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Questions tagged [electric-circuits]

An electronic system, with closed loop current flow, and relative electrical potentials present across electrical components.

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calculate the scattrering matrix of a 3-port lumped element network

I can get S11 and S12 by terminating port 2 and 3 with Z0 and Zr respectively. But when I try to calculate S13 it seems less straight forward. The following is the network, taken from Gao, J. The ...
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1answer
19 views

RC circuits - what can we interpret when the voltage across the capacitor is equal to zero?

When applying the law of addition of voltage in RC series circuit with alternating sinusoidal current, we notice that voltage across the generator is equal to the sum of the voltages across all other ...
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2answers
122 views

How far do electrons actually move along a conductor under an alternating current?

This is more or less a curiosity question. But I have had really good luck with stack exchange so far. If I can expand on my question a little bit - it may not be super important, but I know under say ...
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Why does the computer start from sleep mode when I discharge static electricity about 1 metre away?

The situation When my colleague leaves work for the day he puts his computer in sleep mode. In the mornings I usually arrive to work before him, his computer still being in sleep mode. The floor is ...
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0answers
16 views

Resistance between any two point of an undirected graph [on hold]

Here is the circuit with 11 resistors and each named by its conductance ($G_{1}=\frac{1}{R_{1}}$). Assume the potential of each node ($U_{1}, U_{2},...U_{7}$) and say the current pass each resistor is ...
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38 views

Electrical curcuit problem, calculating current and voltage over resistor [on hold]

Can you help me calculate current over the 20 ohm resistor and voltage over the 30 ohm resistor. Answers are supposed to be 1.8A and 36V but i cant figure out how to get to that answer.
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2answers
58 views

Current without EMF in AC capacitance circuit

In AC circuit with only capacitance, current in circuit is maximum at time t=0 but emf of source is E=0 at t=0 , how can this happen, How can current flows without emf.
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2answers
175 views

Does direction of current in Node Voltage method circuit analysis matter?

I am trying to solve the following circuit using the node voltage method, but I'm having issues with figuring out how current is supposed to flow in and out of nodes. I understand that the current ...
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4answers
110 views

Basics of electric current

I am a mathematician who never took a physics class besides classical mechanics. Recently I have been looking at biophysics for a small project and I have a hard time understanding the basics of ...
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1answer
23 views

How are free electrons produced in a gas?

I'm aware about ionisation but even to ionise a gas you need an electron. So how in a discharge tube, which has a low pressure gas and a high voltage, does current flow? How is the first electron ...
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2answers
182 views

How to vary the current using batteries?

I am making a door bell as a school assignment. It works by having a solenoid produce a magnetic field which attracts a pice of iron attached on a conductor. when the iron is attracted towards the ...
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1answer
180 views

Capacitance dependence on separation between plates

I know that the capacitance of a parallel plate capacitor is $C = \frac{\varepsilon S}{d}$. I am supposed to perform a linear regression to obtain $\varepsilon$, however it turns to be a quadratic ...
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1answer
34 views

Brightness of lamps

The brightness of the lamp (power) depends on two factors: The voltage and the current which flow through it. In Figure(A) as the variable reaistance increases its reaistance, the current which flow ...
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1answer
509 views

Condition for the use of perpendicular symmetry in solving resistance problems

It is possible to use parallel symmetry even when there is more than one battery, but what about perpendicular symmetry? I tried to solve some examples using KVL but it didn't work out as I expected. ...
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1answer
37 views

Self-induction and conservation of energy [closed]

back EMF when switch is open is greater than back EMF when it is on; is that opposes the law of conservation of energy?
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5answers
442 views

Isn't the inductor equation negative?

The inductor "resists" change in current. So say you measure the voltage across the inductor from point A to point B - the current is flowing in from A towards B. Now say the current is increasing. ...
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1answer
27 views

Potential divider

I am just wondering if a potential divider circuit is used to vary the pd across the other resistor from zero to maximum, why can't the variation be done straight from the source?
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4answers
76 views

How does an ideal transformer satisfy law of conservation?

I have read in books and articles that, keeping the power as is transformers increase the voltage and therefore reduce the current as per the formula $$ P = IV $$ and this results to reduction in ...
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1answer
43 views

Does the cold affect electrical conduction?

Disclaimer: I'm not experienced in many phyiscal nor electrical terms and I have no idea where to ask this question elsewhere. I'd be open for a move request if needed. Today when using public ...
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0answers
21 views

Need help in circuits [closed]

So I m not a undergrad student yet and I have some difficulties while measuring the equivalent resistance of a circuit. Specially from those complicated diagrams and so on. So I deadly need advise in ...
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8answers
81k views

Difference between live and neutral wires

In domestic electrical circuits, there are 3 wires - live, earth and neutral. What is the difference between the live and neutral wires? As there is AC supply, it means that there are no fixed ...
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1answer
34 views

What is difference between current and voltage? [closed]

If current passes through the bulb not using it up then how battery becomes weak after sometime?
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1answer
25 views

AC circuit with resistor

As you can see from the image that "potential drop must be equal to the applied emf". This means after that point there is no flow of current as potential difference is zero.But how is this possible?...
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0answers
20 views

Kirchoff's rule DC circuits [closed]

I understand how to solve for the unknown variables but I have trouble labeling the currents and finding the current and junction equations
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4answers
211 views

Connecting a charged capacitor to an uncharged capacitor

I was attending a lecture about capacitors and something confused me. If I charge a capacitor using a DC supply, the capacitor will gain charge $Q_0$. Now, if I discharged it along an uncharged ...
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0answers
82 views

Could the tether of a space elevator be used as a single-wire transmission line?

