thermal ...
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1answer
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Are emitters always receptors?

I had a teacher who told us that every emitter, at least electric powered, can also behave as a receptor. Then he gave us examples of devices producing current : - LEDs when put under a light - ...
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Why does energy flow between a high voltage transmission line and linemen approaching it on a helicopter?

The video High power line workers shows linemen servicing a live high voltage transmission line. As the helicopter approaches the line, the lineman reaches out with a metallic wand that is ...
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Voltage in a circuit

Suppose I have a following circuit: I do not understand, why the potential difference between the points $c$ and $d$ is equal to the potential difference between the points $b$ and $a$? That is, why $...
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1answer
744 views

Joule heating due to the (slow) electron drift velocity?

I understand the concept of why the signal speed is higher than the electron drift velocity, but I can't understand the concept of joule heating. If electrons move slow then how do they produce a lot ...
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Why is there such low current produced when discharging a Van de Graaff Generator, even though voltage is very high?

E.g. putting your finger close to the Generator and seeing a spark between it and your finger. Obviously this does not kill you, but why is there such low current produced when the potential ...
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1answer
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Why high voltage transmission lines?

This is a question which I seem to have tackled multiple times, solved each time after reading a dodgy internet explanation, then partially forgotten about and retackled half a year later. It is time ...
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852 views

What are the current possibilities for large-scale storage of electrical energy?

I'm curious as to how large-scale electrical storage is managed now, how much they can store, how well they scale and for how long (and what is their loss rate). (I'm not asking about technological ...
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Loopy lightning

What causes lightning to follow the path it does ? picture from BBC news: http://news.bbcimg.co.uk/media/images/62891000/jpg/_62891901_untitled-1copy.jpg main page: http://www.bbc.co.uk/news/in-...
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1answer
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Why do plasma filaments of plasma globes move outwards towards the glass?

In plasma bowls, the plasma filaments made of ionized inert gases extend from the central 'mini Tesla coil', all the way to the glass outer sphere. What makes them want to go to the glass? Why don't ...
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Could you use magnetic fields to direct a bolt of lightning?

Would it be possible to direct the flow of electrons in a lightning bolt in a certain direction, using magnetic fields? Furthermore, would it be possible to direct the lightning in a long straight ...
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Why are electrons defined to have negative charge? [duplicate]

We normally think of the "default" or "root" state of things as being on the positive side of the spectrum. For example, we don't normally use a + symbol to ...
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3answers
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Current Through a Circuit with an 8 Resistor Setup

The following is a question from a practice Physics GRE exam (found online at ETS's website). The circuit shown in the figure consists of eight resistors, each with resistance R, and a battery with ...
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Why does current from a grounded electrical outlet flow through me if I stay on the floor made out of a dielectric material?

Why does current from a grounded electrical outlet flow through me when I touch a "hot" terminal, if stay on the floor made of a dielectric material? Consider an AC voltage source, such as a wall ...
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1answer
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How does a crow sitting on one of the electric lines attain the same potential as that of the line so as to prevent a charge flow through its body? [duplicate]

The crow doesn't get an electric shock while sitting on only one electric line because it has the same potential as that of the line. Since there is no potential difference the charge does not flow ...
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Electric power and resistance dependance

According to the equations, $$P=VI =I^2R\,\text{ and voltage } V=IR$$ it seems clear that when the resistance is lower by fixing the voltage at constant, the current is therefore, higher, generating ...
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1answer
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If light is an electric and (magnetic field), how can it be absorbed?

I was wondering how light or any electromagnetic radiation can be "absorbed" if it consists of electric and magnetic fields. For example if there is a charge at point A, and the light reaches point A,...
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1answer
992 views

Why charge distribution is greatest at pointy edges of a surface? [duplicate]

If we have a conductor which is in electrostatic equilibrium, then the charge distribution over this surface $\sigma$ is greatest at the sharp edges of that surface. Why is this the case? I want ...
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2answers
716 views

What happens to a photon after it is absorbed by an antenna?

I recently have read about interception of wireless information, however this mentions that people can intercept the information, and then somehow the recipient also gets the information. Regardless ...
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How do you model the resistance across a symmetric sheet of plastic, streched (think ceran wrap) between a circular anode and cathode?

Here is the problem. You need to model the resistance across a circular sheet of polypropylene (plastic). The resistivity of polypropylene is 1E15 Ohms per meter. The expression for resistance on a ...
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1answer
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Question regarding Drift velocity in general?

The derivation of drift velocity in case of electrons is equivalent to the case of an charged ionic gas and therefore all the arguments also apply there. Now for an ideal "ionic" gas which interacts ...
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1answer
872 views

Why does the characteristic curve (V vs I) for a light bulb backbend?

