A measure of the rate at which electric charge is transported (especially through a circuit), it has units of charge/time.

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28 views

Current and current density

Current, $I$, is generalised as: $I=\iint_{A}^{} \vec{J}. d\vec{A}$. I know that current density always points in the direction of flow of positive charge. I wonder if the infinitesimal element, ...
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4answers
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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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3answers
31 views

What does it mean when we say that power of a bulb is 10 W? Since $V/I=$ resistance is a constant, how can power $=VI$ be a constant?

My question is simple. In Ideal situation, at constant temperature, we know that normal appliances like a filament bulb has straight Voltage vs Current graph, meaning its resistance is constant or ...
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0answers
24 views

What is the physical meaning of $\overline{\Psi} \Psi$ in the Dirac current's “Gordon Decomposition”?

When writing the Dirac (charge) current out in a way that resembles the (charge) current in the Pauli/Schrödinger theories, one obtains the following: $ j^\mu = -\frac{\mathrm{i} e\hbar}{2m} \left[ ...
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4answers
173 views

Ohm's Law Question?

Since $V=IR$, if you were to have say a $5\:\mathrm{V}$ source connected directly to ground, no components in the circuit at all, you would need to add resistance for the current to flow? Or is there ...
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46 views

Is it low current or low voltage that dims a bulb?

At times we have dim bulbs due to some fault from national grid, what I want to know is if this fault is low current or low voltage. Other times electrical appliances blow-up, is that high current or ...
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1answer
33 views

current flow between two points

I can measure a voltage that is induced on my skin from a 50hz electromagnetic field (emf) source using a multimeter, how can I measure the induced current flowing between my finger and ground when I ...
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1answer
34 views

Why does 50/60hz waveform sometimes appear to change direction when imaged on an oscilloscope?

http://www.shutterstock.com/video/clip-32898-stock-footage-an-old-oscilloscope-demonstrates-a-waveform.html The video above displays a waveform moving from left to right. This is generally how my ...
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1answer
109 views

The graph of Alternating current

I am beginner Physics student and currently studying Alternating current. One thing that confuses me about this topic is the graph Voltage/Time. Now I know it is a sinusoid, but it is the ...
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1answer
35 views

Is the magnetic field calculated by Ampere's Law only because of the currents crossing the loop? [duplicate]

In Ampere's Law, $$\oint \vec B \cdot \mathrm d \vec l = \mu I$$ the current outside the curve is not included on the right hand side of the equation.* Does it mean that the magnetic ...
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2answers
112 views

How to use Ampere's Law for a semi-infinite wire with current?

Suppose that there is a semi-infinite wire which extends to infinity only in one direction. There are no other circuit elements at the other end(finite end) of the wire and the current does not loop. ...
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1answer
35 views

Is there an electric field in the direction of a uniform current?

In an infinite plane where uniform current is passing through,is there any electric field? Because i know that charge produces an electric field but in a uniform current and because it is an infinite ...
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0answers
13 views

What is the effect of a magnetic field on a plasma speaker?

I have been doing some research for the past couple years investigating the effect of a magnetic field on a plasma speaker. More specifically, if you apply a relatively uniform magnetic field in no ...
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0answers
37 views

How to apply voltage source in FEM when solve Maxwell equation?

I need to solve the Maxwell equation of electric field by finite element method. In this function, the right hand side is the current density. However, in my problem, the voltage source with 1 MHz ...
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2answers
103 views

Ampere's law of circular path when “bulging” out

The picture shows a use of Ampere's law. A circular path is chosen. $$\oint \vec B \bullet \mathrm{d}\vec l=\mu_0 I_{encl}$$ When using Ampere's law we are talking about the current enclosed. That ...
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1answer
66 views

Filament lamp: Negative part of I/V characteristic?

I don't understand the negative part of the graph. It shows that the resistance is decreasing as the voltage goes from negative towards 0. What does a negative voltage mean and why does the graph ...
2
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2answers
42 views

How is resistance defined when there is no current?

By definition, a component has a resistance of $1\Omega$ if a potential difference of $1$V makes a current of $1$A flow through it. So I was a bit confused when I saw somewhere "a filament lamp has a ...
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1answer
19 views

Inductor in transformer

In an inductor, a current waveform 90 degrees out of phase with the voltage waveform creates a condition where power is alternately absorbed and returned to the circuit by the inductor. If the ...
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0answers
37 views

Is shaking a phone enough energy to charge a smartphone?

Is shaking a phone enough energy to charge a smartphone? For example I'd like to use some kind of power generator to charge smartphone's battery but I'm wondering if shaking (by hand, or motion when ...
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1answer
28 views

How is the efficiency of an electric motor affected? [closed]

Consider that I'm having a motor (AC/DC) and the power switch is on and it's being supplied power and of course the motor is rotating. Now, I continuously start switching off and on the power. For 1 ...
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2answers
52 views

What is the rigorous definition for current in a closed circuit?

I know that $I = \frac {dq} {dt}$, but I have trouble reconciling that derivative definition with my intuitive understanding of current. At an arbitrary point in wire, the current is obviously the ...
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1answer
108 views

does a resistor slow down the flow of electrons or just let less electrons through? or is it both?

I like to have an intuitive grasp of whats happening our tutor said they 'resist' the flow of electrons but I want to understand exactly whats happening.
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1answer
45 views

Can Ampere's Circuital law be used on an infinite number of alternating Helmholtz coils?

I have the following surface current density $$ \bar{\sigma}_s = \hat{\phi} \sin(kz) |\bar{\sigma}_s| $$ to approximate an infinite number of alternating Helmholtz coils stacked along the z-axis with ...
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1answer
114 views

Why is the quench temperature fluctuation not a concern for maglev trains?

