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

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Force between two finite parallel current carrying wires

Remark: This is not a homework question...It is pure out of theoretical interest. I asked this the mathematics-community a couple days ago and got no answer, so I figured I'd try here. Most standard ...
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1answer
23 views

Finding magnetic field inside rotating sphere

A sphere with radius $R$ has constant charge density $\rho$ per unit volume. This sphere rotates as a rigid body with an angular velocity of $\omega$ around an axis that goes through the center. What ...
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2answers
27 views

Why does this capillary action generator not work?

So I was doing a bit of reading. Apparently the the obstacle to generating energy from the forces driving capillary action is breaking the surface tension at the top of a capillary tube. It is just ...
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1answer
20 views

Electric Dipole force exerted by a charged wire on a dipole

Why is force exerted by a charged wire on a dipole given as $F = P×(dE(r))/(dr)$ where P is the dipole moment? Please explain in simple words and avoid using too much technical derivations.I am a ...
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2answers
7k views

Why do two connected 1.5-volt batteries become a 3- volt battery?

Why two connected 1.5 volt battery become 3 volts? 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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3answers
114 views

Which of the following has higher resistance - milliammeter or ammeter?

The question is the same as the heading - Which of the following has higher resistance - milliammeter or ammeter ? Now my teacher's and my answers don't match. It is assumed that the spring constant ...
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25 views

How does one manipulate the frequency of alternating current? [on hold]

Suppose standard voltage, current, and frequency AC power is readily available for a project that requires the available standard voltage and current, but also requires an arbitrary number of cycles ...
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1answer
272 views

How do bathroom scales measure body fat percent?

My bathroom scale has 2 metal strips each for a foot and the producer claims that it can measure body fat percent. There is a warning in the manual, that if I have heart problems (I guess pacemaker), ...
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2answers
56 views

Why do i get a shock if i touch a live wire and my feets are on ground?

I understand that there is a large potential difference but i guess we humans offer a very large resistance.So,shouldn't the current be very small .Small enough not to kill us?
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62 views

Wait… why exactly does farady's ice pail experiment prove Gauss's law?

You'll notice there are no equations in this: that's because this is a question of morale, not of math. But a humble one at that! I come to learn, not to expound. But don't let that limit the form ...
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2answers
71 views

Is it correct to use $P=\large\frac{V^2}{R}$ for AC circuits?

In one of my old notebooks, I found a problem which was basically about finding out resistance of a 100W light bulb (In my country electricity voltage is 220V). Then our teacher had used the equation ...
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How does the specific resistance / conductivity changes, when melting metals?

According to standard school models, the specific resistance (or conductivity) depends on the movability of the charges. In a metal lattice, some electrons can freely move and are only hindered by ...
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How does non-constant conductivity change surface charge on a wire?

Current density is equal to conductivity times electric field. Therefore flux through a cross section of a wire is equal to current over conductivity. Current has to be constant along a wire. This ...
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2answers
57 views

How to check the ohmic contact to the film?

I have a thin semiconductor film deposited on an isolating substrate. I would like to check different metals to find out do they form the ohmic contact or Schottky barrier. What is the best way to do ...
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2answers
42 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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5answers
1k views

How does insulating footwear prevent an electric shock?

The reason I have always heard to explain the reduction of electric shock when we wear insulating footwear goes as follows: When electricity passes from our body to the ground, an electric circuit ...
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3answers
489 views

How the car keys prevent the shock caused by Static Electricity

I was reading around about the Radio Frequency and its effect on the GAS/Petrol forecourts, suddenly i came across something posted by someone, "I personally went on a trip with some friends, one who ...
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2answers
19 views

How to determine whether a statically charged object is positively or negatively charged?

for example, I rub a ballon on carpet. What is an experiment I can do to tell which object is net positive charged and which is negative?
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1answer
357 views

How can one explain the Kelvin Water drop experiment without assuming the container to already possess a positive charge?

Kelvin Water Dropper experiment refers to the electrostatic generator invented by Lord Kelvin, which uses falling water to generate a voltage by electrostatic induction occurring between ...
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1answer
30 views

Can we generate electric current by injecting electrons into a copper panel

Suppose I take a vacuum tube and accelerate electrons in it by electric fields then collide it on a copper plate. Will the electrons then go inside it and generate an electric current by putting a ...
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1answer
22 views

What device is best to prevent electric motors from burning out? [on hold]

Is there any equipment to prevent electric motors from burning out? I have researched about Transient voltage surge suppressor TVSS and also Surge protection device. Can this help?
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1answer
21 views

Can an electric current's magnetic field be represented by a magnet/magnets?

Is it possible to recreate the exact same magnetic field that electric currents have with a magnet and if it is, how?
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1answer
54 views

Potential difference between Earth's surface and 2 meters above

Assuming Earth is a charged sphere of radius $R = 6400\times10^3$ m with uniform surface charge density $\sigma = -10^{-9}$ C/m2 and with $\epsilon_0 = 8.85\times10^{-12}$ F/m I find that ...
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3answers
165 views

Polarization vector of dielectric electrostatics

I know two laws for Polarization vector of a dielectric material $$\vec P = N q \vec{\Delta L}$$ Where N is the number of dipoles per meter cube, $\vec{\Delta L}$ is the vector that represents the ...
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35 views

Electricity should deform the conductor then why doesn't it?