When teaching Ohm's Law, I have students do an exploration of a small, incandescent light bulb with a low frequency (1-2 Hz) sine wave. It's a simple series circuit of source and light bulb, ...
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Why is it possible to pass current to another insulated human while touching a plasma tube?

I work in a museum and we have a large plasma tube (I do not know any details of the current used within the tube). When I place my hand on the tube and then hold out my finger and touch another human ...
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1answer
144 views

What exactly causes an increase in resistivity?

Explain in terms of charge carriers why the resistance of copper is less than that of CuSO4 solution, which is less than that of hydrogen gas This is the question directly asked for homework, and I'm ...
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13k views

What is the dielectric constant of a pure conductor?

Dielectric constant is the ratio of permittivity of a medium to the permittivity of free space. How to find dielectric constant of a conductor?
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771 views

What will happen when lightning strikes on the surface of the deep sea?

The lightning is having enormous amount of energy, as it strikes the ground it makes various electrical and magnetic effects. If the lightning strikes a tree, that will be burst by lightning. Suppose ...
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1answer
19k views

How can a negative charge move towards a position of a higher electric potential but lose electric potential energy?

I understand that the equation is: change in electric potential = (negative)change in electric potential energy / unit charge. And I understand that if q is negative, then the change in electric ...
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1answer
959 views

Electric field generated by a point charge moving at the speed of light

As you see, this is the electric field generated by a point charge moving at constant speed v. I know that when $v$ -> 0, $E$ is just the Coloumb Law. But how do you interpret $E$ when $v$ -> $c$ ? ...
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Heat loss using alternating current

I am looking for somebody who can explain this to me. As I have read in physics books, the Joule-Lenz Law (Joule effect of heating) is represented by the formula: $$ W = I^2Rt $$ which can be ...
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1answer
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Why do we need to have a reference temperature to define thermal coefficient of resistance (TCR)

Why do we need to have a reference temperature to define thermal coefficient of resistance (TCR)? TCR is the change in resistance per change in temperature divided by the resistance at a specified, ...
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2answers
350 views

Resistor and voltage

Considering this simple circuit : The potential is supposed to be constant along each wire (because they're conductors), such that the left wire in its entirety is at the same potential as the ...
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3answers
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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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How to find the distribution of charge on two spheres connected by a conducting wire?

A solid metal sphere of radius $R$ has charge $+2Q$. A hollow spherical shell of radius $3R$, concentric with the first sphere, has net charge $-Q$. What would be the final distribution of the charge ...
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How does electricity travel near the speed of light if electrons drift at a snail pace? [duplicate]

I was in a reddit argument with someone who said electrons didn't flow at the speed of light in a circuit. Then he linked the Wikipedia page for drift velocity and defeated me in one fell swoop. Now, ...
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How do I find the polarity of a U-shaped electromagnet?

How do I find the polarity of a U-shaped electromagnet ? Current flowing clockwise --> South Pole Current flowing anticlockwise --> North Pole However the direction of flow of current changes when ...
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Why is there no change in speed of a charged particle in a uniform perpendicular magnetic field? [duplicate]

When the initial velocity is perpendicular to the magnetic field, the force acting on the charged particle F = qvBsin(theta) ...
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2answers
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Need an intermediate resistivity part/material

I need a part or material for a planned experiment (the experiment is similar to those described in my articles http://arxiv.org/abs/1208.0066 and http://arxiv.org/abs/1109.1626 ). The problem is that ...
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Kirchoff's loop law

Can any one provide me with mathematical proof of kirchoff's loop law? I am not able to understand from where to start.
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Removing a cavity from inside a resistor

Suppose we have a resistor of resistance R. We build a new one by removing a cavity A from inside it, and this new resistor has resistance $R_A$. This cavity can be a sphere or similar region. We then ...
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Why does resistance increase with length?

This is for the case of wires only (pipes are easy). I see that if I place a 9-Volt (constant) battery across any wire, the current is proportional to resistance. I don't understand why, for a ...
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Where does the electricity, generated by a solar panel, go if you don't use the electricity?

I'm sorry if this question is too trivial for this Q&A forum. I am a layman when it comes to physics (though I did cover the high-school physics courses). I was wondering what happens to the ...
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Of all the electrical energy used in a home, is there any portion that won't eventually become thermal energy in the home?

Considering all of the appliances that the average home uses--microwaves, light bulbs, dishwashers, refrigerators--is it safe to say that all of the electrical energy in a home will be converted to ...