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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166 views

How does back-emf oppose the input-current?

A circuit, has current $A$ flowing at a certain $V$. When there is a change in magnetic-flux, based on Faraday's law of induction & Lenz's law, we know that there is change in Potential ...
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0answers
15 views

Field emission beyond Fowler-Nordheim

The Fowler-Nordheim equation describes the emission current accurately in a specific temperature and electric field range [1] and the maximum valid field is about 11 V/nm. Is there a way to find ...
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1answer
60 views

On the power in AC circuits

Say I have an AC circuit with rms voltage $\mathcal{E}_{rms}$, reactance $X$ and resistance $R$. We may say that the average power dissipated in the circuit is as follows: $$P_{avg} = ...
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1answer
145 views

Repulsion and attraction of electric currents

Now, I understand that when a an electron travels, it creates a magnetic field. If you put two wires with current traveling in the same direction they repel, and current traveling in opposite ...
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4answers
1k views

How does the speed of electrons change around a circuit?

I have been thinking about ways of teaching electronics and I'm wondering if the following is true... For starters, when we talk about voltage as energy per unit charge, is this energy manifest ...
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2answers
321 views

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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0answers
32 views

ODE envolving current [closed]

I have to do this exercise: 9. The current $I(t)$ at time $t$ flowing in an electric circuit obeys the differential equation $$I''(t) + RI'(t) + I(t) = \sin \omega t$$ where $R$ and ...
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2answers
92 views

Current in a strip - Scalar or vector [duplicate]

In certain books like David Griffiths electrodynamics , he treats current as a vector in some places. Moreover certain problems like current in a strip are often dealt with by taking current ...
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2answers
1k views

Does the photoelectric effect obey Ohm's law?

So, I've been reading about the photoelectric effect for my modern physics class, and I was confused about how Ohm's law works in relation to it. Say we have a photoelectric apparatus that simply ...
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0answers
25 views

Rapids in a River

Can I expect to have bigger or smaller rapids in a river that has a water level that is higher than average? I am not sure which way this would go. With the lower water, one is closer to the rocks, ...
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1answer
69 views

What happens when a battery is in space? [closed]

What would happen If you take a battery in a space ? Would the current flow between terminals being it in vacuum of space?
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1answer
36 views

Cooling induction current generation

We know that conducting materials can be heated by electromagnetic induction. Is it possible to generate current using a cooling process?
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2answers
535 views

How does AC current flow in an open circuit?

In common house hold wiring we have the hot lead, neutral and ground. If the hot lead in electrical wiring contacts earth ground (perhaps though a short circuit in the chassis of a device) then the ...
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3answers
88 views

Alternating Current Inquires [duplicate]

I have been looking in to alternating current and I am confused. If the voltage reverses doesn't the flow of electrons also reverse? I am aware of another fair answer on this site here. That answer, ...
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1answer
49 views

Is the field generated by an electromagnet always proportional to its current?

Imagine that I use a long wire to create an electromagnet. Let's also assume that the current flowing along the wire is constant, and that the wire is winded on the vacuumm. Is the magnetic field ...
2
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4answers
365 views

Why doesn't current flow when the wire is open?

I realize this may be a hard to answer question but we are currently studying current in our school. One thing that struck me was why the heck doesn't it flow when the wire's closed? When you connect ...
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1answer
430 views

The Voltage and Current of a Discharged Inductor at the Instant the Switch is Closed

The figure shows an LR circuit with a switch and a 240-volt battery. At the instant the switch is closed the current in the circuit and the potential difference between points a and b, Vab, are ...
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0answers
43 views

To find the center of charge in current waveform

I have a circuit for which output current waveform has both positive and negative points. Now I want to get a center of the charge for this waveform (means where my net charge is concentrated ?). Is ...
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1answer
127 views

What is the physical importance of current density?

Why we have used the term current density, Is there any physical significance of it or does it affect anything apart from the drift velocity in equations only. Does it have a major significance like ...
2
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1answer
319 views

Why does a LED light up in reverse direction at a DC generator worked by a falling object

I have done the following experiment: A DC generator is worked by an object falling and connected to the DC generator via string. The DC generator is coupled to a LED in reverse direction. The ...
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1answer
143 views

Why is $I_0\sin\left(\dfrac{\pi}{2}-\omega t\right)$ not used as a solution of the purely capacitive AC circuit?

Why is $I_0\sin\left(\dfrac{\pi}{2}-\omega t\right)$ not used as a solution of the purely capacitive circuit? My Research I've searched on Google, Phys.SE and many school level Physics books ...
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1answer
34 views

Charge density of turning object

A charged disk or sphere will create currents around its turning axis if a rotation is added. The total current can then be calculated by adding all concentric currents together. Every current (I take ...
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2answers
245 views

Does a simple copper wire with an AC current create EM waves?

I want to know if a household AC current flowing through a simple copper wire will radiate EM waves? If yes, up to what range and can they be called radio waves?
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1answer
68 views

Why is the movement of electrons random in electric field?