In electric current both negative particles and positive particles flow but in opposite directions. So why doesn't the conductor's shape deform because its particles are moving here and there? OR is ...
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1answer
333 views

Impurity scattering temperature dependence

Is there any temperature dependence of relaxation time in impurity scattering of conducting electrons? It seems to me that there is none. But, some people claim that there is. So if you could ...
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4answers
373 views

Why wire filament shakes inside a light bulb?

My light bulb cast a clear light toward adjacent wall and I can clearly see, that wire filament inside bulb shakes, though lamp itself does not moves even an inch What is causing this? Is it a ...
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1answer
45 views

If the electric field is not a gradient, can it exist?

We know that the gradient of the electric potential function $V(x,y,z)$ is the electric field. But not all vector fields are gradients, for example $y\hat{i}-x\hat{j}$ is not a gradient. Does this ...
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6answers
634 views

Why is electric potential 0 in this case?

On a test, we had a question where there are 4 point charges at the vertices of a square. The 2 charges at the upper vertices have charges of +q and the 2 charges ...
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2answers
79 views

Why is the voltage of a battery equal to the emf?

We know that there is an electric field inside the battery that works against the moving electrons of a circuit. But there is also the chemical force of the battery that at some point become equal. ...
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7answers
7k views

Why is the charge transferred by electrons and not by protons?

Charges are transferred by electrons which we all know. But why can't it be transferred by protons? Well, I searched on Google where I found similar questions already being asked on many sites. ...
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10 views

How to calculate the capacity of an electroscope out of the geometry and why is it independent on the pointer deflection?

How can one calculate the capacity of an elektroscope like the following from the geometry of the setup? How does this formula implies that the capacity does not (approximatly) depend on the ...
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1answer
2k views

The electric field of a conductive sphere containing a charge - grounded vs not grounded

Let's suppose we have a sphere but unlike theoretical ones it'll have some thickness say $\Delta r$ and inner radius $R$. What I was wondering about is how will it behave if we place some charge $q$ ...
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1answer
22 views

Question regarding Van De Graff generator Belt

I have made a VDG generator with a rubber band as the belt and a glass roller. It doesnt seem to work because I think the rubber band may be conductive. I was thinking of using other materials for ...
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1answer
48 views

Maximum power transfer proof

I have the following homework problem. Consider a power supply with fixed emf $ε$ and internal resistance $r$ causing current in a load resistance $R$. In this problem, $R$ is fixed and $r$ is a ...
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1answer
275 views

Question about superconductivity

A long cylinder of radius $R$ is made from two different material. Its radius $r<r_0$ $(r_0<R)$ part is a material with superconducting transition temperature $T_1$, and its $r_0<r<R$ ...
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2answers
48 views

Is lightning an example of energy emision from accelerated charge?

I have always heard that the inconsistency in explaining atomic models with classical mechanics was that the study of electrical charges had shown that whenever a charge is accelerated, it emits light ...
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1answer
42 views

Can we create a electric field without actually using a battery or power source?

Can we create an electric field without actually using a battery or a source of electricity? Like a take 2 panels, one is positively charged and another is negatively charged and put them parallel to ...
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1answer
32 views

Electricity from electrons

If I was able to get electrons flowing through a glass tube what happens if I put wires connected to a bulb into that stream of electrons?
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2answers
243 views

What happens when you pass a current through a coil made of a ferromagnetic substance?

I know when you pass a current through a length of copper coil or a solenoid, there is the induction of a magnetic flux. But what of the coil made of the ferromagnetic material, the permanent type ...
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1answer
788 views

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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1answer
74 views

Mean-free-time between collisions and drift-velocity

In a physics text book I need help to make sense of the part highlighted in yellow: This is out of context of course, so just to make it clearer: $\tau$ is the mean free time of the electrons in a ...
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1answer
358 views

What determines how much electrical charge an object can hold?

What determines how much electrical charge an object can hold? Does increase voltage force more electrical charge to be store in an object (Van de Graaff generator), since electric field increase as ...
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2answers
42 views

End current without touching wire

Say I am sending a pulse down a five foot wire. If I want the pulse to not reach the end of the wire, but instead reflect at say the two foot mark, how could I achieve this without physically ...
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2answers
39 views

Auto spinning turbine generator

This might be dumb question but I'm so curious to know if this actually works or is impossible. I was researching how you could generate electricity from magnets and copper wire and also have read how ...
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1answer
21 views

Computing energy loss for a hysteresis curve

Let a hysteresis loop be as shown in the figure, with $B_0$ the applied external field due to a current $I$ in a toroid, and $B_M$ the additional field due to the ferromagnetic material. Hence we ...
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1answer
17 views

How can DC-magnetic fields be produced with the aid of currents?

How can DC-magnetic fields be produced with the aid of currents? What's a possible theoretical explanation for this, and how does one achieve this in the real world? What are the maximum fields that ...
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1answer
319 views

How is the current flow perpendicular to the wire?

This answer gives a great explanation of how surface charge builds up to force the current to move perpendicular to the wire: http://physics.stackexchange.com/a/102936/41086 However, it fails to ...
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3answers
9k views

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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1answer
574 views

Why does rubbing feet against sheets create a spark?

Sometimes when I'm in bed at night, I rub my feet against the sheets and light is created. What exactly is going on? I'm familiar with static electricity, and the triboelectric effect (creating a ...