New answers tagged

1 vote

Don’t understand how nonlinear resistors violate ohm’s law

It seems like this is more of a lack of understanding of the mathematical term "directly proportional", rather than a physics question. Two variables are directly proportional if there is ...
0 votes

What is the term "potential difference of voltage" describing exactly?

Near earth gravitational potential is g*h, Potential difference is g(h2-h1) so you can compare Voltage to the difference of height (times g)
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15 votes

Don’t understand how nonlinear resistors violate ohm’s law

What one refers to as "Ohm's law" is a linear relationship between the current and the potential difference, i.e., (see also this answer) $$ V = IR\text{ where } R=const $$ If $R$ is not a ...
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3 votes

Don’t understand how nonlinear resistors violate ohm’s law

Much has been written about Ohm's law and I would suggest you first forget about it and look at the definition of $\text {resistance},\,\Omega= \dfrac{\text{potential difference},\, V}{\rm current,\,A}...
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8 votes

Don’t understand how nonlinear resistors violate ohm’s law

In the words of Captain Barbossa, "they're more like guidelines." Ohm observed that for many elements (things we call "resistive"), there was a linear relationship between voltage ...
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0 votes

Why does this circuit give a quadratic equation for current?

There is an insightful graphical way to see why you get two answers. Let's assume that 60W is the power dissipated by your lamp in this particular circuit (in reality it would be the power dissipated ...
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2 votes

Why does this circuit give a quadratic equation for current?

Here's how I would approach this. First we work out the current through the lamp in the absence of resistive wires. As $I=P/V$, we have $I=60/120 = 0.5A$ As the lamp draws $0.5A$, its resistance is $...
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1 vote

In which cases do KCL and KVL fail to apply in circuits?

I'll focus on KVL. It makes sense to distinguish between "KVL is true" and "KVL is applicable". KVL (shorthand for Kirchhoff's voltage law) is a useful engineering rule inspired by ...
1 vote

In which cases do KCL and KVL fail to apply in circuits?

Item #2 in @Dale's answer is specialized to two-terminal circuit elements: it excludes transformers, for example. To include those you must allow four-terminal elements (2-ports) to represent magnetic ...
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4 votes

In which cases do KCL and KVL fail to apply in circuits?

The assumptions of circuit theory are as follows (see Nilsson and Riedel, Electric Circuits, ch 1): there is no net charge on any circuit element there is no magnetic coupling between circuit ...
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1 vote
Accepted

Why does this circuit give a quadratic equation for current?

The reason is that the power of the bulb is given and no further information, eg working voltage, working current and working resistance, and so the way the problem is set up only current $\times$ ...
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0 votes

Force on a test charge inside a cell

The potential difference caused by the open state electrodes is cancelled by the ion charge net offset, so the macroscale potential gradient is zero. Otherwise, there would be ion electromigration ...
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1 vote
Accepted

Energy transfer in resistors

In a heuristic Drude-like picture, an electron in a circuit moves from the low potential side of the battery through the circuit to the high potential side of the battery, so when it "moves ...
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3 votes
Accepted

Derving circuit equations for $RLC$ circuit such that order does not matter

To solve a series circuit, you solve a system of simultaneous equations: one saying that the sum of the voltage drops across all components is zero, and, for each component, an equation relating the ...
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0 votes

What is electric potential really a measure of?

Electric potential is defined by the amount of work energy required to move a unit of electric charge from a reference point to a specific point in an electric field.
0 votes
Accepted

Electric potential across a resistor

why does the electric potential decrease across the resistor? The short answer is because electrical potential energy is lost as heat dissipated in the resistor. That, in turn, is due to the work per ...
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1 vote

Proving induced voltages will always cancel in circuit that is in a $B$-field

The induced voltage will definitely not cancel in general. That is the entire point of magnetic induction. Unless you specify that the field is constant and the wires don't move, it is actually quite ...
0 votes

Proving induced voltages will always cancel in circuit that is in a $B$-field

The induce voltage is calculated by the change of magnetic flux, if neither the area nor B changes you have no induced voltage
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1 vote

Inductor connected to an AC source

To formalize all the previous answers, model the switch by a step function: $$ H(t) =\begin{cases}0 & t< 0\\1 & t \geq 0\end{cases} $$ And use it to limit the power source to positive t. ...
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0 votes
Accepted

What is electric potential really a measure of?