I am having trouble discovering why temperature fluctuations and magnetic field fluctuations are not a stability concern for superconducting magnetic levitation trains. I am under the impression that ...
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124 views

Earth magnetic field [closed]

I gather that, in these days, the most common explanation for the Earth magnetic field existence and strength is the theory of convetive currents of molten iron and nickel atoms in the outer core of ...
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1answer
40 views

What would you do to measure wind speed below 50m?

Is there a radiation spectrum that could be used near the surface of the earth to fluoresce air enough to see the currents? Is there a complimentary CCD that can image that spectrum? I am looking for ...
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301 views

What would be the resistance of a wire, if it is stretched to double length? [closed]

If there is a wire of resistance $R$ and we stretch it such that it becomes 2 times longer, then what should be the new resistance of the wire?
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2answers
114 views

Does a battery in a vacuum produce a current?

I've always assumed that the strong resistance of air is the reason there is no flow of electrons between the terminals of a battery until a wire is connect. However, in a vacuum there is no resistive ...
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3answers
6k views

Why the bulb glows brighter?

If the total current is divided into the branches in a parallel configuration and it is constant in series. How come the bulbs glow brighter when connected in parallel than when connected in series?
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36 views

Current in discharging capacitor through fixed resistor?

In the textbook I'm using for physics it says that the charge left on the plates of a capacitor after time $t$, that is discharging through a fixed resistor, is $Q=Q_0e^{-t/\tau}$ where $\tau=RC$ is ...
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2answers
516 views

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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4answers
6k views

North and south of magnetic field

The current I is flowing upward in the wire in this figure. The direction of the magnetic filed due to the current can be determined by the right hand rule. Can we determine the north and the south ...
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360 views

How are band gap energy, dielectric constant (permittivity) and resistance related to each other?

The following three properties are related to current flow: Band gap energy Dielectric constant Resistance I would expect them all to have the same trend (i.e. higher band gap energy would cause ...
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31 views

Do these symmetrical looking B field problems have the obvious answer?

I am struggling with two questions whose answers both appear to be trivially zero by symmetry arguments, when I was expecting more derivation to be required. Problem A), An infinitely long wire ...
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1answer
41 views

Interacting magnetic fields

Is there a reason why two magnetic fields perpendicular to each other do not interact? If they are parallel or at a non-90 degree angle they interact. Is it because magnetic field lines can be viwed ...
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0answers
25 views

Non-abelian current transformation

This is my firs post ever here and I just registered on this site but I want to say that this site has helped me a lot and you guys are great! On to the question: I have the equation of motion that ...
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2answers
70 views

Does sending current upwards reduce the voltage?

I was thinking about relativistic situation of having a vertical wire on a neutron star or high-gravity environment and then say if I send current upwards (away from the ground) will the voltage be ...
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1answer
49 views

Cryptic remark in physics revision guide

I am currently revising for my AP physics and I couldn't understand one of the end-of-section summary notes. It says: "Remember that the direction of magnetic field is from North to South, and that ...
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1answer
58 views

Graph of energy stored in capacitor?

Question: In an experiment a capacitor is discharged through a constant current. Draw a graph of how the energy stored in the capacitor varies with time. The answer given is: But I seemed to get a ...
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1answer
17 views

Current function of voltage, Taylor series

Why can the current as a function of voltage for any device be written as a taylor series? Is it because voltage can be written as V0 Cos wt and then I = V/R ? I'm confused on this.
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2answers
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An Ideal Transformer

In a transformer assumed to be transformer, power in the primary is equal to power in the secondary. So in a sense, the power in the secondary is 'fixed'. Output voltage in the secondary is also fixed ...
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1answer
55 views

Question on an unbalanced Wheatstone bridge

Let, P/Q = R/S in the usual sense of a Wheatstone bridge where PQRS are resistances. Then we know that the potential between midpoints of PQ and RS are equal and there's no potential difference ...
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1answer
180 views

Using a circuit to make a magnetic balance to weigh objects

I understand that this is a homework question, but I am learning about magnetic fields and things like that and this certainly wasn't covered in the material, so my question is more about the actual ...
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1answer
55 views

What is the amount of charge density in a metal wire for the electron's which are involved in current flow?

According to the question Why is charge = nALe n is the charge density (number of electrons per unit volume). Which of the electrons in a metal wire are involved in current flow? Only the unbounded to ...
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3answers
396 views

Why does current density have a direction and not current?

As I understand it the definitions are as follows: Current is a scalar $I$ with units of $\mathrm{[J/s]}$. It is defined as $I=\frac{\mathrm{d}Q}{\mathrm{d}t}$. Current density is a vector $\vec{J}$ ...
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1answer
46 views

At what distance is lightning dangerous for someone lying down?

My 8 yo child told me that they learned at school that they should lay down flat on the ground in case of lightning. I told him that the more correct position is crouching down with feet together, but ...
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1answer
55 views

How do the electrons know the route?

I've learnt in classroom that the requirements for electrical current to flow in a circuit are as follows: there must be a closed conducting path between negative and positive terminals there must ...
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1answer
21 views

When I put 2 electrodes into a electrical conductive solution, does the volume of the solution around the electrodes have biological effect?

Media these days give me a common sense that it is able to hurt someone if they step on a wet floor with a electric cord dipped in, several movies have this scene.However, I wonder if I set up an ...
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14 views

How much charge per unit time passes with the Josephson current

I wonder how does the simple defintion of a current $I = \frac{dQ}{dt}$, i.e. charge per unit time which passes through some area, is applied to the Josephson effect. If I have a Josephson junction ...
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2answers
71 views

What is the voltage of an average carpet static shock? Can you make it lethal?

I think I heard somewhere that it was in the thousands of volts, but it had extremely, extremely low amps. Could you somehow transform the current to make it larger or something? Or does the equation ...