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Femme_physics
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Femme_physics said:
You seem to have a short circuit across the capacitor and so across the rest of your circuit apart from the top resistor ( presumably R1)
The two parallel resistor-diode circuits can be analysed by getting the parallel resistor combination and putting it in series with one diode.
You can do this because both diodes will drop 0.7 volts, so you can join their anodes together and no current will flow between points of equal voltage.
For example, let R1 = 1000 ohms, R2 = 2000 ohms, R3 = 3000 ohms Diode voltage = 0.7 volts.
Parallel R2 and R3 = 1200 ohms call this R4
So voltage across R1 and R4 = 15 - diode voltage = 14.3 volts.
Current in R1 and R4 = 14.3 Volts / (1000 ohms + 1200 ohms) = 0.0065 amps
Voltage across R4 if C wasn't there = IR = 0.0065 * 1200 = 7.8 volts
R4 plus diode voltage = 8.5 volts
So this is the maximum voltage that C could charge to, if it didn't have a short circuit on it. :)
vk6kro said:Kirchoff's Laws really only apply to a stable situation, so a charging capacitor would be analysed by taking a "snapshot" of the circuit operation.
Femme_physics said:I.e.
http://img337.imageshack.us/img337/2701/15irk.jpg [Broken]
Is this alright? Is this "legal"?
psparky said:I agree. Kirchoff's laws and Ohm's law apply to 100% of the cases.
Adding a capacitor, inductor, diode...etc does not change that.
Accepting these laws seems to take time...but it shouldn't.
KVL, KCL and V=IR
Just accept it now.
vk6kro said:Not really.
If you have a capacitor that is still charging, you might apply some equations to it for one instant in time, but then it changes as the capacitor charges up some more.
Incidentally, phasors are only relevant for AC circuits. This is DC.
I like Serena said:Yes. Really.
At any point in time, you have voltages and currents to which KVL and KCL apply.
The theory of phasors holds true in DC as well.
With DC, you would need to calculate the inverse Laplace (or Fourier) transform to get the proper result.
psparky said:You know...I have to say.
"I like Serena"
Femme_physics said:Basically, while KLV and KCL always stand true, they won't have me to find a solution with respect to time... which is what I have in my case. But I will post it in the homework section later.. I have more relevant concerns when it comes to electronics, so I'll post this exercise later. Thanks a lot, everyone! :)
Femme_physics said:Basically, while KVL and KCL always stand true
Which is why I Like Serena and Psparky are wrong when the say that Kirchoff's Laws are universal. In non-conservative fields such as time-varying electric/magnetic fields, the voltage between points A and B is not uniquely defined--it depends on the path followed between the points.rbj said:KVL represents conservation of energy in what we call a "conservative field" and KCL represents conservation of charge in a system that doesn't allow too much charge to build up at any particular node.
gnurf said:Which is why I Like Serena and Psparky are wrong when the say that Kirchoff's Laws are universal. In non-conservative fields such as time-varying electric/magnetic fields, the voltage between points A and B is not uniquely defined--it depends on the path followed between the points.
The sum of the voltages around the loop driven by a time-varying magnetic field is non-zero, as shown by Prof. Lewin here:
OP should ignore this post.
Lumped circuit-theory is already based on approximations of Maxwell's equations (e.g. capacitor current I = C*dV/dt is derived by ignoring the effect of the time-varying magnetic field in Faraday's Law). So within lumped circuit theory, I'd say KVL and KCL are always true. That's why I said OP could ignore my post. What is this epsilon you mention?rbj said:you mean your post? for circuits, KVL and KCL are within [itex]\epsilon[/itex] of being precisely true.
gnurf said:Which is why I Like Serena and Psparky are wrong when the say that Kirchoff's Laws are universal. In non-conservative fields such as time-varying electric/magnetic fields, the voltage between points A and B is not uniquely defined--it depends on the path followed between the points.
The sum of the voltages around the loop driven by a time-varying magnetic field is non-zero, as shown by Prof. Lewin here:
OP should ignore this post.
Antiphon said:Here we go again. KVL and KCL are universal and always work. Period.
If EM seems that simple to you, then you either know a lot or more likely nothing.psparky said:It's truly unbelievable that people try to argue this.
Like I said...for some reason it takes a while to sink in.
For the voltage drop across a standard LED, I have seen specs ranging from 1.5 to 5V. Because this depends on the LED used, this parameter should be supplied in this question. I suspect you were thinking of a silicon diode having a typical drop of 0.7 V.vk6kro said:For example, let R1 = 1000 ohms, R2 = 2000 ohms, R3 = 3000 ohms Diode voltage = 0.7 volts.