If you're unfamiliar with single-wire transmission lines: https://en.wikipedia.org/wiki/Single-wire_transmission_line It seems that a material with high conductivity (eg: certain configurations of ...
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0answers
10 views

Finding resistance and how many appliances can be connected

A household electricity system can be modelled as a fuse in series with appliances, all of which are in parallel across the 240V mains supply. In one household there is a 16A fuse in series with the ...
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1answer
213 views

Resistance and permittivity

I saw this formula in a book RC = ρε Can anyone tell me what is the significance of this formula or how is it derived? The capacitance and the resistivity seem to be pretty much unrelated quantities ...
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6answers
49 views

Why doesn’t current decrease when it passes through a resistor?

As far as I know current is the amount of charge passing per unit time. $$I=q/t$$ When current passes through a resistor, the resistor resists the flow of current so the amount of charge going to ...
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2answers
33 views

Hot Wire vs neutral wire [duplicate]

How can a wire be called the "hot" wire and another the "neutral" wire when ac current reverses itself 60x/sec? For first half of a given cycle it flows from the "hot" wire to the load and back out to ...
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1answer
26 views

Why is the open circuit voltage equal to Thevenin's Equivalent Voltage?

When we are taught Thevenin's Theorem, we are taught that the equivalent voltage Vth is the voltage obtained at terminals A-B of the network with terminals A-B open circuited. But why is it true?
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2answers
192 views

Are these capacitors parallel or in series?

I am a bit confused. The authors say these conductors are parallel when the switch is on. But to me it looks like they are in series since the handle is insulating. How can you explain that they are ...
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2answers
52 views

How is a capacitor implemented in Kirchoff's Loop Rule?

I'm real stumped here. I may be missing something, but I'm genuinely perplexed as to how one would add a capacitor to the list of elements being added to an equation through Kirchoff's Loop Rule (KVL)...
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1answer
31 views

How to measure an electron plasma with an electrically biased probe?

I want a crude way to confirm the existence of an electron cloud in a moderate vacuum of 2 mTorr. In my set up, filaments emit electrons into a magnetic trap which concentrates them (similar to an ...
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2answers
730 views

Equivalent Resistance between 2 points

Actually , I'm having trouble in solving for the net resistance of the below network of resistors. Can anyone help me finding the equivalent resistance between points a & b. My answer is $2/3r$ ....
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2answers
56 views

Charging and discharging a capacitor [closed]

So from what I understand, in the case for the charging capacitor, I understand the voltage across the capacitor as well as the charge can be given as the function of time: $$V(t)=\epsilon(1-e^{-t/RC}...
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1answer
196 views

Why can one assume in “infinite grid of resistors” that in the center there are diagonal nodes that have $-1$ and $+1$ currents?

Why can one assume in "infinite grid of resistors" that in the center there are diagonal nodes that have $-1$ and $+1$ currents? As in the matrix $P$ in the following picture:
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3answers
44 views

Is natural frequency of an LC circuit equal to angular frequency ? Why don't the units match?

When I was reading the L-C circuit in my textbook I came across the derivation of equations of instantaneous charge and current. Which is no problem, but when I got to the derivation to current . ...
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2answers
52 views

RC circuit confusion [closed]

I was doing some RC problems in A levels, and in ine of thr problems it was about a circuit attached as a picture: My question is can someone clearly explain to me why when time goes to infinity , ...
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1answer
339 views

Energy dissipation in current flow

In section 4.8, Energy dissipation in current flow, of Purcell and Morin's Electricity in Magnetism, the expression for the power expended by a resistor is derived. The sections includes the following ...
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5answers
202 views

Why isn't there a minus sign in Ohm's law, $V = IR$?

Suppose current runs through a resistor from left to right, and we define the left-to-right direction as positive. Then from left to right, the potential decreases. So $V,$ the voltage across the ...
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3answers
76 views

Electrons and wires [duplicate]

The electrons that carry the electric energy are in wires or in the battery ? battery provides a potential difference but from where electrons flow to make for example a bulb light. If they flow from ...
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5answers
26k views

Detecting if resistances are parallel or series in complex circuits

I know how to detect when resistors are arranged in parallel or series arrangement and I can also find their equivalent resistance in simple circuits or when resistances are connected in form of ...
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0answers
32 views

Why are(n’t) rechargeable batteries damaged by partial charging?

Originally asked in ElectricalEngineering.SE, but I was told Physics.SE might be a better place to ask. Over the years, I've come across websites and people with different opinions on the "correct" ...
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2answers
212 views

Is there an RMS value for power delivered to an inductor?

My textbook defines power delivered to an inductor as: $$P= V_{L\rm\ peak}I_{\rm peak} \cos ( \omega t) \sin( \omega t)$$ where $\omega$ is angular frequency. but makes no mention of $P_{RMS}$. It ...
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1answer
368 views

Why does this derivation/proof of KVL from KCL and Tellegens theorem hold?

I have come across a strange derivation of Kirchhoff's voltage law. You assume that Tellegen's theorem and Kirchhoff's current law both hold. (Bold letters signify a vector.) The proof: Let L be an ...
3
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2answers
949 views

Close or open circuit? [closed]

A electric source provides a non electrostatic influence on the charges inside the source which pushes the positive charges from the negative terminal to the positive one. Does this happen when the ...
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3answers
127 views

Current in infinitely long wire? [closed]

Theoretically, suppose an infinite-length wire with completely no resistance (0Ω) connected to the terminals of a battery, and an electric lamp placed in the "center" of said wire. The question is: ...