Suppose an uncharged conductor is present isolated and there is no net electric field applied. Then by assumption we have the motion of electrons random,i.e. isotropic. But now suppose there is a ...
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0answers
97 views

Magnetic flux density a small distance off axis from a current loop

I'm currently studying physics at University and struggling with the problem mentioned, it's on a past paper I'm trying to do. I have calculated the B field on axis as (sorry don't know how to format ...
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3answers
192 views

Intuition/derivation behind the probability current definition

The definition is: $${\bf{j}} = \frac{\hbar}{2mi} (\psi^* \nabla \psi - \psi \nabla \psi^*)$$ However: Where ever I have looked, the above "pops out of nowhere". I was wondering how can I obtain ...
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1answer
133 views

Force between two current carrying parallel wires, multiple proof? [closed]

Having two current carrying (currents $I'$ and $I$) wires of length $a$ parallel to the $z$-axis, one with end points $(0,0,0)$ and $(0,0,a)$ and one from $(a,0,0)$ to $(a,0,a)$, I'm looking for the ...
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2answers
54 views

Current and magnetism [closed]

When the current in 2 parallel wires flows in same direction they attract each other.Shouldn't the electrons travelling in opposite direction should also repel ?
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1answer
110 views

What exactly is three phase power? [closed]

What is three phase power? How does it differ from normal AC current?
2
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1answer
104 views

Current, Current density

edit: Hi I'm trying to find the magnetic field generated by a time dependent oscillating current in the quasistatic case ($|z|,r <<c\omega$) where r is the perpendicular distance from the ...
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2answers
359 views

time dependent current/ magnetic field

Is there a general way to calculate the magnetic field for a time dependent current of a long thing wire? For ex: If the current is $$ I(t)=I\sin wt, $$ is there a general method to use in order to ...
2
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0answers
164 views

The anapole moment, derivation from Dirac current density

Basically I am looking for a way to expand the electromagnetic interaction energy $W = A_{\mu}j^{\mu}$ (both $A$ and $j$ obtained from the Dirac equation) similar to the classical expansion in ...
0
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1answer
153 views

Maximum magnetic strength achievable

What will be the maximum magnetic strength to which a piece of ferromagnetic substance can be magnetised? What will the maximum strength possible for a piece of iron? How would the shape of the ...
0
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3answers
287 views

Why is emf equal to PD when circuit is “open”?

I understand that the set up of a battery consists of very good, but not ideal conductor, and therefore, some internal resistance exists. Also, I get that emf is the PD that would exist if the ...
4
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1answer
70 views

Why is Kirchoff's Current Rule valid?

The fact that Kirchoff's Current Rule is valid means that whatever current flows in flows out. But this will only be valid for a steady current circuit, that is, when there is no accumulation of ...
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2answers
400 views

Why does the coil in this apparatus reverse its direction of oscillation?

I've been given some notes and I have to 'unscramble' them and put them in order. They are supposed to describe what happens in the diagram below: The notes to unscramble and form a proper answer ...
3
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1answer
330 views

If I charge a battery using a much higher current, can it explode?

If I have a 12V 4Ah lead acid battery and use a battery charger that, let's say for example, can charge 10A, 50A, or 100A. If I theoretically turned it to 100A will the battery explode? I understand ...
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2answers
352 views

Potentiometer at null pointer!

Consider the situation when a cell of an unknown emf is being measured using a potentiometer. We slide the jockey so as to obtain the null point. Now, is there any current in the potentiometer wire at ...
4
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1answer
492 views

Child-Langmuir Space Charge Law for Non-Zero Cathode Potential (Non-Zero Initial Electron Velocity)

I'm trying to reconcile some conflicting results that I've found in publications that address the idea of the current in a vacuum diode in the case where the cathode has a non-zero potential, in other ...
2
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4answers
135 views

Faraday's Law - recursive?

So we know that the EMF is induced by change of flux. The thing that was always confusing me is the following: we start changing the magnetic field which in turn induces electric field which makes ...
2
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3answers
105 views

Resistor and LED - together and separated

Something very basic which I don't get no matter how much I read. circuit A: only a resistor and power supply - the resistor will burn. circuit B: only a LED and power supply - the LED will damaged. ...
0
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1answer
2k views

How to calibrate an Ammeter?

Calibrating an ammeter is tricky thing to do. The obvious way would be connecting it to another good ammeter and see if they get the same result. However, people have been telling you can use a ...
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1answer
1k views

How can rheostat change the current without changing int voltage

NCERT science of 10th, in chapter electricity on page 205, there, a is line written: a component use to regulate the current without changing the voltage ...
0
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1answer
143 views

Finding current in a sphere while given a changing current density

im trying to calculate the current running through a sphere while given a current density $$J=Jcos(2\theta)e^{-t/r} \hat r $$ J is given in spherical coordinates. I know that: $$I=\int J\,da $$ But I ...
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2answers
62 views

Back $emf$ and power?

The formula relating current to back emf is $V-E=IR$ where $V$ is the source pd and $E$ is the back Emf. Thus it can be seen that as the back emf increases the current decreases. But what is the ...
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2answers
754 views

A confusion in ohms law $V = IR$

ohms law clearly states that potential difference is directly proportional to the current flowing through the wire or V directly proportional to I ______(i) so the formula which is ...
4
votes
2answers
168 views

Why can't an excess of electrons or holes by themselves cause current flow?

I am a beginner in electrical engineering. Often times (most cases actually), the underlying physics aren't really explained to us and we are just left to assume that it works "because it works." This ...
1
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1answer
25 views

Relative perimittivity of conductor?

Why is the relative permittivity $\epsilon_r$ infinite for a conductor? Where does it come from mathematically and does it make physical sense?
4
votes
1answer
216 views

DC current in ideal conductor and skin effect?

I know that the skin depth derived for AC current goes as $$\delta \propto \sqrt{\frac{1}{\omega \sigma}}$$ where $\omega$ is the angular frequency of the field and $\sigma$ is the conductivty. Now: ...
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2answers
239 views

Faraday's law and “infinite induction”

I may have confused after thinking too much about Faraday's law. If an emf is induced in a circuit due to some changing magnetic field, the induced current will be in a direction such that the ...