I like to think of electric potential as "pressure". By Ohm's law, $$I = V/R$$ Here, $V$ is the potenial difference. The current carrying wire lets in more current, if the potential ...
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0 votes

What is electric potential really a measure of?

It is a scalar function whose gradient gives the force a charge would experience were it placed at a point. It is a function which measures the change in energy if a charge were to be brought from ...
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0 votes

What is electric potential really a measure of?

Electric potential is the ability of a system to perform work on a charge. There do not have to be other charges in the system itself.
1 vote
Accepted

Inductor connected to an AC source

Both your equations are correct as they are for different situations with the equation the general one $i=\frac{V_0}{L\omega }(1-\cos\omega t) = \frac{V_0}{L\omega } - \frac{V_0}{L\omega }\cos\omega t$...
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2 votes

Inductor connected to an AC source

it is printed in my textbook that the value of $c$ equals zero, This is incorrect. It should say that $c$ is the value that makes the current equal to zero at $t=0$. $$i(t=0) =-\frac{V_0}{L\omega }\...
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1 vote

Inductor connected to an AC source

The typical textbook treatment of an inductor to which $V=V_0 \sin\ \omega t$ is applied 'shows' the current to be $I=I_0 \frac 1{L\omega} \sin\ (\omega t-\frac{\pi}2)=-I_0\frac 1{L\omega} \cos \omega ...
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0 votes

What is capacitance, in general?

In a general physical system we generally have through variables and across variables. We also have flow and effort variables. See Vibration and Shock Handbook - Silva. In an electrical network ...
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2 votes
Accepted

Does EMF depend on distance between electrodes in a battery?

"E.m.f of a cell is defined as work done to carry a unit charge from cathode to anode in internal circuit." You need to insert the words "at very low speed of charge-carriers", in ...
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1 vote

Does EMF depend on distance between electrodes in a battery?

I suspect you're confusing EMF (Electromotive force) with an actual force. It is not a force in the newtonian sense, so if you presume that there's some "$\mathcal{E}\cdot\vec{d}$" term that ...
0 votes

Kirchhoff's voltage law, signs?

Problem here is with sign convention in closed loop. We know that when current pass through resistor or any passive component, there is energy loss or voltage drop. Now this voltage drop, in case of ...
2 votes

Kirchhoff's voltage law, signs?

According to the diagram the potential $v_1$ is greater than the potential $v_2$ by 0.5 $v_x$. So $$v_1-v_2=0.5\ v_x\ \ \ \ \ \text{that is}\ \ \ \ \ -v_1+v_2+0.5v_x=0$$ We have not appealed to ...
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2 votes
Accepted

Kirchhoff's voltage law, signs?

Can someone explain what convention has been used to determine the signs in the equation? Sure. First note that the equation is for the loop clockwise (the signs would be reversed if counter-...
2 votes

Kirchhoff's voltage law, signs?

Kirchoff's Voltage law deals with the sum of voltages across a closed loop, and the region between the two nodes 1 and 2 is not one. In between the two nodes (with potentials $v_1$ and $v_2$), this ...
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1 vote
Accepted

What effects do inductors have on circuits?

Inductors only play a part in a circuit if the current through them is changing and then the act ny producing an emf so as to try and oppose the change. If the current through an inductor is ...
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0 votes

Why short circuit doesn't takes places in a homopolar motor?

It is not a dead short. There is some resistance at the contacts of the moving parts and also the internal resistance of the battery. The battery can warm up but also drain fairly quickly due to the ...
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0 votes

Why $di/dt$ in inductor circuit reduces gradually after closing the switch?

