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

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How can I prevent being zapped by static electricity every time I touch a doorknob or handle in the office?

I don't know what it is about this office, but it seems everything I touch (doorknob, bathroom faucet, edge of kitchen sink in the break room), I get zapped by static electricity. It's getting old. ...
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What happens to a fully charged battery, when you continue to charge it?

If I leave my cell-phone charging the whole night, it will be fully charged after a while. What happens with the battery and the excess energy I add? Also, I noticed my charger emitting a different ...
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How fast do electrons move through a conductor?

If I apply $1 \text{ V}$ across a $1 \text{ }\Omega$ resistance, I'd get $1 \text{ A}$ flowing. $1 \text{ A}$ is defined as $1 \frac{\text{C}}{\text{s}}$, and $1 \text{ C}$ is equivalent to ...
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Why doesn't the electric field inside a wire in a circuit fall off with distance from the battery?

We studied electric fields due to point charges. The magnitude of these fields decreases with the square of the distance from the point charge. It seems to me that we could treat the positive ...
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Why does a faraday cage protect you from high currents?

In an electrostatic case it is clear that that in a space enclosed with a conductor (without charge in it) the electric field is zero. This is often demonstrated in physics shows like on the ...
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Where do electrons in electricity come from?

Where do the electrons come from when an electric generator is making electricity? Is from the air? Would a generator work in a vacuum? Electrons have mass so where would they be pulled from if ...
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Salt water conduction

Does salt water conduct mostly by the ions travelling through the solution, or by electrons collectively flowing or hopping through the solution like in metals?
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How much of current flows through a bird sitting on a power line?

I've been googling for hours and went through over a hundred answers. Now, some say the bird doesn't form a closed loop, some say the current is so small that it doesn't kill the bird. From as much as ...
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Calculating engine starter’s energy use

During a discussion on start-and-stop vehicle technology some bloke began pushing the point that re-starting the car uses stored energy from the battery, which needs to be replenished by increased ...
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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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What's the difference between Capacitors, Ultra-Capacitors and Batteries

Capacitors are known to hold and release energy very quickly, unlike the slower release that batteries exhibit. If one were to bunch many (1000's of) capacitors together could they function as a ...
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How to calculate required current and specifications for a “wet coil” to generate x Tesla of magnetic field?

A water management project requires a "wet coil" (coil will be submerged in aqueous media) designed to generate a steady-state electromagnetic field of adjustable magnetic magnetic flux density at the ...
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Why and how exactly is electric motor torque limited?

Inspired by this question and specifically this answer to it. From my experience there's always some very specific limit to how much torque an electric motor can output. For example, an electric ...
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Charge signs in current

I've had recently an argument with my friend about different charge carriers in an electric current. Suppose that electrons and holes are moving in the same direction. It effectively means we have ...
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How Special Relativity causes magnetism

So my physics teacher assigned us an article about how special relativity causes magnetism in a wire with a current, even with the low drift velocities of electrons in a current. It seemed that the ...
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998 views

The possibility of free electrical energy?

Please excuse my lack of knowledge/understanding. Question: Why Nikola Tesla's Free Energy concept was never worked upon? Even today. Context: Now that we know Nikola Tesla was a genius and did ...
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155 views

Can I feel (the results of) electromagnetic induction from overhead rail lines in the human body?

A few weeks back, I was standing at a Hornsey rail station (in the UK) which uses overhead lines, and particularly has a number of parallel rail lines, all close to the platform. While I was standing ...
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Does current flow back to the source through earth?

We know that if Single Line to Fault occurs, then fault current flows to the earth. I want to know whether the current will return to the source or not. For the current to flow we need a closed path. ...
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How does current flow in a irregularly shaped heterogeneous resistor?

The motivation for my question is understanding how electricity gets through your skin as opposed to running along it, and how the presence of things like water on the skin affect the relative ...
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This sentence makes no sense, electrostatics and electrons moving in a conductor - current

I highlighted the part where the confusion is. The sentence said that the potential difference is 0, yet it then immediately talks about how electrons can have motion. What are they trying to ...
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Are square wave really square or are they always relative approximations using harmonics

I'm studying the properties of waves through different mediums, and got hung up on this. Is a square wave always a sum of harmonics or can we produce a square wave by quickly changing voltage? Is ...
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Why don't electric workers get electrocuted when only touching one wire? [duplicate]

I know that when electricians work on the poles on the streets, if they only touch one wire at a time they will be fine. However, from my understanding, the negative wire is connected to a large ...
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Is a billion volt electrical transmission line possible?

Because $$ P_{\,\displaystyle\rm loss}=\frac{P^2R}{V^2} $$ in an electrical line where $R$ is the total resistance Why not use a $1$ billion volt line instead of a $800\,\rm kv$ line? Is there a ...
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Can the lightning be captured and used as power source?

I would like to update my knowledge in this area, that is really out-of-dated and stopped somewhere like ten years ago. I asked the very same question on my physics lecture at my studies and got the ...
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Why doesn't an electron accelerate in a circuit?