:)
gnurf said:Antiphon: Here you go again--I must have missed something! Did you read my follow-up comment, though? I tried to communicate that Kirchoff's Laws apply in lumped circuit theory precisely for the reasons you mentioned.
If I understand you correctly, you're saying that Kirchoff's laws are "universal" because they should only be applied to lumped circuits. So when Prof. Lewin replaces the DC source with a time varying magnetic field to induce a voltage in the loop, he re-applies KVL to a problem that should no longer be solved with KVL? Is this what you're saying?
OP, sorry for messing up your thread.
Ouabache said:For the voltage drop across a standard LED, I have seen specs ranging from 1.5 to 5V. Because this depends on the LED used, this parameter should be supplied in this question. I suspect you were thinking of a silicon diode having a typical drop of 0.7 V.
psparky said:It's truly unbelievable that people try to argue this.
Like I said...for some reason it takes a while to sink in.
I feel bad for the OP tho, your thread is kind of off track, but I think you got your answer that KVL and KCL can be used with a capacitor.
psparky said:The thread is right on track.
She originally asked "Am I allowed to use KLV and KCL with a capacitor in the circuit?"
Some of you guys are more or less saying we can't used KVL when calculating drag on an airplane at 600 mph. Ya, we get that.
The KVL, KCL, and V=IR refer to circuits. There is never a case in a CIRCUIT where these are not true.
Show me a circuit where the sum of the voltage drops in a loop don't add up to be the voltage sorce.
Show me a circuit where current in doesn't equal current out of a node.
Show me a circuit where V does not equal IR.
Thank you in advance.
DragonPetter said:https://ece.uwaterloo.ca/~dwharder/NumericalAnalysis/10RootFinding/Newton/diode.png
Solve for the voltage and current in the diode using only KVL, KCL, and ohm's law :D
psparky said:Comon!
KCL, KVL and V=IR are all true! You just proved my point.
Just because the voltage source is not strong enough to turn on the circuit...that does not mean the laws are cancelled!
It is a FACT you may not be able to solve all circuits with those three tools...but those three tools are at your disposal at any times...and there laws always hold true!
AHHHHHHHHHHHHHHHHHHHHH!
psparky said:If the diode is on at .5 volts...and has a trickle current...it also has a resistance.
V=IR~!
psparky said:Comon!
https://ece.uwaterloo.ca/~dwharder/N...wton/diode.png [Broken]
KCL, KVL and V=IR are all true! You just proved my point.
Just because the voltage source is not strong enough to turn on the circuit...that does not mean the laws are cancelled!
It is a FACT you may not be able to solve all circuits with those three tools...but those three tools are at your disposal at any times...and there laws always hold true!
AHHHHHHHHHHHHHHHHHHHHH!
psparky said:You made the point that the diode has a non IV relationship. Ok...so what.
At any point in time that voltage or current is based on the resistance of the diode.
In the circuit you showed...let's say there is a trickle current for the diode.
This trickle current will also trickle thru the resistor.
The current thru the diode multipled thru the resistance at that time...is the voltage drop.
The trickle current thru the resistor has a voltage drop based on same rules.
The sum of the two voltage drops equals the voltage source (KVL)Current into the source equals current out.(KCL) V=IR on all things described.
You have not convinced me...nor will you ever.
KLV (Kirchhoff's Voltage Law) and KCL (Kirchhoff's Current Law) are fundamental laws in circuit analysis that describe the conservation of energy and charge, respectively. These laws are used to analyze and solve complex circuits.
Yes, KLV and KCL can be applied to circuits that contain capacitors. However, special considerations must be made for capacitors, such as taking into account the charging and discharging of the capacitor.
To apply KLV and KCL to a circuit with a capacitor, you must first identify all the branches and nodes in the circuit. Then, you can use KLV to analyze the voltage drops across the branches and KCL to analyze the current at each node, taking into account the behavior of the capacitor.
While KLV and KCL can be used with capacitors, there are some limitations. For example, these laws assume ideal conditions and may not accurately reflect the behavior of real-world circuits with non-ideal components. Additionally, KVL and KCL may not be applicable to circuits with non-linear elements, such as diodes.
Using KLV and KCL with a capacitor in a circuit allows for accurate analysis and understanding of the circuit's behavior. It can help in designing and troubleshooting circuits, as well as predicting the behavior of the circuit under different conditions. Additionally, KLV and KCL are fundamental laws in circuit analysis and are essential for understanding and solving complex circuits.