When you close the circuit the current starts to rise, and it wants to rise very fast, ideally as a step function. But there are two phenomena that prevent this sudden rise: the first is that the ...
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0 votes

Why $di/dt$ in inductor circuit reduces gradually after closing the switch?

First, $$\frac{dI}{dt} = -\frac{\epsilon_L}{L}$$ where $L$ is the inductance and $\epsilon_L$ is the induced emf. As the current starts to increase in the circuit when the switch closes, an induced ...
0 votes

Why $di/dt$ in inductor circuit reduces gradually after closing the switch?

But what does the rate of change of current depends on? The inductance of the inductor coil. The relationship between the voltage across and current through an ideal inductor are $$v_{L}(t)=L\frac{...
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0 votes

Why $di/dt$ in inductor circuit reduces gradually after closing the switch?

The rate of change of current depends on the inductance of the coil- lots of inductance means the rate changes slowly (for no inductance at all, the rate of change is very high!) and the voltage of ...
0 votes

Why does a Resistor cause a potential drop?

I think it's because the build up of electrons on the (-) side of the resistor (like a damn) more readily fills charge holes reducing potential on that side, meaning, making that area where the ...
2 votes

If resistance in an electric current is 0 (ideally) then would there even be current flow?

No, in fact it would be quite the opposite. The current would be so high that all of the voltage would be dropped over the internal resistance of the battery. Thus the terminals would be at the same ...
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2 votes

How do signals propagate down unballanced coaxial transmission lines?

Perhaps my idea is not a good answer, but due to the limited size of the comment field, I will put my thoughts here. What you wrote "coaxial cable ... being that both lines have different ...
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0 votes

Going through resonance without damping by controlling the coupling between the oscillator and the generator

The main problem I see in your (not)equivalent models is that you start with a forced model and try to find an equivalent model with no external force. Comparison between the original system and the ...
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0 votes

Doubling the length of a solenoid doubles its inductance. Two identical solenoids in series have up to four times the inductance due to M. Why?

The formula you're using is an approximation that assumes $l>>r$. But in that approximation, there is negligible mutual inductance, since the flux spreads out at the end on a scale of $\approx r$...
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1 vote

Doubling the length of a solenoid doubles its inductance. Two identical solenoids in series have up to four times the inductance due to M. Why?

But this case is identical to the first case where we simply doubled the length of the original inductor and doubled the number of turns to get $L_{new}=2L_0$ so we have a contradiction. How can this ...
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0 votes

Voltmeter in series with a component?

@Jdeep - the voltmeter should read close to the output potential of the cell/battery, assuming the other components in the circuit have a total resistance that is significantly smaller than that of ...
3 votes

How do you solve any partial differential equation using equivalent circuits? How is this possible?

Prior to the ascendency of digital computation, analog computers were built for the express purpose of solving big differential equations. You implemented the equation through the use of resistors, ...
0 votes

Current sign convention RC and LC circuits

The difficultly here is that the sign convention used by the OP is not clear. Many Physicists use a sign convention which is dictated by the labelling of the capacitor with the left-hand diagram ...
  • 81k
2 votes

Surface charge on a current carrying conductor is impossible?

There is another way of looking at the problem that was pointed out in the 1941 paper The Electric Field Associated with a Steady Current in Long Cylindrical Conductor by Alexander Marcus. Like the ...
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0 votes

Current sign convention RC and LC circuits

You are correct and your book is wrong. For a discharge: $$I = -(dQ/dt)_{\rm capacitor}$$ For a charge: $$I = (dQ/dt){\rm capacitor}$$ While the current in the circuit is: $$I= \int_ \Sigma \...
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1 vote

Current sign convention RC and LC circuits

I don't understand what you mean by "the rate at which charge is leaving the capacitor should equal the rate at which the circuit is gaining the charge". Perhaps that's why I can't follow ...
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