Why don't electrons accelerate when a voltage is applied between two points in in a circuit? All the textbooks I've referred conveyed the meaning that when an electron traveled from negative potential ...
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Sign of Work and potential energy in electrostatics

Conceptual question: Suppose we have a configuration of point charges. If the potential of the energy of the system is negative, this means work is positive. I'm kind of rusty with my mechanics, ...
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How do stun guns not kill people?

Now, I've seen a lot of answers to this sort of question, but most of them provide answers that don't actually make sense from a physics perspective. As an example of such an answer, I've commonly ...
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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, ...
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Different batteries connected in parallel

If we have 2 batteries one of emf x and the other is of emf y and we connect them in series we get an effective emf of x+y. But what if we connect them in parallel, how to calculate the emf now?
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How much energy is in a lightning strike?

According to Wikipedia an average lightning has 1TW, the whole world used 16TW in 2006. (I suppose this means the same as 16TWh in one year?) Sometimes the lightning reaches 100kA. This peak last for ...
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Voltage drop due to a resistor

I'm afraid this question is going to sound rather dumb but here it is: Suppose I have a very simple circuit: one battery and one resistor. The sum of the voltages in this circuit has to equal zero. ...
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Why is the voltage drop across an ideal wire zero?

I'm having trouble conceptualizing why the voltage drop between two points of an ideal wire (i.e. no resistance) is $0~V$. Using Ohm's Law, the equation is such: $$ V = IR \\ V = I(0~\Omega) \\ V = ...
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Why does connecting a battery's positive terminal to the negative terminal of another battery not create a short circuit?

This is a question regarding the physics behind the observation. I have guessed the answer to the question, but I may be wrong, so I want to wait for the responses before posting it. Some major ...
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Why two connected 1.5 volt battery become 3 volt

Why two connected 1.5 volt battery become 3 volt? If one battery plus side is connect to minus side of another battery, there should be a current flow between them until no free charge move anymore, ...
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How does lightning “know” where to go?

If lightning comes down in, say, a large flat field with a lightning rod sticking out of the middle, the lightning will strike the rod. How does it "know" the rod is there? Will it always strike the ...
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Why are $\mu_0$ and $\epsilon_0$, which appear in electrostatics and magnetostatics, related to the speed of light which appears in electrodynamics?

$\epsilon_0$ and $\mu_0$ appear in electrostatics and magnetostatics. When we include time varying fields we have electrodynamics and the appearance of c which turns out to be related to $\epsilon_0$ ...
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Why do batteries work?

Consider diagram A. In diagram A a car is at the top of a cliff. Gravity is pulling it down, but it does't move since its on a flat surface. Even though it would be at a lower potential at point B, ...
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Is the Earth negatively or positively charged?

The Earth carries a negative electric charge of roughly 500 thousand Coulombs. Does that mean the Earth is negatively charged?
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How can there be a Current and an Electric field in an idealized wire with no voltage drop?

In an ideal circuit, How can there be a current b/w points a & b, when there is no potential difference and thus no electric field between a & b? If there is no current, then where does ...
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Capacitor circuits with light bulb

Let's say we have a normal circuit with a light bulb, with wires and a battery. When one places a capacitor in this circuit, how is the light bulb able to light up, even when the capacitor prevents ...
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why do lightbulbs sometimes unscrew by themselves?

I've seen some light fixtures which exhibit the behavior of lightbulbs gradually unscrewing by themselves.
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General integral to find resistance

My question is: is there a simple and truly general equation for the resistance between two electrical equipotential surfaces?. Obviously, if so, what is it, and if not, why? It would be very ...
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How can there be a path to ground with thick shoes and a carpet?

I'm connecting a test light to one pin of an halogen lamp. When I touch the metallic part on the back of the test light, the light glows, as it is supposed to. However, I have thick shoes and I am ...
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Why does welding produce UV light?

Looking directly at a welder is dangerous because large amounts of UV light is produced. What makes this light? Is it electrons from the current that excites metal atoms, and these atoms sends out UV ...
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What happens when the frequency of A.C becomes infinite?

In an alternating current, the flow of electric charge periodically reverses direction, and the number of times it does that is called the frequency of the current. However, if the frequency of an AC ...
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Electric Field from Dielectric Shell

This is a question taken from a past E&M exam A thick spherical shell (inner radius $R_1$ and outer radius $R_2$) is made of a dielectric material with a "frozen in" polarization ...
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Will there be any force of attraction or repulsion between an electrified body and a non-electrified body?

Up to my knowledge an electrified (charged) body can attract a non-electrified (neutral) body. I thought this because, when we bring a charged (suppose negatively charged) body near a neutral one. ...
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Speakers and Static from Hands

Why is it that when you touch the a bare male end of a speaker feed that the speaker makes hissing noises? Is it just (eddy?) currents running through you?
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How can you have a negative voltage?

How can you have a negative voltage? I don't really understand the concept of negative voltage, how can it exist?
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Sum total distance of electrons on a spherical surface

What is the sum total distance between every possible pair of point charges when there are n point charges on a spherical surface? All point charges can only and are located on the infinitesimal ...